centre of the ellipse_, or _orbit_; while the imaginary straight line
which passes through both foci and the centre, with its ends at opposite
points of the ellipse, is called "_the major axis_" of the orbit. It is
also known as "_the line of the apsides_." The ideal straight line
which, in passing through the centre of the orbit, cuts the major axis
at right angles, and is prolonged on either side to opposite points on
the ellipse, is called "_the minor axis_" of the orbit.
When a planet reaches that extremity of the major axis of its orbit
which is the nearest to the Sun, it is said to be in its "_perihelion_;"
while, when it arrives at the other extremity, which is farthest from
this body, it is said to be in its "_aphelion_." When a planet reaches
either of the two opposite points of its orbit situated at the
extremities of its minor axis, it is said to be at its _mean distance_
from the Sun.
The rapidity with which the planets move on their orbits varies with
their distance from the Sun; the farther they are from this body, the
more slowly they move. The rapidity of their motion is greatest when
they are in perihelion, and least when they are in aphelion, having its
mean rate when these bodies are crossing either of the extremities of
the minor axes of their orbits.
The imaginary line which joins the Sun to a planet at any point of its
orbit, and moves with this planet around the Sun, is called "_the radius
vector_." According to Kepler's second law, whatever may be the distance
of a planet from the Sun, the radius vector sweeps over equal areas of
the plane of the planet's orbit in equal times.
There is a remarkable relation between the distance of the planets from
the Sun and their period of revolution, in consequence of which the
squares of their periodic times are respectively equal to the cubes of
their mean distances from the Sun. From this third law of Kepler, it
results that the mere knowledge of the mean distance of a planet from
the Sun enables one to know its period of revolution, and _vice versa_.
The orbit described by the Earth around the Sun in a year, or the
apparent path of the Sun in the sky, is called "_the ecliptic_." Like
that of all the planetary orbits, the plane of the ecliptic passes
through the Sun's centre. The ecliptic has a great importance in
astronomy, inasmuch as it is the fundamental plane to which the orbits
and motions of all planets are referred.
The orbits of the larger planets are not quite parallel to the ecliptic,
but more or less inclined to this plane; although the inclination is
small, and does not exceed eight degrees. On account of this inclination
of the orbits, the planets, in accomplishing their revolutions around
the Sun, are sometimes above and sometimes below the plane of the
ecliptic. A belt extending 8° on each side of the ecliptic, and,
therefore, 16° in width, comprises within its limits the orbits of all
the principal planets. This belt is called "_the Zodiac_."
Since all the planets have the Sun for a common centre, and have their
orbits inclined to the ecliptic, it follows that each of these orbits
must necessarily intersect the plane of the ecliptic at two opposite
points situated at the extremities of a straight line passing through
the Sun's centre. The two opposite points on a planetary orbit where its
intersections with the ecliptic occur, are called "_the Nodes_," and the
imaginary line joining them, which passes through the Sun's centre, is
called "the line of the nodes." The node situated at the point where a
planet crosses the ecliptic from the south to the north, is called "_the
ascending node_" while that situated where the planet crosses from north
to south, is called "_the descending node_."
The planets circulating around the Sun are eight in number, but, beside
these, there is a multitude of very small planets, commonly called
"asteroids," which also revolve around our luminary. The number of
asteroids at present known surpasses two hundred, and constantly
increases by new discoveries. In their order of distance from the Sun
the principal planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus and Neptune. The orbits of the asteroids are comprised between
the orbits of Mars and Jupiter.
When the principal planets are considered in regard to their differences
in size, they are separated into two distinct groups of four planets
each, viz.: the small planets and the large planets. The orbits of the
small planets are wholly within the region occupied by the orbits of the
asteroids, while those of the large planets are wholly without this
region.
When the planets are considered in regard to their position with
reference to the Earth, they are called "inferior planets" and "superior
planets." The inferior planets comprise those whose orbits are within
the orbit of our globe; while the superior planets are those whose
orbits lie beyond the orbit of the Earth.
Since the orbits of the inferior planets lie within the orbit of the
Earth, the angular distances of these bodies from the Sun, as seen from
the Earth, must always be included within fixed limits; and these
planets must seem to oscillate from the east to the west, and from the
west to the east of the Sun during their sidereal revolution. In this
process of oscillation these planets sometimes pass between the Earth
and the Sun, and sometimes behind the Sun. When they pass between us and
the Sun they are said to be in "inferior conjunction," while, when they
pass behind the Sun, they are said to be in "superior conjunction." When
such a planet reaches its greatest distance, either east or west, it is
said to be at its greatest elongation east or west, as the case may be,
or in quadrature.
