We now take up the various planets singly. The nearest to the sun, as

far as known, is Mercury, its average distance being only 36,000,000

miles. But its orbit is so eccentric that the distance varies from

28,500,000 miles at perihelion to 43,500,000 at aphelion. In consequence

its speed in its orbit is very variable, and likewise the amount of heat

and light received by it from the sun. On the average it gets more than

6½ times as much solar light and heat as the earth gets. But at

perihelion it gets 2½ times as much as at aphelion, and the time which

it occupies in passing from perihelion to aphelion is only six weeks,

its entire year being equal to 88 of our days. Being situated so much

nearer the sun than the earth is, Mercury is never visible to us except

in the morning or the evening sky, and then not very far from the sun.

Its diameter is about 3000 miles, but its mass is not certainly known

from lack of knowledge of its mean density. This lack of knowledge is

due to the fact that Mercury has no satellite. When a planet has a

satellite it is easy to calculate its density from its measured diameter

combined with the orbital speed of its satellite. Certain considerations

have led some to believe that the mean density of Mercury may be very

great, perhaps as great as that of lead, or of the metal mercury itself.

Not knowing the mass, we cannot say exactly what the weight of bodies on

Mercury is. We are also virtually ignorant of the condition of the

surface of this planet, the telescope revealing very little detail, but

it is generally thought that it bears a considerable resemblance to the

surface of the moon. There is another way in which Mercury is remarkably

like the moon. The latter, as we have seen, always keeps the same side

turned toward the earth, which is the same thing as saying that it turns

once on its axis, while going once around the earth. So Mercury keeps

always the same side toward the sun, making one rotation on its axis in

the course of one revolution in its orbit. Consequently, one side of

Mercury is continually in the sunlight, while the opposite side is

continually buried in night. There must, however, be regions along the

border between these two sides, where the sun does rise and set once in

the course of one of Mercury's years. This arises from the eccentricity

of the orbit, and the consequent variations in the orbital velocity of

the planet, which cause now a little of one edge and now a little of the

other edge of the dark hemisphere to come within the line of sunlight.

(The same thing occurs with the moon, though to a less degree owing to

the smaller eccentricity of the moon's orbit, which, however, is

sufficient to enable us to see at one time a short distance round one

side of the moon and at another time a short distance round the opposite

side.) This phenomenon is known as libration. Mercury apparently

possesses an atmosphere, but we know nothing certain concerning its

density.

* * * * *

The next planet, in the order of distance from the sun, is Venus, whose

average distance is 67,200,000 miles. The orbit of Venus is remarkable

for its small eccentricity, so that the difference between its greatest

and least distances from the sun is less than a million miles. The

period, or year, of Venus is 225 of our days. Owing to her situation

closer to the sun, she gets nearly twice as much light and heat as the

earth gets. In size Venus is remarkably like the earth, her diameter

being 7713 miles, which differs by only 205 miles from the mean diameter

of the earth. Her axis is nearly perpendicular to the plane of her

orbit. Her globe is a more perfect sphere than that of the earth, being

very little flattened at the poles or swollen at the equator. Although

Venus, like Mercury, has no satellite, her mean density has been

calculated by other means, and is found to be 0.89 that of the earth.

From this, in connection with her measured diameter, it is easy to

deduce her mass, and the force of gravity on her surface. The latter

comes out at about 0.85 that of the earth, _i. e._ a body weighing 100

pounds on the earth would weigh 85 pounds if removed to Venus. She

possesses an atmosphere denser and more extensive than would

theoretically have been expected—indicating, perhaps, a difference of

constitution. Her atmosphere has been estimated to be twice as dense as

ours, a great advantage, it may be remarked, from the point of view of

aëronautics. But this dense and abundant atmosphere renders Venus a very

difficult object for the telescope on account of the brilliance of its

reflection. In consequence, we know but little of the surface of the

planet.

One important result of this is that the question remains undecided

whether Venus rotates on her axis at a rate closely corresponding with

that of the earth, as some observers think, or whether, as others think,

she, like Mercury, turns only once on her axis in going once round the

sun. The importance of the question in its bearing on the habitability

of Venus is apparent, for if she keeps one face always sunward, then on

one side there is perpetual day and on the other perpetual night. On the

other hand, if she has days and nights approximately equal in length to

those of the earth, it may well be thought that she is habitable by

beings not altogether unlike ourselves, because the force of gravity on

her surface is not much less than on the earth, and her dense

atmosphere, filled with clouds, might tend to shield her inhabitants

from the effects of the greater amount of heat poured upon her by the

sun. As her orbit is inside that of the earth, Venus, like Mercury, is

only visible either in the evening or in the morning sky, but owing to

her greater actual distance from the sun, her apparent distance from it

in the sky is greater than that of Mercury.

[Illustration:

=The Great Nebula in Orion=

Photographed at the Lick Observatory by J. E. Keeler, with the

Crossley reflector. Exposure one hour.

]

Both of these planets, in consequence of passing alternately between the

sun and the earth and round the opposite side of the sun, present phases

resembling those of the moon. The reader can explain these to himself by

means of the experiment, before mentioned, with a billiard ball and a

lamp. In this case let the observer remain seated in his chair while

another person carries the ball round the lamp in such a manner that it

shall alternately pass between the lamp and the observer and round the

other side of the lamp. When Venus comes nearly in line between the

earth and the sun, she becomes an exceedingly brilliant object in either

the evening or the morning sky, although at such times we see, in the

form of a crescent, only a part of that half of her surface which is

illuminated. Her increase of brightness at such times is due to her

greater nearness to the earth. When between the earth and the sun she

may be only about 26,000,000 miles away, while when she is on the other

side of the sun she may be over 160,000,000 miles away. Both Venus and

Mercury when passing exactly between the sun and the earth are seen, in

the form of small black circles, moving slowly across the sun's disk.

These occurrences are called transits, and in the case of Venus have

been before referred to. They are more frequent with Mercury than with

Venus, but Mercury's transits are not utilisable for parallax

observations. The latest transit of Venus occurred in 1882, and there

will not be another until 2004. The latest transit of Mercury occurred

in 1907, and there will be another in 1914.

* * * * *

The earth is the third planet in order of distance, and then comes Mars,

whose average distance from the sun is 141,500,000 miles. The orbit of

Mars is so eccentric that the distance varies between 148,000,000 and

135,000,000 miles. Its period or year is about 687 of our days. In

consequence of its distance, Mars gets, on the average, a little less

than half as much light and heat as the earth gets. When it is on the

same side of the sun with the earth, and nearly in line with them, it is

said to be in opposition. At such times it is manifestly as near the

earth as it can come, and thus an opposition of Mars offers a good

opportunity for the telescopic study of its surface. These oppositions

occur once in about 780 days, but they are not all of equal importance,

because the distance between the two planets is not the same at

different oppositions. The cause of the difference of distance is the

eccentricity of the orbit. If an opposition occurs when Mars is in

aphelion its distance from the earth will be about 61,000,000 miles, but

if the opposition occurs when Mars is in perihelion the distance will be

only about 35,000,000 miles. The average distance at an opposition is

about 48,500,000 miles. The most favourable oppositions always occur in

August or September, and are repeated at an interval of from fifteen to

seventeen years. But at some of the intervening oppositions the distance

of the planet is not too great to afford good views of its surface. The

diameter of Mars is about 4330 miles, with a similar polar flattening to

that of the earth. Its density is 0.71 that of the earth, and the force

of gravity on its surface 0.38. A body weighing 100 pounds on the earth

would weigh 38 pounds on Mars. The evidence in regard to its atmosphere

is conflicting, but the probability is that it has an atmosphere not

denser than that existing on our highest mountain peaks. Opinions

concerning the existence of water vapour on Mars are also conflicting.

One fact tending to show that its atmosphere must be very rare and

cloudless is that its surface features are very plainly discernible with

telescopes.

[Illustration:

_Fig. 16. Orbits of Mars and the Earth._

Inspection shows at once why the oppositions of Mars which occur in

August and September are the most favourable because Mars being then

near the perihelion point of its elongated orbit is comparatively

near the earth, while oppositions which occur in February and March

are very unfavourable because then Mars is near the aphelion point

of its orbit, and its distance from the earth is much greater. The

oppositions occur along the more favourable part of the orbit about

two years and two months apart. Thus the figure shows that the

opposition of September 24, 1909 was followed by one on November 25,

1911.

]

About each pole, as it happens to be turned earthward, is to be seen a

round white patch (supposed to be snow), and this gradually disappears

as the summer advances in that hemisphere of the planet—for Mars has

seasons very closely resembling our seasons, except that they are about

twice as long. The inclination of the axis of Mars to the plane of its

orbit is about 24° 50′, which is not very different from the inclination

of the earth's axis. Moreover, Mars rotates in a period of 24 hours, 37

min., 22 sec., so that the length of day and night upon its surface is

very nearly the same as upon the earth. The surface of the planet is

marked by broad irregular areas of contrasting colour, or tone, some of

them being of a slightly reddish, or yellowish, hue, and others of a

neutral dusky tint. The general resemblance to a globe of the earth,

with differently shaped seas and oceans, is striking.

On account of the many likenesses between Mars and the earth, some

astronomers are disposed to think that Mars may be a habitable planet.

