earth varies within much wider limits than that of Mercury. When Venus
is at inferior conjunction, her distance from the earth is ninety-two
million miles _minus_ sixty-seven million miles, or twenty-five million
miles; and when at superior conjunction it is ninety-two million miles
_plus_ sixty-seven million miles, or a hundred and fifty-nine million
miles. Venus is considerably more than _six times_ as far off at
superior conjunction as at inferior conjunction.
[Illustration: Fig. 258.]
227. _Apparent Size of Venus._--Owing to the great variation in the
distance of Venus from the earth, her apparent diameter varies from
about ten seconds to about sixty-six seconds. Fig. 258 shows the
apparent size of Venus at her extreme and mean distances from the earth.
228. _Volume and Density of Venus._--The real size of Venus is about the
same as that of the earth, her diameter being only about three hundred
miles less. The comparative sizes of the two planets are shown in Fig.
259. The density of Venus is a little less than that of the earth.
[Illustration: Fig. 259.]
229. _The Greatest Elongation of Venus._--Venus, like Mercury, appears
to oscillate to and fro across the sun. The angular value of the
greatest elongation of Venus varies but slightly, its greatest value
being about forty-seven degrees.
230. _Sidereal and Synodical Periods of Venus._--The _sidereal_ period
of Venus, or that of a complete revolution around the sun, is about two
hundred and twenty-five days; her orbital motion being at the rate of
nearly twenty-two miles a second. Her _synodical_ period, or the time it
takes her to pass around from her greatest eastern elongation to the
same elongation again, is about five hundred and eighty-four days, or
eighteen months. Venus is a hundred and forty-six days, or nearly five
months, in passing from her greatest elongation east through inferior
conjunction to her greatest elongation west.
231. _Venus as a Morning and an Evening Star._--For a period of about
nine months, while Venus is passing from superior conjunction to her
greatest eastern elongation, she will be east of the sun, and will
therefore set after the sun. During this period she is the _evening
star_, the _Hesperus_ of the ancients. While passing from inferior
conjunction to superior conjunction, Venus is west of the sun, and
therefore rises before the sun. During this period of nine months she is
the _morning star_, the _Phosphorus_, or _Lucifer_, of the ancients.
232. _Brilliancy of Venus._--Next to the sun and moon, Venus is at times
the most brilliant object in the heavens, being bright enough to be seen
in daylight, and to cast a distinct shadow at night. Her brightness,
however, varies considerably, owing to her phases and to her varying
distance from the earth. She does not appear brightest when at full, for
she is then farthest from the earth, at superior conjunction; nor does
she appear brightest when nearest the earth, at inferior conjunction,
for her phase is then a thin crescent (see Fig. 258). She is most
conspicuous while passing from her greatest eastern elongation to her
greatest western elongation. After she has passed her eastern
elongation, she becomes brighter and brighter till she is within about
forty degrees of the sun. Her phase at this point in her orbit is shown
in Fig. 260. Her brilliancy then begins to wane, until she comes too
near the sun to be visible. When she re-appears on the west of the sun,
she again becomes more brilliant; and her brilliancy increases till she
is about forty degrees from the sun, when she is again at her brightest.
Venus passes from her greatest brilliancy as an evening star to her
greatest brilliancy as a morning star in about seventy-three days. She
has the same phase, and is at the same distance from the earth, in both
cases of maximum brilliancy. Of course, the brilliancy of Venus when at
the maximum varies somewhat from time to time, owing to the
eccentricities of the orbits of the earth and of Venus, which cause her
distance from the earth, at her phase of greatest brilliancy, to vary.
She is most brilliant when the phase of her greatest brilliancy occurs
when she is at her aphelion and the earth at its perihelion.
[Illustration: Fig. 260.]
233. _The Atmosphere and Surface of Venus._--Schröter believed that he
saw shadings and markings on Venus similar to those on Mercury,
indicating the presence of an atmosphere and of elevations on the
surface of the planet. Fig. 261 represents the surface of Venus as it
appeared to this astronomer. By watching certain markings on the disk of
Venus, Schröter came to the conclusion that Venus rotates on her axis in
about twenty-four hours.
[Illustration: Fig. 261.]