The superior planets, whose orbits lie beyond that of the Earth and
enclose it, present a different appearance. A superior planet never
passes between the Earth and the Sun, since its orbit lies beyond that
of our globe, and, therefore, no inferior conjunction of such a planet
can ever occur. When one of these planets passes beyond the Sun, just
opposite to the place occupied by the Earth, the planet is said to be in
"conjunction;" while, when it is on the same side of the Sun with our
globe, it is said to be in "opposition." While occupying this last
position, the planet is most advantageously situated for observation,
since it is then nearer to the Earth. The period comprised between two
successive conjunctions, or two successive oppositions of a planet, is
called its "synodical period." This period differs for every planet.
It is supposed that all the planets rotate from west to east, like our
globe; although no direct evidence of the rotation of Mercury Uranus,
and Neptune has yet been obtained, it is probable that these planets
rotate like the others. It results from the rotation of the planets that
they have their days and nights, like our Earth, but differing in
duration for every planet.
The axes of rotation of the planets are more or less inclined to their
respective orbits, and this inclination varies but little in the course
of time. From the inclination of the axes of rotation of the planets to
their orbits, it results that these bodies have seasons like those of
the Earth; but, of course, they differ from our seasons in duration and
intensity, according to the period of revolution and the inclination of
the axis of each separate planet.
THE PLANET MARS
PLATE VIII
Mars is the fourth of the planets in order of distance from the sun;
Mercury, Venus and the Earth being respectively the first, second and
third.
Owing to the great eccentricity of its orbit, the distance of Mars from
the Sun is subject to considerable variations. When this planet is in
its aphelion, its distance from the Sun is 152,000,000 miles, but at
perihelion it is only 126,000,000 miles distant, the planet being
therefore 26,000,000 miles nearer the Sun at perihelion than at
aphelion. The mean distance of Mars from the Sun is 139,000,000 miles.
Light, which travels at the rate of 185,000 miles a second, occupies
12½ minutes in passing from the Sun to this planet.
While the distance of Mars from the Sun varies considerably, its
distance from the Earth varies still more. When Mars comes into
opposition, its distance from our globe is comparatively small,
especially if the opposition occurs in August, as the two planets are
then as near together as it is possible for them to be, their distance
apart being only 33,000,000 miles. But if the opposition occurs in
February, the distance may be nearly twice as great, or 62,000,000
miles. On the other hand, when Mars is in conjunction in August, the
distance between the two planets is the greatest possible, or no less
than 245,000,000 miles; while, when the conjunction occurs in February,
it is only 216,000,000 miles. Hence the distance between Mars and the
Earth varies from .33 to 245 millions of miles; that is, this planet may
be 212 million miles nearer to us at its nearest oppositions than at its
most distant conjunctions.
From these varying distances of Mars from the Earth, necessarily result
great variations in the brightness and apparent size of the planet, as
seen from our globe. When nearest to us it is a very conspicuous object,
appearing as a star of the first magnitude, and approaching Jupiter in
brightness; but when it is farthest it is much reduced, and is hardly
distinguishable from the stars of the second and even third magnitude.
In the first position, the apparent diameter of Mars is 26", in the last
it is reduced to 3" only.
The orbit of Mars has the very small inclination of 1° 51' to the plane
of the ecliptic. The planet revolves around the Sun in a period of 687
days, which constitutes its sidereal year, the year of Mars being only
43 days less than two of our years.
Mars travels along its orbit with a mean velocity of 15 miles per
second, being about ⁸⁄₁₀ of the velocity of our globe in its
orbit. The synodical period of Mars is 2 years and 48 days, during which
the planet passes through all its degrees of brightness.
Mars is a smaller planet than the Earth, its diameter being only 4,200
miles, and its circumference 13,200 miles. It seems well established
that it is a little flattened at its poles, but the actual amount of
this flattening is difficult to obtain. According to Prof. Young, the
polar compression is ¹⁄₂₁₉.
The surface of this planet is a little over ²⁸⁄₁₀₀ of the
surface of our globe, and its volume is 6½ times less than that of the
Earth. Its mass is only about ⅒ while its density is about ¾ that of
the Earth. The force of gravitation at its surface is nearly ¾ of what
it is at the surface of our globe.
The planet Mars rotates on an axis inclined 61° 18' to the plane of its
orbit, so that its equator makes an angle of 28° 42' with the same
plane. The period of rotation of this planet, which constitutes its
sidereal day, is 24 h. 37 m. 23 s.