The terms “seas” and “continents” were formerly applied to the

contrasted areas just spoken of, but now it is believed that there are

no large bodies of water on Mars. Crossing the light, or

reddish-coloured, areas there are sometimes seen great numbers of

intersecting lines, very narrow and faint, which have received the name

of “canals.” Some speculative minds find in these ground for believing

that they are of artificial origin, and a theory has been built up,

according to which the so-called canals are “irrigated bands,” the

result of the labours of the inhabitants. The argument of the advocates

of this theory is put about as follows: Mars is evidently a nearly

dried-up planet, and most of the water left upon it is periodically

locked up in the polar snows. As these snows melt away in the summer

time, now in one hemisphere and now in the other, the water thus formed

is conducted off toward the tropical and equatorial zones by innumerable

canals, too small to be seen from the earth. The lands irrigated by

these canals are narrow strips, whose situation is determined by local

circumstances, and which cross one another in all directions. Within

these bands, which enlarge into rounded “oases” where many of them

intersect, vegetation pushes, and its colour causes them to appear as

dark lines and patches on the surface of the planet. The fact that the

lines make their appearance gradually, after the polar caps begin to

disappear, is regarded as strongly corroborative of the theory. In

answer to the objection that works so extensive as this theory of

irrigation calls for would be practically impossible, it is replied that

the relatively small force of gravity on Mars not only immensely

diminishes the weight of all bodies there, but also renders it possible

for animal forms to attain a greater size, with corresponding increase

of muscular power. It is likewise argued that Mars may have been longer

inhabited than the earth, and that its inhabitants may consequently have

developed a more complete mastery over the powers of nature than we as

yet possess. Many astronomers reject these speculations, and even aver

that the lines called “canals” (and it must be admitted that many

powerful telescopes show few or none of them) have no real existence,

what is seen, or imagined to be seen, being due to some peculiarity of

the soil, rocks, or atmosphere.

Mars has two small satellites, revolving round it with great speed at

close quarters. The more distant satellite, Deimos, is 14,600 miles from

the centre of Mars and goes round it in 30 hours, 18 min. The nearer

one, Phobos, is only 5800 miles from the planet's centre, and its period

of revolution is only 7 hours, 39 min., so that it makes more than three

circuits while the planet is rotating once on its axis. Both of the

satellites are minute in size, probably under ten miles in diameter.

* * * * *

Beyond Mars, at an average distance of about 246,000,000 miles from the

sun, is a system of little planets called asteroids. More than 600 are

now known, and new ones are discovered every year, principally by means

of photography. Only four of these bodies are of any considerable size,

and they were, naturally, the first to be discovered. They are Ceres,

diameter 477 miles; Pallas, 304 miles; Vesta, 239 miles; and Juno, 120

miles. Many of the others have a diameter of only about ten, or even,

perhaps, as little as five, miles. Their orbits are more eccentric than

those of any of the large planets, and one of them, Eros, has a mean

distance of 135,000,000 miles, and a least distance of only 105,000,000,

so that it is nearer to the sun than Mars is. Eros may, under favourable

circumstances, approach within 14,000,000 miles of the earth. This fact,

as already mentioned, has been taken advantage of for measuring its

distance from the earth, from which the distance of the sun may be

calculated with increased accuracy. Eros and some others of the

asteroids seem to be of an irregular or fragmentary form, and this has

been used to support a theory, which is not, however, generally

accepted, that the asteroids are the result of an explosion, by which a

larger planet was blown to pieces.

* * * * *

Sixth in order of distance from the sun (counting the asteroids as

representing a single body) is the greatest of all the planets, Jupiter.

His average distance from the sun is 483,000,000 miles, but the

eccentricity of his orbit causes him to approach within 472,500,000

miles at perihelion, and to recede to 493,500,000 miles at aphelion.

When in opposition, Jupiter's mean distance from the earth is

390,000,000 miles. This gigantic planet has a mean diameter of 87,380

miles, but is so flattened at the poles and bulged round the equator

that the polar diameter is only 84,570 miles, while the equatorial

diameter is 90,190 miles, a difference of 5680 miles. This peculiar form

is doubtless due to the planet's swift rotation. The axis, like that of

Venus, is nearly perpendicular to the plane of the orbit. He makes a

complete turn on his axis in a mean period of 9 hours, 55 minutes. The

reason for saying “a mean period” will appear in a moment. Jupiter's

year is equal to 11.86 of our years, but it comes into opposition to the

sun, as seen from the earth, once in every 399 days.

The volume of Jupiter is about 1300 times that of the earth, _i.e._ it

would take 1300 earths rolled into one to equal Jupiter in size. But its

mean density is slightly less than one quarter of the earth's, so that

its mass is only 316 times greater than the earth's. The force of

gravity on its surface is 2.64 times the earth's. A body weighing 100

pounds on the earth would weigh 264 pounds on Jupiter. It will be

observed that Jupiter's mean density is very nearly the same as that of

the sun, and we conclude that it cannot be a solid, rigid globe like the

earth. This conclusion is made certain by the fact that its period of

rotation on its axis is variable, another resemblance to the sun. The

equatorial parts go round in a shorter period than parts situated some

distance north or south of the equator. It may be supposed that there is

a solid nucleus within, but if so, no direct evidence of its existence

has been found.

Nevertheless, although Jupiter appears to be in a cloud-like state, it

does not shine with light of its own, so that its temperature, while no

doubt higher than that of the earth, cannot approach anywhere near that

of the sun. We do not know of what materials Jupiter is composed, for

spectroscopic analysis applies especially to bodies which shine with

their own light. When they shine only by reflected light received from

the sun, their spectra resemble the regular solar spectrum, except for

the presence of faint bands due to absorption in the planet's

atmosphere. It may be that there are no elements of great atomic

density, such as iron or lead, in the globe of Jupiter. Yet in the

course of long ages the planet may become smaller and more condensed, in

consequence of the escape of its internal heat. In this way Jupiter may

be regarded as representing an intermediate stage of evolution between

an altogether vaporous and very hot body like the sun, and a cool and

solid one like the earth.

Jupiter presents a magnificent appearance in a good telescope. Its

oblong disk is seen crossed in an east and west direction, and parallel

to its equator, by broad, vari-coloured bands, called belts. These

frequently change in form and, to some extent, in situation, as well as

in number. But there are always at least two wide belts, one on each

side of the equator. In 1878 a very remarkable feature was noticed just

south of the principal south belt of Jupiter, which has become

celebrated under the name of the Great Red Spot. In a few years after

its discovery its colour faded, but it still remains visible, with

varying degrees of distinctness, as an oblong marking, about 30,000

miles long and 7000 miles broad. The outer border of the great south

belt bends away from the spot, as if some force of repulsion acted

between them, or as if the spot were an elevation round which the clouds

of the belt flowed like a river round a projecting headland. The nature

of this curious spot is unknown. Other smaller spots, sometimes white,

sometimes dusky, occasionally make their appearance, but they do not

exhibit the durability of the Great Red Spot.

Jupiter has eight satellites, four of which, known since the time of

Galileo, are conspicuous objects in the smallest telescope. All but one

of these four are larger than our moon, while the other four are

extremely insignificant in size. The four principal satellites are

designated by Roman numerals, I, II, III, IV, arranged in the order of

distance from the planet. They also have names which are seldom used.

Satellite I (Io) has a diameter of 2452 miles, and revolves in a period

of 1 day, 18 hours, 27 min., 35.5 sec., at a mean distance of 261,000

miles; II (Europa) is 2045 miles in diameter, and revolves in 3 days, 13

hours, 13 min., 42.1 sec., at a mean distance of 415,000 miles; III

(Ganymede) has a diameter of 3558 miles, a period of seven days, 3

hours, 42 min., 33.4 sec., and a mean distance of 664,000 miles; IV

(Callisto) is 3345 miles in diameter, has a period of 16 days, 16 hours,

32 min., 11.2 sec., and a mean distance of 1,167,000 miles. The object

of giving the periods with extreme accuracy will appear when we speak of

the use made of observations of Jupiter's satellites. The first of the

four small satellites, discovered by Barnard in 1892, is probably less

than 100 miles in diameter, and has a mean distance of 112,500 miles,

and a period of only 11 hours, 57 min., 22.6 sec. The other small

satellites are much more distant than any of the large ones, the latest

to be discovered, the eighth, being situated at a mean distance of about

15,000,000 miles, but travelling in an orbit so eccentric that the

distance ranges between 10,000,000 and 20,000,000 miles. The period is

about two and a fifth years. But the most remarkable fact is that this

satellite revolves round Jupiter from east to west, a direction contrary

to that pursued by all the others, and contrary to the direction which

is almost universal among the rotating and revolving bodies of the solar

system.

The large satellites are very interesting objects for the telescope.

When they come between the sun and Jupiter their round black shadows can

be plainly seen moving across his disk, and when they pass round into

his shadow they are suddenly eclipsed, emerging after a time out of the

other side of the shadow. These phenomena are known as transits and

eclipses, and their times of occurrence are carefully predicted in the

_American Ephemeris and Nautical Almanac_, published at Washington for

the benefit of astronomers and navigators, because these eclipses can be

employed in comparing local time with standard meridian time. They were

formerly utilised to determine the velocity of light, in this way:

As the earth goes round its orbit inside that of Jupiter the latter is

seen in opposition to the sun at intervals of 399 days. When it is thus

seen the earth must be between the sun and Jupiter, and the distance

between the two planets is the least possible. But when the earth has

passed round to the other side of the sun from Jupiter this distance

becomes the greatest possible. The increase of distance between the two

planets, as the earth goes from the nearest to the farthest side of its

orbit, is about 186,000,000 miles. Now it was noticed by the Danish

astronomer, Roemer, that as the earth moved farther and farther from

Jupiter the times of occurrence of the eclipses kept getting later and

later, until when the earth arrived at its greatest distance the

eclipses were about 16 minutes behind time. He correctly inferred that

the retardation of the time was due to the increase of the distance, and

that the 16 minutes by which the eclipses were behindhand when the

distance was greatest represented the time taken by light to cross the

186,000,000 miles of space by which the earth had increased its distance

from Jupiter. In other words, light must travel 186,000,000 miles in

about sixteen minutes, from which it was easy to calculate its speed per

second—which we now know to be 186,330 miles. Our knowledge of the

velocity of light furnishes one of the means of calculating the distance

of the sun.