It is now generally conceded that Venus has a dense atmosphere; but
Schröter's observations of the spots on her disk have not been verified
by modern astronomers, and we really know nothing certainly of her
rotation.
234. _Transits of Venus._--When Venus happens to be near one of the
nodes of her orbit when she is in inferior conjunction, she makes a
transit across the sun's disk. These transits occur in pairs, separated
by an interval of over a hundred years. The two transits of each pair
are separated by an interval of eight years, the dates of the most
recent being 1874 and 1882. Venus, like Mercury, appears surrounded with
a border on passing across the sun's disk, as shown in Fig. 262.
[Illustration: Fig. 262.]
Mars.
235. _The Orbit of Mars._--The orbit of Mars is more eccentric than that
of any of the larger planets, except Mercury; its eccentricity being
about one-eleventh. The inclination of the orbit to the ecliptic is
somewhat under two degrees. The mean distance of Mars from the sun is
about a hundred and forty million miles; but, owing to the eccentricity
of his orbit, the distance varies from a hundred and fifty-three million
miles to a hundred and twenty-seven million miles.
[Illustration: Fig. 263.]
236. _Distance of Mars from the Earth._--It will be seen, from Fig. 263,
that a _superior_ planet (or one outside the orbit of the earth), like
Mars, is nearer the earth, by the whole diameter of the earth's orbit,
when in opposition than when in conjunction. The mean distance of Mars
from the earth, at the time of opposition, is a hundred and forty
million miles _minus_ ninety-two million miles, or forty-eight million
miles. Owing to the eccentricity of the orbit of the earth and of Mars,
the distance of this planet when in opposition varies considerably. When
the earth is in aphelion, and Mars in perihelion, at the time of
opposition, the distance of the planet from the earth is only about
thirty-three million miles. On the other hand, when the earth is in
perihelion, and Mars in aphelion, at the time of opposition, the
distance of the planet is over sixty-two million miles.
The mean distance of Mars from the earth when in conjunction is a
hundred and forty million miles _plus_ ninety-two million miles, or two
hundred and thirty-two million miles. It will therefore be seen that
Mars is nearly five times as far off at conjunction as at opposition.
[Illustration: Fig. 264.]
237. _The Apparent Size of Mars._--Owing to the varying distance of Mars
from the earth, the apparent size of the planet varies almost as much as
that of Venus. Fig. 264 shows the apparent size of Mars at its extreme
and mean distances from the earth. The apparent diameter varies from
about four seconds to about thirty seconds.
[Illustration: Fig. 265.]
238. _The Volume and Density of Mars._--Among the larger planets Mars is
next in size to Mercury. Its real diameter is somewhat more than four
thousand miles, and its bulk is about one-seventh of that of the earth.
Its size, compared with that of the earth, is shown in Fig. 265.
[Illustration: Plate 4.]
The density of Mars is only about three-fourths of that of the earth.
239. _Sidereal and Synodical Periods of Mars._--The _sidereal_ period of
Mars, or the time in which he makes a complete revolution around the
sun, is about six hundred and eighty-seven days, or nearly twenty-three
months; but he is about seven hundred and eighty days in passing from
opposition to opposition again, or in performing a _synodical_
revolution. Mars moves in his orbit at the rate of about fifteen miles a
second.
240. _Brilliancy of Mars._--When near his opposition, Mars is easily
recognized with the naked eye by his fiery-red light. He is much more
brilliant at some oppositions than at others, for reasons already
explained (236), but always shines brighter than an ordinary star of the
first magnitude.
241. _Telescopic Appearance of Mars._--When viewed with a good telescope
(see Plate IV.), Mars is seen to be covered with dusky, dull-red
patches, which are supposed to be continents, like those of our own
globe. Other portions, of a greenish hue, are believed to be tracts of
water. The ruddy color, which overpowers the green, and makes the whole
planet seem red to the naked eye, was believed by Sir J. Herschel to be
due to an ochrey tinge in the general soil, like that of the red
sandstone districts on the earth. In a telescope, Mars appears less red,
and the higher the power the less the intensity of the color. The disk,
when well seen, is mapped out in a way which gives at once the
impression of land and water. The bright part is red inclining to
orange, sometimes dotted with brown and greenish points. The darker
spaces, which vary greatly in depth of tone, are of a dull gray-green,
having the aspect of a fluid which absorbs the solar rays. The
proportion of land to water on the earth appears to be reversed on Mars.