The year of Mars, which is composed of 669⅔ of these Martial days,
equals 687 of our days, this planet rotating 669⅔ times upon its axis
during this period. But owing to the movement of Mars around the Sun,
the number of solar days in the Martial year is only 668⅔, while,
owing to the same cause, the solar day of Mars is a little longer than
its sidereal day, and equals 24 h. 39 m. 35 s.
The days and nights on Mars are accordingly nearly of the same length as
our days and nights, the difference being a little less than
three-quarters of an hour. But while the days and nights of Mars are
essentially the same as ours, its seasons are almost twice as long as
those of the Earth. Their duration for the northern hemisphere,
expressed in Martial days, is as follows: Spring, 191; Summer, 181;
Autumn, 149; Winter, 147. While the Spring and Summer of the northern
hemisphere together last 372 days, the Autumn and Winter of the same
hemisphere last only 296 days, or 76 days less. Since the summer seasons
of the northern hemisphere correspond to the winter seasons of the
southern hemisphere, and vice versa, the northern hemisphere, owing to
its longer summer, must accumulate a larger quantity of heat than the
last. But on Mars, as on the Earth, there is a certain law of
compensation resulting from the eccentricity of the planet's orbit, and
from the fact that the middle of the summer of the southern hemisphere
of this planet, coincides with its perihelion. From the greater
proximity of Mars to the Sun at that time, the southern hemisphere then
receives more heat in a given time than does the northern hemisphere in
its summer season. When everything is taken into account, however, it is
found that the southern hemisphere must have warmer summers and colder
winters than the northern hemisphere.
Seen with the naked eye, Mars appears as a fiery red star, whose
intensity of color is surpassed by no other star in the heavens. Seen
through the telescope, it retains the same red tint, which, however,
appears less intense, and gradually fades away toward the limb, where it
is replaced by a white luminous ring.
[PLATE VIII.--THE PLANET MARS.
Observed September 3, 1877, at 11h. 55m. P.M.]
Mars is a very difficult object to observe, the atmosphere surrounding
it being sometimes so cloudy and foggy that the sight can hardly
penetrate through its vapors. When this planet is observed under
favorable atmospheric conditions, and with sufficient magnifying power,
its surface, which is of a general reddish tint, is found to be
diversified by white, gray and dark markings. The dark markings, which
are the most conspicuous, almost completely surround the planet. They
are of different forms and sizes, and very irregular, as can be seen on
Plate VIII., which represents one of the hemispheres of this planet.
Many of them, especially those situated in the tropical regions of the
planet, form long narrow bands, whose direction is in the main parallel
to the Martial equator.
The dusky spots differ very much, both from one another and in their
several parts, as regards intensity of shade. Some appear almost black,
while others which appear grayish, are so faint, that they can seldom be
seen. In the southern hemisphere, the darkest part of the spots is
generally found along their northern border; especially where there are
deep indentations.
Some observers have described these spots as being greenish or bluish,
but I have never been able to see the faintest trace of these colors in
them, except when they were observed close to the limb, and involved in
the greenish tinted ring which is always to be seen there. It is
probably an effect of contrast, since green and red are complementary
colors, and since this greenish tinge around the limb covers all kinds
of spots, whether white or dark. When such dark spots, involved in the
greenish tint, are carried by the rotation towards the centre of the
disk, they no longer show this greenish color. To me, these spots have
always appeared dark, and of such tints as would result from a mixture
of white and black in different proportions; except that on their
lighter portions they show some of the prevalent reddish tint of the
Martial surface. It is to be remarked that in moments of superior
definition of the telescopic image, the intensity of darkness of all the
spots is considerably increased--some of them appearing almost perfectly
black.
The markings on the surface of Mars are now tolerably well
known--especially those of its southern hemisphere, which, owing to the
greater proximity of the planet to our globe when this hemisphere is
inclined towards the earth, have been better studied. Those of the
northern hemisphere are not so well known, since when this hemisphere is
inclined towards us, the distance of Mars from the Earth is 26,000,000
miles greater, so that the occasions for observing them are not so
favorable.
Several charts of Mars are in existence, but as the same nomenclature
has not been employed in all of them, some confusion has arisen in
regard to the names given to the most remarkable features of the
planet's surface. In order to give clearness to the subject, it will be
necessary here to give a brief description of the principal markings
represented on Plate VIII. In this the nomenclature will be employed
which has been adopted by the English observers in the fine chart of Mr.