We come next to the beautiful planet Saturn, whose mean distance from

the sun is 886,000,000 miles. The distance varies between 911,000,000

and 861,000,000 miles. Saturn's year is equal to 29.46 of our years. It

comes into opposition every 378 days. The most surprising feature of

Saturn is the system of immense rings surrounding it above the equator.

The globe of the planet is 76,470 miles in equatorial diameter, and

69,780 miles in polar diameter, a difference of 6690 miles, so that

Saturn is even more compressed at the poles and swollen at the equator

than Jupiter. The axis of rotation is inclined 27° from a perpendicular.

The rings are three in number, very thin in proportion to their vast

size, and placed one within another in the same plane. The outer

diameter of the outer ring, called Ring A, is about 168,000 miles. Its

breadth is about 10,000 miles. Then comes a gap, about 1600 miles

across, separating it from Ring B, the brightest of the set. This is

about 16,500 miles broad, and at its inner edge it gradually fades out,

blending with Ring C, which is called the crape, or gauze, ring, because

it has a dusky appearance, and is so translucent that the globe of the

planet can be seen through it. This ring is about 10,000 miles broad,

and its inner edge comes within a distance of between 9000 and 10,000

miles of the surface of the planet. Ring A apparently has a very narrow

gap running round at about a third of its breadth from the outer edge.

This, known as Encke's Division, is not equally plain at all times.

Occasionally observers report the temporary appearance of other thin

gaps.

The mean density of Saturn is less than that of any other planet, being

but 0.13 that of the earth, or 0.72 that of water. It follows that this

great planet would float in water. The weight of bodies at its surface

would be a little less than three-quarters of their weight on the

surface of the earth. The globe of Saturn, like that of Jupiter, is

marked by belts parallel with the equator, but they are less definite in

outline and less conspicuous than the belts of Jupiter. The equatorial

zone often shows a beautiful pale salmon tint, while the regions round

the poles are faintly bluish. Light spots are occasionally seen upon the

planet, and it appears to rotate more rapidly at the equator than in the

higher latitudes. There seems to be every reason to think that Saturn,

also, is of a vaporous constitution, although it may have a relatively

condensed nucleus.

But while the globe of the planet appears to be vaporous, the same is

not true of the rings. We have already mentioned the fact that they are

exceedingly thin in proportion to their great size and width. The

thickness has not been determined with exactness, but it probably does

not exceed, on the average, one hundred miles. There appear to be

portions of the rings which are thicker than the average, as if the

matter of which they are composed were heaped up there. This matter

evidently consists of an innumerable multitude of small bodies. In other

words, the rings are composed of swarms of what may be called meteors.

That their composition must be of this nature, although the telescope

does not reveal it, has been proved in two ways: first, by mathematical

calculation, which shows that if the rings were all of a piece, whether

solid or liquid, they would be destroyed by the contending forces of

attraction to which they are subject; and, second, by spectroscopic

observation, which proves, in a way that will be shown when we come to

deal with the stars, that the rings rotate with velocities proportional

to the distances of their various parts from the centre of the planet.

Hence it is inferred that they must consist of a vast number of small

bodies or particles.

Saturn has ten satellites, all revolving outside the rings. The names of

nine of these in the order of increasing distance are: Mimas, distance

117,000 miles; Enceladus, distance 157,000 miles; Tethys, distance

186,000 miles; Dione, distance 238,000 miles; Rhea, distance 332,000

miles; Titan, distance 771,000 miles; Hyperion, distance 934,000 miles;

Japetus, distance 2,225,000 miles; and Phœbe, distance 8,000,000 miles.

The last, like the eighth satellite of Jupiter, revolves in a retrograde

direction. Only Titan and Japetus are conspicuous objects. The period of

Mimas is only about 22½ hours; that of Titan is 15 days, 22 hours, 41

min., and that of Japetus about 79 days, 8 hours. Barnard's measurements

indicate for Titan a diameter of 2720 miles. Japetus is probably about

two-thirds as great in diameter as Titan.

[Illustration:

=Photographs of Mars=

Made at the Yerkes Observatory by E. E. Barnard, with the forty-inch

refractor, September 28, 1909.

]

Beyond Saturn, in the order named, are Uranus and Neptune. The mean

distance of the former from the sun is 1,782,000,000 miles, and that of

the latter 2,791,500,000 miles. The orbit of Uranus is more eccentric

than that of Neptune. The diameter of Uranus is about 32,000 miles and

that of Neptune about 35,000 miles. The year of Uranus is equal to 84 of

our years, and that of Neptune to 164.78. These planets are so remote,

and so poorly illuminated by the sun, that the telescope reveals very

little detail on their surfaces. Their density is somewhat less than

that of Jupiter. Uranus has four satellites, Ariel, Umbriel, Titania,

and Oberon, situated at the respective distances of 120,000, 167,000,

273,000, and 365,000 miles. Neptune has one, nameless, satellite, at a

distance of 225,000 miles.

The most remarkable thing about these two planets is that their axes of

rotation, as compared with those of all the other planets, are tipped

over into a different plane, so that they rotate in a retrograde or

backward direction, and their satellites, in like manner, revolve from

east to west. The axis of Uranus is not far from upright to the plane of

the ecliptic, so that the motion of its satellites carries them

alternately far northward and far southward of that plane, but the axis

of Neptune is tipped so far over that the retrograde, or east to west,

motion is very pronounced. Neptune is celebrated for having been

discovered by means of mathematical calculations, based on its

disturbing attraction on Uranus. These calculations showed where it

ought to be at a certain time, and when telescopes were pointed at the

indicated spot the planet was found. Similar disturbances of the motions

of Neptune lead some astronomers to think that there is another, yet

undiscovered, planet still more distant.

7. =Comets.= Comets are the most extraordinary in appearance of all

celestial objects visible to the naked eye. Great comets have been

regarded with terror and superstitious dread in all ages of the world,

wherever ignorance of their nature has prevailed. They have been taken

for prognosticators of wars, famines, plagues, the death of rulers, the

outbreak of revolutions, and the subversion of empires. One reason for

this, aside from their strange and menacing appearance, is, no doubt,

the rarity of very great and conspicuous comets. It was not until Newton

had demonstrated the law of gravitation that the fact began to be

recognised that comets are controlled in their motions by the sun. We

now know that they travel in orbits, frequently, and perhaps always,

elliptical, having the sun in one of the foci. Comets are habitually

divided into two classes: first, periodical comets, meaning those which

have been observed at more than one return to perihelion; and, second,

non-periodical comets, meaning those which have been seen but once, but

which, nevertheless, may return to perihelion in a period so long that a

second return has not been observed. A better division is into comets of

short period, and comets of long, or unknown, periods.

[Illustration:

_Fig. 17. Ellipse, Parabola, and Hyperbola._

The figure shows graphically why it is so difficult to tell exactly

the form of a comet's orbit. The three kinds of curves are nearly of

the same form near the focus (the Sun), and it is only in that part

of its orbit that the comet can be seen. Moreover a comet is, at

best, a misty and indefinite object, which renders it so much the

more difficult to obtain good observations of its precise position

and movement.

]

Still, many astronomers are disposed to think that the majority of

comets do not travel in elliptical but in parabolic, and a few in

hyperbolic, orbits. This calls for a few words of explanation. Ellipses,

parabolas, and hyperbolas are all conic-section curves, but the ellipse

alone returns into itself, or forms a closed circuit. In each case the

sun is situated at the focus where the perihelion, or nearest approach,

of the comet occurs, but only comets travelling in elliptical orbits

return again after having once been seen. A comet moving in a parabola

would go back into the depths of space nearly in the direction from

which it had come, and would never be seen again; and if it moved in a

hyperbola it would go off toward another quarter of the celestial

sphere, and likewise would never return. Now it is true that the forms

calculated for the orbits of the majority of comets that have been

observed appear to be parabolic (a very few seem to be hyperbolic), and

if this is the fact such comets cannot be permanent members of the solar

system, but must enter it from far-off regions of space, and having

visited the sun must return to such regions without any tendency to come

back again. In that case they may pay similar visits to other suns.

But it is quite possible that what appear to be parabolic orbits may, in

reality, be ellipses of very great eccentricity. The difficulty in

determining the precise shape of a comet's orbit arises from the fact

that all three of the curves just mentioned closely approximate to one

another in the neighbourhood of their common focus, the sun, and it is

only in that part of their orbits that comets are visible. The whole

question is yet in abeyance, but, as we have said, it seems likely that

all comets really move in elliptical orbits, and consequently never get

entirely beyond the control of the sun's attraction. But in all cases

the orbits of comets are much more eccentric than those of the large

planets. The famous comet of Halley, for instance, which has the longest

period of any of the known periodical class, about seventy-five years,

is 3,293,000,000 miles from the sun when in aphelion, and only

54,770,000 miles when in perihelion.

Comets, when near the sun, are greatly affected by the disturbing

attraction of large planets, and especially of the most massive of them

all, Jupiter. The effect of this disturbance is to change the form of

their orbits, with the not infrequent result that the latter are altered

from apparent parabolas into unquestionable ellipses, and thus the

comets concerned are said to be “captured,” or made prisoners to the

sun, by the influence of the disturbing planet. About twenty small

comets are known as “Jupiter's Comet Family,” because they appear to

have been “captured” in this way by him. A few others are believed to

have been similarly captured by Saturn, Uranus, and Neptune.

The orbits of comets differ from those of the planets in other ways

beside their greater eccentricity. The planets all move round the sun

from west to east, but comets move in both directions. The orbits of the

planets, with the exception of some of the asteroids, all lie near one

common plane, but those of comets are inclined at all angles to this

plane, some of them coming down from the north side of the ecliptic and

others up from the south side.