On the earth every continent is an island; on Mars all seas are lakes.
Long, narrow straits are more common than on the earth; and wide
expanses of water, like our Atlantic Ocean, are rare. (See Fig. 266.)
[Illustration: Fig. 266.]
[Illustration: Fig. 267.]
Fig. 267 represents a chart of the surface of Mars, which has been
constructed from careful telescopic observation. The outlines, as seen
in the telescope, are, however, much less distinct than they are
represented here; and it is by no means certain that the light and dark
portions are bodies of land and water.
In the vicinity of the poles brilliant white spots may be noticed, which
are considered by many astronomers to be masses of snow. This conjecture
is favored by the fact that they appear to diminish under the sun's
influence at the beginning of the Martial summer, and to increase again
on the approach of winter.
242. _Rotation of Mars._--On watching Mars with a telescope, the spots
on the disk are found to move (as shown in Fig. 268) in a manner which
indicates that the planet rotates in about twenty-four hours on an axis
inclined about twenty-eight degrees from a perpendicular to the plane of
its orbit. The inclination of the axis is shown in Fig. 269. It is
evident from the figure that the variation in the length of day and
night, and the change of seasons, are about the same on Mars as on the
earth. The changes will, of course, be somewhat greater, and the seasons
will be about twice as long.
[Illustration: Fig. 268.]
[Illustration: Fig. 269.]
[Illustration: Fig. 270.]
243. _The Satellites of Mars._--In 1877 Professor Hall of the Washington
Observatory discovered that Mars is accompanied by two small moons,
whose orbits are shown in Fig. 270. The inner satellite has been named
_Phobos_, and the outer one _Deimos_. It is estimated that the diameter
of the outer moon is from five to ten miles, and that of the inner one
from ten to forty miles.
Phobos is remarkable for its nearness to the planet and the rapidity of
its revolution, which is performed in seven hours thirty-eight minutes.
Its distance from the centre of the planet is about six thousand miles,
and from the surface less than four thousand. Astronomers on Mars, with
telescopes and eyes like ours, could readily find out whether this
satellite is inhabited, the distance being less than one-sixtieth of
that of our moon.
It will be seen that Phobos makes about three revolutions to one
rotation of the planet. It will, of course, rise in the west; though the
sun, the stars, and the other satellite rise in the east. Deimos makes a
complete revolution in about thirty hours.
III. THE ASTEROIDS.
244. _Bode's Law of Planetary Distances._--There is a very remarkable
law connecting the distances of the planets from the sun, which is
generally known by the name of _Bode's Law_. Attention was drawn to it
in 1778 by the astronomer Bode, but he was not really its author.
To express this law we write the following series of numbers:--
0, 3, 6, 12, 24, 48, 96;
each number, with the exception of the first, being double the one which
precedes it. If we add 4 to each of these numbers, the series becomes--
4, 7, 10, 16, 28, 52, 100;
which series was known to Kepler. These numbers, with the exception of
28, are sensibly proportional to the distances of the principal planets
from the sun, the actual distances being as follows:--
Mercury. Venus. Earth. Mars. ---- Jupiter. Saturn.
3·9 7·2 10 15·2 52·9 95·4
245. _The First Discovery of the Asteroids._--The great gap between Mars
and Jupiter led astronomers, from the time of Kepler, to suspect the
existence of an unknown planet in this region; but no such planet was
discovered till the beginning of the present century. _Ceres_ was
discovered Jan. 1, 1801, _Pallas_ in 1802, _Juno_ in 1804, and _Vesta_
in 1807. Then followed a long interval of thirty-eight years before
_Astræa_, the fifth of these minor planets, was discovered in 1845.
246. _Olbers's Hypothesis._--After the discovery of Pallas, Olbers
suggested his celebrated hypothesis, that the two bodies might be
fragments of a single planet which had been shattered by some explosion.
If such were the case, the orbits of all the fragments would at first
intersect each other at the point where the explosion occurred. He
therefore thought it likely that other fragments would be found,
especially if a search were kept up near the intersection of the orbits
of Ceres and Pallas.