A comet consists of two distinct portions: first, the head, or nucleus;

and, second, the tail. The latter only makes its appearance when the

comet is drawing near the sun, and, as a whole, it is always directed

away from the sun, but usually more or less curved backward along the

comet's course, as if the head tended to run away from it. The

appearance of a comet's tail at once suggests that it is produced by

some repulsive force emanating from the sun. Recently there has been a

tendency to explain this on the principle of what is known as the

pressure of light. This demands a brief explanation. Light is believed

to be a disturbance of the universal ether in the form of waves which

proceed from the luminous body. These waves possess a certain mechanical

energy tending to drive away bodies upon which they impinge. The energy

is relatively slight, and in ordinary circumstances produces no

perceptible effect, but when the body acted upon by the light is

extremely small the pressure may become so great relatively to

gravitation as to prevail over the latter. To illustrate this, let us

recall two facts—first, that gravitation acts upon the mass, _i.e._ all

the particles of a body throughout its entire volume; and, second, that

pressure acts only upon the exterior surface. Consequently gravitation

is proportional to the volume, while the pressure of light is

proportional to the surface of the body acted upon. Now the mass, or

volume, of any body varies as the cube of its diameter, and the surface

only as the square. If, then, we have two bodies, one of which has twice

the diameter of the other, the mass of the second will be eight times

less than that of the first, but the surface will be only four times

less. If the second has only one-third the diameter of the first, then

its mass will be twenty-seven times less, but its surface only nine

times less. Thus we see that as we diminish the size of the body, the

mass falls off more rapidly than the area of the surface, and

consequently the pressure gains relatively to the gravitation.

Experiment has corroborated the conclusions of mathematics on this

subject, and has shown that when a particle of matter is only about one

one-hundred-thousandth of an inch in diameter the pressure of light upon

it becomes greater than the force of gravitation, and such a particle,

situated in open space, would be driven away from the sun by the light

waves. This critical size would vary with the density of the matter

composing the particle, but what we have said will serve to convey an

idea of its minuteness.

Now in applying this to comets' tails it is only necessary to remark

that they are composed of either gaseous or dusty particles, or both,

rising from the nucleus, probably under the influence of the heat or the

electrical action of the sun, and these particles, being below the

critical size, are driven away from the sun, and appear in the form of a

tail following the comet. It may be added that the same principle has

been evoked to explain the corona of the sun, which may be composed of

clouds of gas or dust kept in suspension by the pressure of light.

The nuclei of comets contain nearly their whole mass. The actual mass of

no comet is known, but it can in no case be very great. Moreover, it is

probable that the nucleus of a comet does not consist of a single body,

either solid or liquid, but is composed of a large number of separate

small bodies, like a flock of meteors, crowded together and constantly

impinging upon one another. As the comet approaches the sun the nucleus

becomes violently agitated, and then the tail begins to make its

appearance.

The possibility exists of an encounter between the earth and the head of

a comet, but no such occurrence is known. Two or three times, however,

the earth is believed to have gone through the tail of a comet, the last

time in 1910, when Halley's comet passed between the earth and the sun,

but no certain effects have been observed from such encounters. The

spectroscope shows that comets contain various hydrocarbons, sodium,

nitrogen, magnesium, and possibly iron, but we know, as yet, very little

about their composition. The presence of cyanogen gas was reported in

Halley's comet at its last appearance. We are still more ignorant of the

origin of comets. We do know, however, that they tend to go to pieces,

especially those which approach very close to the sun. The great comet

of 1882, which almost grazed the sun, was afterward seen retreating into

space scattered into several parts, each provided with a tail. In at

least one case, several comets have been found travelling in the same

track, an indication that one large original comet has been separated

into three or four smaller ones. This appears to be true of the comets

of 1843, 1880, and 1882,—and perhaps the comet of 1576 should be added.

But the most remarkable case of disruption is that of Biela's Comet,

which first divided into two parts in 1846 and then apparently became

scattered into a swarm of meteors which was encountered by the earth in

1872, when it passed near the old track of the comet. This leads us to

our next subject.

=8. Meteors.= Everybody must, at some time, have beheld the phenomenon

known as a falling, or shooting, star. A few of these objects can be

seen darting across the sky on almost any clear night in the course of

an hour or two of watching. Sometimes they appear more numerously, and

at intervals they are seen in “showers.” They are called meteors, and it

is believed that they are minute solid bodies, perhaps averaging but a

small fraction of an ounce in weight, which plunge into the atmosphere

with velocities varying from twenty to thirty or more miles per second,

and are set afire and consumed by the heat of friction developed by

their rush through the air. Anybody who has seen a bullet melted by the

heat suddenly developed when it strikes a steel target has had a graphic

illustration of the transformation of motion into heat. But if we could

make the bullet move fast enough it would _melt in the air_, the heat

being developed by the constant friction.

The connection of meteors with comets is very interesting. In the year

1833, a magnificent and imposing display of meteors, which, for hours,

on the night between the 13th and 14th of November, filled the sky with

fire-balls and flaming streaks, astonished all beholders and filled many

with terror. It was found that these meteors travelled in an orbit

intersecting that of the earth at the point where the latter arrived in

the middle of November, and also that they had a period of revolution

about the sun of 33¼ years, and were so far scattered along their orbit

that they required nearly three years to pass the point of intersection

with the orbit of the earth. Thus it was concluded that for three years

in succession, in mid-November, there should be a display of the meteors

plunging into the earth's atmosphere. But only in the year when the

thickest part of the swarm was encountered by the earth would the

display be very imposing. Upon this it was predicted that there would be

a recurrence of the phenomenon of 1833 in the year 1866. It happened as

predicted, except that the number of meteors was not quite so great as

before. In the meantime, it had been discovered that these meteors

followed in the track of a comet known as Temple's Comet, and also that

certain other meteors, which appear every year in considerable numbers

about the 10th of August, followed the track of another comet called

Tuttle's Comet. Then in 1872 came the display, mentioned in the last

section, of meteors which were evidently the debris of the vanished

comet of Biela. The inference from so many similar cases was

irresistible that the meteors must be fragments of destroyed or

partially destroyed comets. Several other cases of identity of orbits

between meteors and comets have been discovered.

It has been said that the August meteors appear every year. The

explanation of this is that they have, in the course of many ages, been

scattered around the whole circuit of their orbit, so that each year,

about the 10th of August, when the earth crosses their track, some of

the meteors are encountered. They are like an endless railroad train

travelling upon a circular track. The November meteors also appear, in

small numbers, every year, a fact indicating that some of them, too,

have been scattered all around their orbit, although the great mass of

them is still concentrated in an elongated swarm, and a notable display

can only occur when this swarm is at the crossing simultaneously with

the earth. These meteors were eagerly awaited in 1899, when it was hoped

that the splendid displays of 1833 and 1866 might be repeated, but,

unfortunately, in the meantime the planets Jupiter and Saturn, by their

disturbing attractions, had so altered the position of the path of the

meteors in space that the principal swarm missed the connection. There

are many other periodical meteor showers, generally less brilliant than

those already mentioned, and some astronomers think that all of them had

their origin from comets.

It is not known that any meteor from any of these swarms has ever

reached the surface of the earth. The meteors appear to be so small that

they are entirely burnt up before they can get through the atmosphere,

which thus acts as a shield against these little missiles from outer

space. But there is another class of meteoric bodies, variously known as

meteorites, aërolites, uranoliths, or bolides, which consists of larger

masses, and these sometimes fall upon the earth, after a fiery passage

through the air. Specimens of them may be seen in many museums. They are

divided into two principal classes, according to their composition:

first, stony meteorites, which are by far the most numerous; and,

second, iron meteorites, which consist of almost pure iron, generally

alloyed with a little nickel. The stony meteorites, which usually

contain some compound of iron, consist of a great variety of substances,

including between twenty and thirty different chemical elements.

Although they resemble in many ways minerals of volcanic origin on the

earth, they also possess certain characteristics by which they can be

recognised even when they have not been seen to fall.

When a meteorite passes through the air it makes a brilliant display of

light, and frequently bursts asunder, with a tremendous noise,

scattering its fragments about. The largest fragment of a meteorite

actually seen to fall, weighs about a quarter of a ton. Upon striking

the ground the meteorite sometimes penetrates to a depth of several

feet, and some have been picked up which were yet hot on the surface,

although very cold within. It is not known that meteorites have any

connection with comets, and their origin can only be conjectured. Among

the various suggestions that have been made the following may be

mentioned: (1) that they have been shot out of the sun—particularly the

iron meteorites; (2) that they were cast into space by lunar volcanoes

when the moon was still subject to volcanic action; (3) that they are

the products of explosion in the stars. But some astronomers are

disposed to think that they originated in a similar manner to other

members of the solar system, although it is difficult, on this

hypothesis, to account for their great density. The opinion that the

iron meteorites have come from the sun, or some other star, is enforced

by the fact that they contain hydrogen, carbon, and helium, in forms

suggesting that these gases were absorbed while the bodies were immersed

in a hot, dense atmosphere.

------------------------------------------------------------------------

PART IV.

THE FIXED STARS.

------------------------------------------------------------------------

PART IV.

THE FIXED STARS.

=1. The Stars.= The stars are distant suns, varying greatly in

remoteness, in magnitude, and in condition. Many of them are much

smaller than our sun, and many others are as much larger. They vary,

likewise in age, or state of development. Some are relatively young,

others in a middle stage, and still others in a condition that may be

called solar decrepitude. These proofs of evolution among the stars, the

knowledge of which we owe mainly to spectroscopic analysis, serve to

establish more firmly the conclusion, to which the simple aspect of the

heavens first leads us, that the universe is a connected system,

governed everywhere by similar laws and consisting of like materials.

The number of stars visible to the naked eye is about six thousand, but

telescopes show tens of millions. It is customary to divide the stars

into classes, called magnitudes, according to their apparent brightness.

By a system of photometry, or light-measurement, they are grouped into

stars of the first, second, third, etc., magnitude. With the naked eye

no stars fainter than the sixth magnitude are visible, but very powerful

telescopes may show them down to the eighteenth magnitude. Each

magnitude is about two and a half times brighter than the next magnitude

below in the scale. A first-magnitude star is about one hundred times

brighter than one of the sixth magnitude. But, in reality, the variation

of brightness is gradual, and for very accurate estimates fractions of a

magnitude have to be employed. There are about twenty first-magnitude

stars, but they are not all of equal brightness. A more accurate

photometry assumes a zero magnitude, very nearly, represented by the

star Arcturus, and makes the ratio 2.512. Thus a star, nearly

represented by Aldebaran or Altair, which is 2.512 times fainter than

the zero magnitude, is of the first magnitude, and a star, nearly

represented by the North Star, which is 2.512 times fainter than the

first magnitude, is of the second magnitude. Counting in the other

direction, a star, like Sirius, which is brighter than the zero

magnitude, is said to be of a negative magnitude. The magnitude of

Sirius is—1.6. There is only one other star of negative magnitude,

Canopus, whose magnitude is—0.9. But for ordinary purposes one need not

trouble himself with these refinements.

[Illustration:

=Schiaparelli’s Chart of Martian “Canals.”=

]

The stars are divided into five principal types, according to their

spectra. These are:

I. White stars, having a bluish tinge, in which the spectrum is

characterised by broad dark bands, due apparently to an extensive

atmosphere of hydrogen, while there are but few lines indicating the

presence of metallic vapours. About half the stars whose spectra have

been studied belong to Type I.

II. Yellowish-white stars, resembling the sun in having their spectra

crossed with a great number of lines produced by metallic vapours, while

the hydrogen lines are less conspicuous. These are often called solar

stars, and they, too, are very numerous.

III. Orange and slightly reddish stars, whose spectra contain mostly

broad bands instead of narrow lines, the bands being situated toward the

blue end of the spectrum, whence the prevailing colour, since the blue

light is thus cut off. Only a few hundred of these stars are known, but

they include most of the well-known variable stars.

IV. Small deep-red stars having dark bands absorbing the light of the

red end of the spectrum. Less than a hundred of these stars are known.

V. Stars whose spectra are characterised by bright instead of dark

lines, although they also show dark bands. The bright lines indicate

that the atmospheric vapours producing them are at a higher temperature

than the body of the star. Stars of this type are sometimes called

Wolf-Rayet stars and they are few in number.

Various modifications of these main types exist, but we cannot here

enter into an account of them. In a general way, although there are

exceptions depending upon the precise nature of each spectrum, the white

stars are thought to be younger than the yellowish ones, and the red

stars older.

In speaking of the “size” of the stars we really mean their luminosity,

or the amount of light radiated from them. When a star is said to be a

thousand times greater than the sun, the meaning is that the amount of

light that it gives would, if both were viewed from the same distance,

be equal to a thousand times the amount given by the sun. We have no

direct knowledge of the actual size of the stars as globes, because the

most powerful telescope is unable to reveal the real disk of a star. In

comparing the luminosity of a star with that of the sun its distance

must be taken into account. Most of the stars are so far away that we

really know nothing of their distances, but there are fifty or more

which lie within a distance not too great to enable us to obtain an

approximate idea of what it is. The nearest star in the northern sky is

so far from being the brightest that it can barely be seen with the

naked eye. It must be very much less luminous than the sun. On the other

hand, some very bright stars lie at a distance so immense that it can

hardly be estimated, and they must exceed the sun in luminosity hundreds

and even thousands of times.

The question of the distance of the stars has already been treated in

the section on Parallax. In employing our knowledge of star distances

for the purpose of comparing their luminosity with that of the sun, we

must first ascertain, as accurately as possible, the actual amount of

light that the star sends to the earth as compared with the actual

amount of light that the sun sends. The star Arcturus gives to our eyes

about one forty-billionth as much light as the sun does. Knowing this,

we must remember that the intensity of light varies, like gravitation,

inversely as the square of the distance. Thus, if the sun were twice as

far away as it is, the amount of its light received on the earth would

be reduced to one fourth, and if its distance were increased three

times, the amount would be reduced to one ninth. If the sun were 200,000

times as far away, its light would be reduced to one forty-billionth, or

the same as that of Arcturus. At this point the actual distance of

Arcturus enters into the calculation. If that distance were 200,000

times the sun's distance, we should have to conclude that Arcturus was

exactly equal to the sun in luminosity, since the sun, if removed to the

same distance, would give us the same amount of light. But, in fact, we

find that the distance of Arcturus, instead of being 200,000 times that

of the sun, is about 10,000,000 times. In other words, it is fifty times

as far away as the sun would have to be in order that it should appear

to our eyes no brighter than Arcturus. From this it follows that the

real luminosity of Arcturus must be the square of 50, or 2500, times

that of the sun. In the same manner we find that Sirius, which to the

eye appears to be the brightest star in the sky (much brighter than

Arcturus because much nearer), is about thirty times as luminous as the

sun.

Many of the stars are changeable in brightness, and those in which the

changes occur to a notable extent, and periodically, are known as

variable stars. It is probable that all the stars, including the sun,

are variable to a slight degree. Among the most remarkable variables are

Mira, or Omicron Ceti, in the constellation Cetus, which in the course

of about 331 days rises from the ninth to the third magnitude and then

falls back again (the maxima of brightness are irregular); and Algol, or

Beta Persei, in the constellation Perseus, which, in a period of 2 days,

20 hours, 49 minutes, changes from the third to the second magnitude and

back again. In the case of Mira the cause of the changes is believed to

lie in the star itself, and they may be connected with its gradual

extinction. The majority of the variable stars belong to this class. As

to Algol, the variability is apparently due to a huge dark body circling

close around the star with great speed, and periodically producing

partial eclipses of its light. There are a few other stars with short

periods of variability which belong to the class of Algol.

When examined with telescopes many of the stars are found to be double,

triple, or multiple. Often this arises simply from the fact that two or

more happen to lie in nearly the same line of sight from the earth, but

in many other cases it is found that there is a real connection, and

that the stars concerned revolve, under the influence of their mutual

gravitation, round a common centre of force. When two stars are thus

connected they are called a binary. The periods of revolution range from

fifty to several hundred years. Among the most celebrated binary stars

are Alpha Centauri, in the southern hemisphere, the nearest known star

to the solar system, whose components revolve in a period of about

eighty years; Gamma Virginis, in the constellation Virgo, period about

one hundred and seventy years; and Sirius, period about fifty-three

years. In the case of Sirius, one of the components, although perhaps

half as massive as its companion, is ten thousand times less bright.

There is another class of binary stars, in which one of the companions

is invisible, its presence being indicated by the effects of its

gravitational pull upon the other. Algol may be regarded as an example

of this kind of stellar association. But there are stars of this class,

where the companion causes no eclipses, either because it is not dark,

or because it never passes over the other, as seen from the earth, but

where its existence is proved, in a very interesting way, by the

spectroscope. In these stars, called spectroscopic binaries, two bright

components are so close together that no telescope is able to make them

separately visible, but when their plane of revolution lies nearly in

our line of sight the lines in their combined spectrum are seen

periodically split asunder. To understand this, we must recall the

principles underlying spectroscopic analysis and add something to what

was said before on that subject.

Light consists of waves in the ether of different lengths and making

upon the eye different impressions of colour according to the length of

the waves. The longest waves are at the red end of the spectrum and the

shortest at the blue, or violet, end. But since they all move onward

with the same speed, it is clear that the short blue waves must fall in

quicker succession on the retina of the eye than the long red waves. Now

suppose that the source of light from which the waves come is

approaching very swiftly; it is easy to see that all the waves will

strike the eye with greater rapidity, and that the whole spectrum will

be shifted toward the blue, or short-wave, end. The Fraunhofer lines

will share in this shifting of position. Next suppose that the source of

the light is retreating from the eye. The same effect will occur in a

reversed sense, for now there will be a general shift toward the red end

of the spectrum. A sufficiently clear illustration, by analogy, is

furnished by the waves of sound. We know that low-pitched sounds are

produced by long waves, and high-pitched ones by short waves; then if

the source of the sound, such as a locomotive whistle, rapidly

approaches the ear the waves are crowded together, or shifted as a whole

toward the short end of the gamut, whereupon the sound rises to a shrill

scream. If, on the contrary, the source of sound is retreating, the

shift is in the other direction, and the sound drops to a lower pitch.

This is precisely what happens in the spectrum of a star which is either

approaching or receding from the eye. If it is approaching, the

Fraunhofer lines are seen shifted out of their normal position toward

the blue, and if it is receding they are shifted toward the red. The

amount of shifting will depend upon the speed of the star's motion. If

that motion is across the line of sight there will be no shifting,

because then the source of light is neither approaching nor receding.

Now take the case of a binary star whose components are too close to be

separated by a telescope. If they happen to be revolving round their

common centre in a plane nearly coinciding with the line of sight from

the earth, one of them must be approaching the eye at the same time that

the other is receding from it, and the consequence is that the spectral

lines of the first will be shifted toward the blue, while those of the

second are shifted toward the red. The colours of the two intermingled

spectra blend into each other too gradually to enable this effect to be

detected by their means, but the Fraunhofer lines are sharply defined,

and in them the shift is clearly seen; and since there is a simultaneous

shifting in opposite directions the lines appear split. But when the two

stars are in that part of their orbit where their common motion is

across the line of sight the lines close up again, because then there is

no shift. This phenomenon is beautifully exhibited by one of the first

spectroscopic binaries to be discovered, Beta Aurigæ. In 1889, Prof. E.