Professor Newcomb makes the following observations concerning this
hypothesis:--
"The question whether these bodies could ever have formed a single
one has now become one of cosmogony rather than of astronomy. If a
planet were shattered, the orbit of each fragment would at first
pass through the point at which the explosion occurred, however
widely they might be separated through the rest of their course;
but, owing to the secular changes produced by the attractions of the
other planets, this coincidence would not continue. The orbits would
slowly move away, and after the lapse of a few thousand years no
trace of a common intersection would be seen. It is therefore
curious that Olbers and his contemporaries should have expected to
find such a region of intersection, as it implied that the explosion
had occurred within a few thousand years. The fact that the required
conditions were not fulfilled was no argument against the
hypothesis, because the explosion might have occurred millions of
years ago; and in the mean time the perihelion and node of each
orbit would have made many entire revolutions, so that the orbits
would have been completely mixed up.... A different explanation of
the group is given by the nebular hypothesis; so that Olbers's
hypothesis is no longer considered by astronomers."
247. _Later Discoveries of Asteroids._--Since 1845 over two hundred
asteroids have been discovered. All these are so small, that it requires
a very good telescope to see them; and even in very powerful telescopes
they appear as mere points of light, which can be distinguished from the
stars only by their motions.
To facilitate the discovery of these bodies, very accurate maps have
been constructed, including all the stars down to the thirteenth
magnitude in the neighborhood of the ecliptic. A reduced copy of one of
these maps is shown in Fig. 271.
[Illustration: Fig. 271.]
Furnished with a map of this kind, and with a telescope powerful enough
to show all the stars marked on it, the observer who is searching for
these small planets will place in the field of view of his telescope six
spider-lines at right angles to each other, and at equal distances
apart, in such a manner that several small squares will be formed,
embracing just as much of the heavens as do those shown in the map. He
will then direct his telescope to the region of the sky he wishes to
examine, represented by the map, so as to be able to compare
successively each square with the corresponding portion of the sky. Fig.
272 shows at the right hand the squares in the telescopic field of view,
and at the left hand the corresponding squares of the map.
[Illustration: Fig. 272.]
He can then assure himself if the numbers and positions of the stars
mapped, and of the stars observed, are identical. If he observes in the
field of view a luminous point which is not marked in the map, it is
evident that either the new body is a star of variable brightness which
was not visible at the time the map was made, or it is a planet, or
perhaps a comet. If the new body remains fixed at the same point, it is
the former; but, if it changes its position with regard to the
neighboring stars, it is the latter. The motion is generally so
sensible, that in the course of one evening the change of position may
be detected; and it can soon be determined, by the direction and rate of
the motion, whether the body is a planet or a comet.
IV. OUTER GROUP OF PLANETS.
Jupiter.
248. _Orbit of Jupiter._--The orbit of Jupiter is inclined only a little
over one degree to the ecliptic; and its eccentricity is only about half
of that of Mars, being less than one-twentieth. The mean distance of
Jupiter from the sun is about four hundred and eighty million miles;
but, owing to the eccentricity of his orbit, his actual distance from
the sun ranges from four hundred and fifty-seven to five hundred and
three million miles.
249. _Distance of Jupiter from the Earth._--When Jupiter is in
opposition, his mean distance from the earth is four hundred and eighty
million miles _minus_ ninety-two million miles, or three hundred and
eighty-eight million miles, and, when he is in conjunction, four hundred
and eighty million miles _plus_ ninety-two million miles, or five
hundred and seventy-two million miles. It will be seen that he is less
than twice as far off in conjunction as in opposition, and that the
ratio of his greatest to his least distance is very much less than in
the case of Venus and Mars. This is owing to his very much greater
distance from the sun. Owing to the eccentricities of the orbits of the
earth and of Jupiter, the greatest and least distances of Jupiter from
the earth vary somewhat from year to year.
[Illustration: Fig. 273.]
250. _The Brightness and Apparent Size of Jupiter._--The apparent
diameter of Jupiter varies from about fifty seconds to about thirty
seconds. His apparent size at his extreme and mean distances from the
earth is shown in Fig. 273.
Jupiter shines with a brilliant white light, which exceeds that of every
other planet except Venus. The planet is, of course, brightest when near
opposition.