C. Pickering noticed that the spectral lines of this star appeared split

every second night, from which he inferred that it consisted of two

stars revolving round a common centre in a period of four days.

This spectroscopic method has been applied to determine the speed with

which certain single stars are approaching or receding from the solar

system. It has also served to show, what we have before remarked, that

the inner parts of Saturn's rings travel faster than the outer parts.

Moreover, it has been used in measuring the rate of the sun's rotation

on its axis, for it is plain that one edge of the sun approaches us

while the opposite edge is receding. Even the effect of the rotation of

Jupiter has been revealed in this way, and the same method will probably

settle the question whether Venus rotates rapidly, or keeps the same

face always toward the sun.

Not only do many stars revolve in orbits about near-by companions, but

all the stars, without exception, are independently in motion. They

appear to be travelling through space in many different directions, each

following its own chosen way without regard to the others, and each

moving at its own gait. These movements of the stars are called proper

motions. The direction of the sun's proper motion is, roughly speaking,

northward, and it travels at the rate of twelve or fourteen miles per

second, carrying the earth and the other planets along with it. Some

stars have a much greater speed than the sun, and some a less speed. As

we have said, these motions are in many different directions, and no

attempt to discover any common law underlying them has been entirely

successful, although it has been found that in some parts of the sky a

certain number of stars appear to be travelling along nearly parallel

paths, like flocks of migrating birds. In recent years some indications

have been found of the possible existence of two great general currents

of movement, almost directly opposed to each other, part of the stars

following one current and part the other. But no indication has been

discovered of the existence of any common centre of motion. Several

relatively near-by stars appear to be moving in the same direction as

the sun. Stars that are closely grouped together, like the cluster of

the Pleiades, seem to share a common motion of translation through

space. We have already remarked that when stars are found to be moving

toward or away from the sun, spectroscopic observation of the shifting

of their lines gives a means of calculating their velocity. In other

cases, the velocity across the line of sight can be calculated if we

know the distance of the stars concerned. One interesting result of the

fact that the earth goes along with the sun in its flight is that the

orbit of the earth cannot be a closed curve, but must have the form of a

spiral in space. In consequence of this we are continually advancing, at

the rate of at least 400,000,000 miles per year, toward the northern

quarter of the sky. The path pursued by the sun appears to be straight,

although it may, in fact, be a curve so large that we are unable in the

course of a lifetime, or many lifetimes, to detect its departure from a

direct line. At any rate we know that, as the earth accompanies the sun,

we are continually moving into new regions of space.

It has been stated that many millions of stars are visible with

telescopes—perhaps a hundred millions, or even more. The great majority

of these are found in a broad irregular band, extending entirely round

the sky, and called the Milky Way, or the Galaxy. To the naked eye the

Milky Way appears as a softly shining baldric encircling the heavens,

but the telescope shows that it consists of multitudes of faint stars,

whose minuteness is probably mainly due to the immensity of their

distance, although it may be partly a result of their relative lack of

actual size, or luminosity. In many parts of the Milky Way the stars

appear so crowded that they present the appearance of sparkling clouds.

The photographs of these aggregations of stars in the Milky Way, made by

Barnard, are marvellous beyond description. In the Milky Way, and

sometimes outside it, there exist globular star-clusters, in which the

stars seem so crowded toward the centre that it is impossible to

separate them with a telescope, and the effect is that of a glistering

ball made up of thousands of silvery particles, like a heap of

microscopic thermometer bulbs in the sunshine. A famous cluster of this

kind is found in the constellation Hercules.

The Milky Way evidently has the form of a vast wreath, made up of many

interlaced branches, some of which extend considerably beyond its mean

borders. Within, this starry wreath space is relatively empty of stars,

although some thousands do exist there, of which the sun is one. We are

at present situated not very far from the centre of the opening within

the ring or wreath, but the proper motion of the sun is carrying us

across this comparatively open space, and in the course of time, if the

direction of our motion does not change, we shall arrive at a point not

far from its northern border. The Milky Way probably indicates the

general plan on which the visible universe is constructed, or what has

been called the architecture of the heavens, but we still know too

little of this plan to be able to say exactly what it is.

The number of stars in existence at any time varies to a slight degree,

for occasionally a star disappears, or a new one makes its appearance.

These, however, are rare phenomena, and new stars usually disappear or

fade away after a short time, for which reason they are often called

temporary stars. The greatest of these phenomena ever beheld was Tycho

Brahe's star, which suddenly burst into view in the constellation

Cassiopeia in 1572, and disappeared after a couple of years, although at

first it was the brightest star in the heavens. Another temporary star,

nearly as brilliant, appeared in the constellation Perseus, in 1901, and

this, as it faded, gradually turned into a nebula, or a star surrounded

by a nebula. It is generally thought that outbursts of this kind are

caused by the collision of two or more massive bodies, which were

invisible before their disastrous encounter in space. The heat developed

by such a collision would be sufficient to vaporise them, and thus to

produce the appearance of a new blazing star. It is possible that space

contains an enormous number of great obscure bodies,—extinguished suns,

perhaps—which are moving in all directions as rapidly as the visible

stars.

=2. The Nebulæ.= These objects, which get their name from their

cloud-like appearance, are among the most puzzling phenomena of the

heavens, although they seem to suggest a means of explaining the origin

of stars. Many thousands of nebulæ are known, but there are only two or

three bright enough to be visible to the naked eye. One of these is in

the “sword” of the imaginary giant figure marking the constellation

Orion, and another is in the constellation Andromeda. They look to the

unaided eye like misty specks, and require considerable attention to be

seen at all. But in telescopes their appearance is marvellous. The Orion

nebula is a broad, irregular cloud, with many brighter points, and a

considerable number of stars intermingled with it, while the Andromeda

nebula has a long spindle shape, with a brighter spot in the centre. It

is covered and surrounded with multitudes of faint stars. It was only

after astronomical photography had been perfected that the real shapes

of the nebulæ were clearly revealed. Thousands of nebulæ have been

discovered by photography, which are barely if at all visible to the

eye, even when aided by powerful telescopes. This arises from the fact

that the sensitive photographic plate accumulates the impression that

the light makes upon it, showing more and more the longer it is exposed.

Plates placed in the focus of telescopes, arranged to utilise specially

the “photographic rays,” are often exposed for many hours on end in

order to picture faint nebulæ and faint stars, so that they reveal

things that the eye, which sees all it can see at a glance, is unable to

perceive.

Nebulæ are generally divided into two classes—the “white” nebulæ and the

“green” nebulæ. The first, of which the Andromeda nebula is a striking

example, give a continuous spectrum without dark lines, as if they

consisted either of gas under high pressure, or of something in a solid

or liquid state. The second, conspicuously represented by the Orion

nebula, give a spectrum consisting of a few bright lines, characteristic

of such gases as hydrogen and helium, together with other substances not

yet recognised. But there is no continuous spectrum like that shown by

the white nebulæ, from which it is inferred that the green nebulæ, at

least, are wholly gaseous in their constitution. The precise

constitution of the white nebulæ remains to be determined.

It is only in relatively recent years that the fact has become known

that the majority of nebulæ have a spiral form. There is almost

invariably a central condensed mass from which great spiral arms wind

away on all sides, giving to many of them the appearance of spinning

pin-wheels, flinging off streams of fire and sparks on all sides. The

spirals look as if they were gaseous, but along and in them are arrayed

many condensed knots, and frequently curving rows of faint stars are

seen apparently in continuation of the nebulous spirals. The suggestion

conveyed is that the stars have been formed by condensation from the

spirals. These nebulæ generally give the spectra of the white class, but

there are also sometimes seen bright lines due to glowing gases. The

Andromeda nebula is sometimes described as spiral, but its aspect is

rather that of a great central mass surrounded with immense elliptical

rings, some of which have broken up and are condensing into separate

masses. The Orion nebula is a chaotic cloud, filled with partial

vacancies and ribbed with many curving, wave-like forms.

There are other nebulæ which have the form of elliptical rings,

occasionally with one or more stars near the centre. A famous example of

this kind is found in the constellation Lyra. Still others have been

compared in shape to the planet Saturn with its rings, and some are

altogether bizarre in form, occasionally looking like glowing tresses

floating among the stars.

The apparent association of nebulæ with stars led to the so-called

nebular hypothesis, according to which stars are formed, as already

suggested, by the condensation of nebulous matter. In the celebrated

form which Laplace gave to this hypothesis, it was concerned specially

with the origin of our solar system. He assumed that the sun was once

enormously expanded, in a nebulous state, or surrounded with a nebulous

cloud, and that as it contracted rings were left off around the

periphery of the vast rotating mass. These rings subsequently breaking

and condensing into globes, were supposed to have given rise to the

planets. It is still believed that the sun and the other stars may have

originated from the condensation of nebulæ, but many objections have

been found to the form in which Laplace put his hypothesis, and the

discovery of the spiral nebulæ has led to other conjectures concerning

the way in which the transformation is brought about. But we have not

here the space to enter into this discussion, although it is of

fascinating interest.

A word more should be said about the use of photography in astronomy. It

is hardly going too far to aver that the photographic plate has taken

the place of the human retina in recording celestial phenomena,

especially among the stars and nebulæ. Not only are the forms of such

objects now exclusively recorded by photography, but the spectra of all

kinds of celestial objects—sun, stars, nebulæ, etc.—are photographed and

afterward studied at leisure. In this way many of the most important

discoveries of recent years have been made, including those of variable

stars and new stars. Photographic charts of the heavens exist, and by

comparing these with others made later, changes which would escape the

eye can be detected. Comets are sometimes, and new asteroids almost

invariably, discovered by photography. The changes in the spectra of

comets and new stars are thus recorded with an accuracy that would be

otherwise unattainable. Photographs of the moon excel in accuracy all

that can be done by manual drawing, and while photographs of the planets

still fail to show many of the fine details visible with telescopes,

continual improvements are being made. Many of the great telescopes now

in use or in course of construction are intended specially for

photographic work.

=3. The Constellations.= The division of the stars into constellations

constitutes the uranography or the “geography of the heavens.” The

majority of the constellations are very ancient, and their precise

origin is unknown, but those which are invisible from the northern

hemisphere have all been named since the great exploring expeditions to

the south seas. There are more than sixty constellations now generally

recognised. Twelve of these belong to the zodiac, and bear the same

names as the zodiacal signs, although the precession of the equinoxes

has drifted them out of their original relation to the signs. Many of

the constellations are memorials of prehistoric myths, and a large

number are connected with the story of the Argonautic expedition and

with other famous Greek legends. Thus the constellations form a

pictorial scroll of legendary history and mythology, and possess a deep

interest independent of the science of astronomy. For their history and

for the legends connected with them, the reader who desires a not too

detailed résumé, may consult _Astronomy with the Naked Eye_, and for

guidance in finding the constellations, _Astronomy with an Opera-glass_,

or _Round the Year with the Stars_. The quickest way to learn the

constellations is to engage the aid of some one who knows them already,

and can point them out in the sky. The next best way is to use star

charts, or a star-finder or planisphere.

A considerable number of the brighter and more important stars are known

by individual names, such as Sirius, Canopus, Achernar, Arcturus, Vega,

Rigel, Betelgeuse, Procyon, Spica, Aldebaran, Regulus, Altair, and

Fomalhaut. Astronomers usually designate the principal stars of each

constellation by the letters of the Greek alphabet, α, β, γ, etc., the

brightest star in the constellation bearing the name of the first

letter, the next brightest that of the second letter, and so on.

The constellations are very irregular in outline, and their borders are

only fixed with sufficient definiteness to avoid the inclusion of stars

catalogued as belonging to one, within the limits of another. In all

cases the names come from some fancied resemblance of the figures formed

by the principal stars of the constellation to a man, woman, animal, or

other object. In only a few cases are these resemblances very striking.

The most useful constellations for the beginner are those surrounding

the north celestial pole, and we give a little circular chart showing

their characteristic stars. The names of the months running round the

circle indicate the times of the year when these constellations are to

be seen on or near the meridian in the north. Turn the chart so that the

particular month is at the bottom, and suppose yourself to be facing

northward. The hour when the observation is supposed to be made is, in

every case, about 9 o'clock in the evening, and the date is about the

first of the month. The top of the chart represents the sky a little

below the zenith in the north, and the bottom represents the horizon in

the north.

[Illustration:

_Fig. 18. The North Circumpolar Stars._

]

The apparent yearly revolution of the heavens, resulting from the motion

of the earth in its orbit, causes the constellations to move westward in

a circle round the pole, at the rate of about 30° per month. But the

daily rotation of the earth on its axis causes a similar westward motion

of the heavens, at the rate of about 30° for every two hours. From this

it results that on the same night, after an interval of two hours, you

will see the constellations occupying the place that they will have, at

the original hour of observation, one month later. Thus, if you observe

their positions at 9 P.M. on the first of January, and then turn the

chart so as to bring February at the bottom, you will see the

constellations around the north pole of the heavens placed as they will

be at 11 P.M. on the first of January.

[Illustration:

_Fig. 19. Key to North Circumpolar Stars._

]

Only the conspicuous stars have been represented in the chart, just

enough being included to enable the learner to recognise the

constellations by their characteristic star groups, from which they have

received their names. The chart extends to a distance of 40° from the

pole, so that, for observers situated in the mean latitude of the United

States, none of the constellations represented ever descends below the

horizon, those that are at the border of the chart just skimming the

horizon when they are below the pole.

On the key to the chart the Greek-letter names of the principal stars

have been attached, but some of them have other names which are more

picturesque. These are as follows: In Ursa Major (the Great Bear, which

includes the Great Dipper), α is called Dubhe, β Merak, γ Phaed, δ

Megrez, ε Alioth, ζ Mizar, and η Benetnash. The little star close by

Mizar is Alcor. In Cassiopeia, α is called Schedar, β Caph, and δ

Ruchbar. In Ursa Minor, the Little Bear, α is called Polaris, or the

North Star, and β Kochab. In Draco, α is called Thuban, and γ Eltanin.

In Cepheus, α is called Alderamin, and β Alfirk. These names are nearly

all of Arabic origin. It will be observed that Merak and Dubhe are the

famous “Pointers,” which serve to indicate the position of the North

Star, while Thuban is the “star of the pyramid,” before mentioned. The

north celestial pole is situated almost exactly on a straight line drawn

from Mizar through the North Star to Ruchbar, and a little more than a

degree from the North Star in the direction of Ruchbar. This furnishes a

ready means for ascertaining the position of the meridian. For instance,

about the middle of October, Mizar is very close to the meridian below

the pole, and Ruchbar equally close to it above the pole, and then,

since the North Star is in line with these two, it also must be

practically on the meridian, and its direction indicates very nearly

true north. The same method is applicable whenever, at any other time of

the year or of the night, Mizar and Ruchbar are observed to lie upon a

vertical line, no matter which is above and which below. It is also

possible to make a very good guess at the time of night by knowing the

varying position of the line joining these stars.

The star Caph is an important landmark because it lies almost on the

great circle of the equinoctial colure, which passes through the vernal

and autumnal equinoxes.

On the key, the location of the North Pole of the Ecliptic is shown, and

the greater part of the circle described by the north celestial pole in

the period of 25,800 years.

While the reader who wishes to pursue the study of the constellations in

detail must be referred to some of the works before mentioned, or others

of like character, it is possible here to aid him in making a

preliminary acquaintance with other constellations beside those included

in our little chart, by taking each of the months in turn, and

describing the constellations which he will see on or near the meridian

south of the border of the chart at the same time that the polar

constellations corresponding to the month selected are on or near the

meridian in the north. Thus, at 9 P.M. about the first of January, the

constellation Perseus, lying in a rich part of the Milky Way, is nearly

overhead and directly south of the North Star. This constellation is

marked by a curved row of stars, the brightest of which, of the second

magnitude, is Algenib, or α Persei. A few degrees south-west of Algenib

is the wonderful variable Algol. East of Perseus is seen the very

brilliant white star Capella in the constellation Auriga. This is one of

the brightest stars in the sky. Almost directly south of Perseus, the

eye will be caught by the glimmering cluster of the Pleiades in the

constellation Taurus. A short distance south-east of the Pleiades is the

group of the Hyades in Taurus, shaped like the letter V, with the

beautiful reddish star Aldebaran in the upper end of the southern branch

of the letter. The ecliptic runs between the Pleiades and the Hyades.

Still lower in the south will be seen a part of the long-winding

constellation Eridanus, the River Po. Its stars are not bright but they

appear in significant rows and streams.

About the first of February the constellation Auriga is on the meridian

not far from overhead, Capella lying toward the west. Directly under

Auriga, two rather conspicuous stars mark the tips of the horns of

Taurus, imagined as a gigantic bull, and south of these, with its centre

on the equator, scintillates the magnificent constellation Orion, the

most splendid in all the sky, with two great first-magnitude stars, one,

in the shoulder of the imaginary giant, of an orange hue, called

Betelguese, and the other in the foot, of a blue-white radiance, called

Rigel. Between these is stretched the straight line of the “belt,”

consisting of three beautiful second-magnitude stars, about a degree and

a half apart. Their names, beginning with the western one, are Mintaka,

Alnilam, and Alnitah. Directly under the belt, in the midst of a short

row of faint stars called the “sword,” is the great Orion nebula. It

will be observed that the three stars of the belt point, though not

exactly, toward the brightest of all stars, Sirius, in the constellation

Canis Major, the Great Dog, which is seen advancing from the east. Under

Orion is a little constellation named Lepus, the Hare.

The first of March the region overhead is occupied by the very faint

constellation Lynx. South of it, and astride the ecliptic, appear the

constellations Gemini, the Twins, and Cancer, the Crab. These, like

Taurus, belong to the zodiac. The Twins are westward from Cancer, and

are marked by two nearly equal stars, about five degrees apart. The more

westerly and northerly one is Castor and the other is Pollux. Cancer is

marked by a small cluster of faint stars called Præsepe, the Manger

(also sometimes the Beehive). Directly south of the Twins, is the bright

lone star Procyon, in the constellation Canis Minor, the Little Dog.

Sirius and the other stars of Canis Major, which make a striking figure,

are seen south-west of Procyon.

The first of April the zodiac constellation Leo is near the meridian,

recognisable by a sickle-shaped figure marking the head and breast of

the imaginary Lion. The bright star at the end of the handle of the

sickle is Regulus. Above Leo, between it and the Great Dipper, appears a

group of stars belonging to the small constellation Leo Minor, the

Little Lion. Farther south is a winding ribbon of stars indicating the

constellation Hydra, the Water Serpent. Its chief star, Alphard, of a

slightly reddish tint, is seen west of the meridian and a few degrees

south of the equator.

At the beginning of May, when the Great Dipper is nearly overhead, the

small constellation Canes Venatici, the Hunting Dogs, is seen directly

under the handle of the Dipper, and south of that a cobwebby spot,

consisting of minute stars, indicates the position of the constellation

Coma Berenices, Berenice's Hair. Still farther south, where the ecliptic

and the equator cross, at the autumnal equinox, is the large

constellation Virgo, the Virgin, also one of the zodiacal band. Its

chief star Spica, a pure white gem, is seen some 20° east of the

meridian. Below and westward from Virgo, and south of the equator, are

the constellations Crater, the Cup, and Corvus, the Crow. The stars of

Hydra continue to run eastward below these constellations. The

westernmost, Crater, consists of small stars forming a rude semicircle

open toward the east, while Corvus, which possesses brighter stars, has

the form of a quadrilateral.