251. _The Volume and Density of Jupiter._--Jupiter is the "giant planet"
of our system, his mass largely exceeding that of all the other planets
combined. His mean diameter is about eighty-five thousand miles; but the
equatorial exceeds the polar diameter by five thousand miles. In volume
he exceeds our earth about thirteen hundred times, but in mass only
about two hundred and thirteen times. His specific gravity is,
therefore, far less than that of the earth, and even less than that of
water. The comparative size of Jupiter and the earth is shown in Fig.
274.
[Illustration: Fig. 274.]
252. _The Sidereal and Synodical Periods of Jupiter._--It takes Jupiter
nearly twelve years to make a _sidereal_ revolution, or a complete
revolution around the sun, his orbital motion being at the rate of about
eight miles a second. His _synodical_ period, or the time of his passage
from opposition to opposition again, is three hundred and ninety-eight
days.
253. _The Telescopic Aspect of Jupiter._--There are no really permanent
markings on the disk of Jupiter; but his surface presents a very
diversified appearance. The earlier telescopic observers descried dark
belts across it, one north of the equator, and the other south of it.
With the increase of telescopic power, it was seen that these bands were
of a more complex structure than had been supposed, and consisted of
stratified, cloud-like appearances, varying greatly in form and number.
These change so rapidly, that the face of the planet rarely presents the
same appearance on two successive nights. They are most strongly marked
at some distance on each side of the planet's equator, and thus appear
as two belts under a low magnifying power.
Both the outlines of the belts, and the color of portions of the planet,
are subject to considerable changes. The equatorial regions, and the
spaces between the belts generally, are often of a rosy tinge. This
color is sometimes strongly marked, while at other times hardly a trace
of it can be seen. A general telescopic view of Jupiter is given in
Plate V.
[Illustration: Plate 5.]
254. _The Physical Constitution of Jupiter._--From the changeability of
the belts, and of nearly all the visible features of Jupiter, it is
clear that what we see on that planet is not the solid nucleus, but
cloud-like formations, which cover the entire surface to a great depth.
The planet appears to be covered with a deep and dense atmosphere,
filled with thick masses of clouds and vapor. Until recently this
cloud-laden atmosphere was supposed to be somewhat like that of our
globe; but at present the physical constitution of Jupiter is believed
to resemble that of the sun rather than that of the earth. Like the sun,
he is brighter in the centre than near the edges, as is shown in the
transits of the satellites over his disk. When the satellite first
enters on the disk, it commonly seems like a bright spot on a dark
background; but, as it approaches the centre, it appears like a dark
spot on the bright surface of the planet. The centre is probably two or
three times brighter than the edges. This may be, as in the case of the
sun, because the light near the edge passes through a greater depth of
atmosphere, and is diminished by absorption.
It has also been suspected that Jupiter shines partly by his own light,
and not wholly by reflected sunlight. The planet cannot, however, emit
any great amount of light; for, if it did, the satellites would shine by
this light when they are in the shadow of the planet, whereas they
totally disappear. It is possible that the brighter portions of the
surface are from time to time slightly self-luminous.
[Illustration: Fig. 275.]
Again: the interior of Jupiter seems to be the seat of an activity so
enormous that it can be ascribed only to intense heat. Rapid movements
are always occurring on his surface, often changing its aspect in a few
hours. It is therefore probable that Jupiter is not yet covered by a
solid crust, and that the fiery interior, whether liquid or gaseous, is
surrounded by the dense vapors which cease to be luminous on rising into
the higher and cooler regions of the atmosphere. Figs. 275 and 276 show
the disk of Jupiter as it appeared in December, 1881.
[Illustration: Fig. 276.]
255. _Rotation of Jupiter_.--Spots are sometimes visible which are much
more permanent than the ordinary markings on the belts. The most
remarkable of these is "the great red spot," which was first observed in
July, 1878, and is still to be seen in February, 1882. It is shown just
above the centre of the disk in Fig. 275. By watching these spots from
day to day, the time of Jupiter's axial rotation has been found to be
about nine hours and fifty minutes.
The axis of Jupiter deviates but slightly from a perpendicular to the
plane of its orbit, as is shown in Fig. 277.
[Illustration: Fig. 277.]
THE SATELLITES OF JUPITER.
[Illustration: Fig. 278.]