The first of June the great golden star Arcturus, whose position may be

found by running the eye along the curve of the handle of the Great

Dipper, and continuing onward a distance equal to the whole length of

the Dipper, is seen approaching the meridian from the east and high

overhead. This superb star is the leader of the constellation Boötes,

the Bear-Driver. Spica in Virgo is now a little west of the meridian.

The first of July, when the centre of Draco is on the meridian north of

the zenith, the exquisite circlet of stars called Corona Borealis, the

Northern Crown, is nearly overhead. A short distance north-east of it

appears a double-quadrilateral figure, marking out the constellation

Hercules, while directly south of the Crown a crooked line of stars

trending eastward indicates the constellation Serpens, the Serpent.

South-west of Serpens, two widely separated but nearly equal stars of

the second magnitude distinguish the zodiacal constellation Libra, the

Balance; while lower down toward the south-east appears the brilliant

red star Antares, in the constellation Scorpio, likewise belonging to

the zodiac.

On the first of August the head of Draco is on the meridian near the

zenith, and south of it is seen Hercules, toward the west, and the

exceedingly brilliant star Vega, in the constellation Lyra, the Lyre,

toward the east. Vega, or Alpha Lyræ, has few rivals for beauty. Its

light has a decided bluish-white tone, which is greatly accentuated when

it is viewed with a telescope. South of Hercules two or three rows of

rather large, widely separated stars mark the constellation Ophiuchus,

the Serpent-Bearer. This extends across the equator. Below it, in a rich

part of the Milky Way, is Scorpio, whose winding line, beginning with

Antares west of the meridian, terminates a considerable distance east of

the meridian in a pair of stars representing the uplifted sting of the

imaginary monster.

The first of September the Milky Way runs directly overhead, and in the

midst of it shines the large and striking figure called the Northern

Cross, in the constellation Cygnus, the Swan. The bright star at the

head of the Cross is named Denib. Below the Cross and in the eastern

edge of the Milky Way is the constellation Aquila, the Eagle, marked by

a bright star, Altair, with a smaller one on each side and not far away.

Low in the south, a little west of the meridian and partly immersed in

the brightest portion of the Milky Way, is the zodiacal constellation

Sagittarius, the Archer. It is distinguished by a group of stars several

of which form the figure of the upturned bowl of a dipper, sometimes

called the Milk Dipper. East of Cygnus and Aquila a diamond-shaped

figure marks the small constellation Delphinus, the Dolphin.

At the opening of October, when Denib is near the meridian, the sky

directly in the south is not very brilliant. Low down, south of the

equator, is seen the zodiacal constellation Capricornus, the Goat, with

a noticeable pair of stars in the head of the imaginary animal.

On the first of November, when Cassiopeia is approaching the meridian

overhead, the Great Square, in the constellation Pegasus, is on the

meridian south of the zenith, while south-west of Pegasus the zodiacal

constellation Aquarius, the Water-Bearer, appears on the ecliptic. A

curious scrawling Y-shaped figure in the upper part of Aquarius serves

as a mark to identify the constellation. Thirty degrees south of this

shines the bright star Fomalhaut, in the constellation Piscis Australis,

the Southern Fish. The two stars forming the eastern side of the Great

Square of Pegasus are interesting because, like Caph in Cassiopeia, they

lie close to the line of the equinoctial colure. The northern one is

called Alpheratz and the southern Gamma Pegasi. Alpheratz is a star

claimed by two constellations, since it not only marks one corner of the

square of Pegasus, but it also serves to indicate the head of the maiden

in the celebrated constellation of Andromeda.

The first of December, Andromeda is seen nearly overhead, south of

Cassiopeia. The constellation is marked by a row of three

second-magnitude stars, beginning on the east with Alpheratz and

terminating near Perseus with Almaack. The central star is named Mirach.

A few degrees north-west of Mirach glimmers the great Andromeda nebula.

Below Andromeda, west of the meridian, appears the zodiacal

constellation Aries, the Ram, indicated by a group of three stars,

forming a triangle, the brightest of which is called Hamal.

South-westerly from Aries is the zodiacal constellation Pisces, the

Fishes, which consists mainly of faint stars arranged in pairs and

running far toward the west along the course of the ecliptic, which

crosses the equator at the vernal equinox, near the western end of the

constellation. South of Pisces and Aries is the broad constellation

Cetus, the Whale, marked by a number of large quadrilateral and

pentagonal figures, formed by its stars. Near the centre of this

constellation, but not ordinarily visible to the naked eye, is the

celebrated variable Mira, also known as Omicron Ceti.

With a little application any person can learn to recognise these

constellations, even with the slight aid here offered, and if he does,

he will find the knowledge thus acquired as delightful as it is useful.

------------------------------------------------------------------------

INDEX

Aberration, 88

Alpha Centauri, 145

Alpha Draconis, 62

Altitude, 13, 14

Andromeda, nebula in, 236

Antares, 24

Aphelion, 8

April, aspect of sky in, 251

Arcturus, 224

Asteroids, the, 187

Atmosphere, the, 82

August, aspect of sky in, 253

Azimuth, 13, 14

Beta Aurigæ, 230

Binary (and multiple) stars, 226

Binaries, spectroscopic, 227

Calendar, the, 117

Calorie, measure of sun's heat, 129

Cassiopeia, 22

Cheops, pyramid of, 62

Chromosphere, of sun, 135

Circles, vertical, 15;

altitude, 14;

division of, 16

Circumpolar stars, 224

Clock, the astronomical, 41, 90

Comets, 201, 203

Constellations, the, 241

Corona, of the sun, 135

Coronium, 152

Corona Borealis, 24

Day, change of, 99, 101

Day and night, 96

December, aspect of sky in, 256

Declination, 31, 35

Dipper, the Great, 21;

the Little, 23

Earth, description of the, 67;

weight of, 73

Eclipses, 163

Ecliptic, 43

Elements, chemical, in the sun, 151

Equator, 38

Equinoxes, 45

Eros, the asteroid, 142

February, aspect of sky in, 249

Galaxy, the, 233

Gravitation, laws of, 70

“Greenwich of the Sky,” 32

Heavens, apparent motion of, 19

Helium, 152

Horizon, 10, 12;

dip of, 86

Hour Circles, 31

January, aspect of sky in, 248

July, aspect of sky in, 252

June, aspect of sky in, 252

Jupiter, the planet, 189

Kepler, laws of, 173

Latitude, terrestrial, 30;

celestial, 49, 51

Light, pressure of, 206

Longitude, terrestrial, 30;

celestial, 49, 51

Magnitudes, stellar, 220

March, aspect of sky in, 250

Mars, the planet, 181

May, aspect of sky in, 251

Mercury, the planet, 175

Meteors, 210

Meteorites, 214

Milky Way, the, 233

Molecules, escape from planetary atmospheres, 158

Moon, description of, 153;

how earth controls, 75

Motion, proper, of solar system, 232

Nadir, 11

Nebulæ, 236

Nebular Hypothesis, 239

Neptune, the planet, 199

Noon-line, 12

North Point, 20

North Pole, 27, 37

North Star, 20, 27, 61

November, aspect of sky in, 255

Nutation, 63

Oblique sphere, 39

October, aspect of sky in, 254

Oppositions, of Mars, 183

Orbits of planets, 7, 52

Orion, nebula in, 236

Parallax, 136, 139

Parallel sphere, 38

Perihelion, 8

Phases of the moon, 160

Photography, in astronomy, 240

Photometry, of stars, 220

Photosphere of sun, 134

Planets, description of the, 172

Polar circles, 111

Precession of the Equinoxes, 55

Refraction, 83, 85

Right Ascension, 31, 35

Right sphere, 39

Rotation, effect of earth's, 5

Saturn, description of, 195

Seasons, the, 104, 107;

secular change of, 115

September, aspect of sky in, 254

Signs of Zodiac, 54

Spectroscopic analysis, 145, 147;

shifting of lines, 227

Sphere, celestial, 9

Solstices, the, 47

Stars, the fixed, 219;

daily revolution of, 25;

how to locate, 29;

luminosity of, 222;

spectra of, 221;

variable, 225;

temporary, 235;

double and multiple, 226

Sun, the description of, 127

Sun-spots, 131

Temperature, of sun, 130

Temporary stars, 235

Tides, the, 76, 79

Time, 89;

sidereal and solar, 93

Transits, of Venus, 140, 141

Tropics, the, 111

Universe, appearance of, 8

Uranus, description of, 199

Ursa Major, 21

Variable stars, 225

Vega, 63

Venus, the planet, 178

Vernal Equinox, 32, 42, 46

Zenith, 10

Zodiac, 50

------------------------------------------------------------------------

● Transcriber’s Notes:

○ New original cover art included with this eBook is granted to the

public domain.

○ Missing or obscured punctuation was silently corrected.

○ Typographical errors were silently corrected.

○ Inconsistent spelling and hyphenation were made consistent only

when a predominant form was found in this book.

○ Text that:

was in italics is enclosed by underscores (_italics_);

was in bold by is enclosed by “equal” signs (=bold=).

○ The use of a caret (^) before a letter, or letters, shows that the

following letter or letters was intended to be a superscript, as

in S^t Bartholomew or 10^{th} Century.

○ Superscripts are used to indicate numbers raised to a power. In

this plain text document, they are represented by characters like

this: “P^3” or “10^{18}”, i.e. P cubed or 10 to the 18th power.

○ Variables in formulæ sometimes use subscripts, which look like

this: “A_{0}”. This would be read “A sub 0”.