Nath. Green. The large dark spot represented on the left-hand side of
the plate is called De La Rue Ocean. The dark oval spot, isolated in the
vicinity of the centre of the disk, is called Terby Sea; while the dark,
irregular form on the right, near the border, represents the western
extremity of Maraldi Sea.
The dusky spots of Mars seem to be permanent, and to form a part of the
general surface of the planet. That several among them, at least, are
permanent, is proved by the fact that they have been observed in the
same position, and with the same general form, for over two centuries.
Yet, if we are to depend upon the drawings made fifty years ago by Beer
and Maedler, it would seem that the permanency of some of them does not
exist, since a very large spot represented by these astronomers on their
chart of Mars is not visible now. This object, which, on their map, has
its middle at 270°, should be precisely under the prominent dark oval
spot called Terby Sea, seen near the centre of the picture, and would
extend down almost as far as the northern limb. This can hardly be
attributed to an error of observation, since these observers were both
careful, and had great experience in this class of work. It is a very
singular fact that, at the very same place where Maedler represented the
spot in question, I found a conspicuous dark mark on December 16, 1881,
which was certainly not visible in 1877, during one of the most
favorable oppositions which can ever occur. The object, which is still
visible (Feb., 1882), consists of an isolated spot situated a little to
the north of Terby Sea. During the memorable opposition of 1877, I
investigated thoroughly the markings of Mars, and made over 200 drawings
of its disk, 32 of which represent the Terby Sea; but this isolated spot
was not to be seen, unless it be identified with the faint mark,
represented on the plate, which occupied its place. There cannot be the
slightest doubt that a change has occurred at that place. Changes in the
markings have also been suspected on the other hemisphere of the planet.
The well-known fact that the continents, and especially the mountainous
and denuded districts of our globe, reflect much more light than the
surfaces covered by water, has led astronomers to suppose that the dark
spots on Mars are produced by a liquid strongly absorbing the rays of
light, like the liquids on the surface of the Earth. According to this
theory the dark spots observed are supposed to be lakes, seas, and
oceans, similar to our own seas and oceans, while the reddish and
whitish surfaces separating these dark spots, are supposed to be
islands, peninsulas, and continents. This supposition seems certainly to
have a great deal of probability in its favor, although some of the
lighter markings may have a different origin, and perhaps be due to
vegetation; but no observer has yet seen in them any of the changes
which ought to result from change of seasons. Some of the changes in the
dark spots might also be attributed to the flooding or drying up of
marshes and low land. The change which I have observed lately might be
attributed to such a cause, especially as my observation was made
shortly after the spring equinox of the northern hemisphere of Mars,
which occurred on December 8th.
Besides the dark spots just described, there are markings of a different
character and appearance. Among the most conspicuous are two very
brilliant white oval spots, which always occupy opposite sides of the
planet. These two bright spots, which correspond very closely with the
poles of rotation of Mars, have been called "polar spots."
On account of the inclination of the axis of rotation of this planet to
the ecliptic, it is rare that both of these spots are visible on the
disk at the same time; and when this occurs, they are seen considerably
foreshortened, as they are then both on the limb of the planet. Usually
only one spot is visible, and it appears to its best advantage when the
region to which it belongs attains its maximum of inclination towards
the Earth.
The polar spots change considerably in size, as they do also in form.
Sometimes they occupy nearly one-third of the disk, as is proved by many
of my observations; while at other times they are so much reduced as to
be totally invisible. It is to be remarked that the reduction of these
spots generally corresponds with the summer seasons, and their
enlargement with the winter seasons of the hemispheres to which they
respectively belong. From these well-observed facts it would appear that
a relation exists between the temperature of the two hemispheres of Mars
and the variations of the white spots observed at its poles. A similar
relation is known to exist on our globe between the progress of the
seasons and the melting away and the accumulation of snow in the polar
regions. Astronomers have been led, accordingly, to attribute the polar
spots of Mars, with all their variations, to the alternate accumulation
and melting of snows. On this account, the polar spots of Mars are
sometimes also called "snow-spots."
Errors have certainly been made by astronomers in some of their
observations of the so-called polar snow-spots, other objects occupying
their place having been mistaken for them. A regular series of
observations on this planet, which I have now continued for seven years,
has revealed the fact that during the winter seasons of the southern
hemisphere of Mars, the polar spots are most of the time invisible,
being covered over by white, opaque, cloud-like forms, strongly
reflecting light. In 1877, during more than a month, I, myself, mistook
for the polar spots such a canopy of clouds, which covered at least
one-fifth of the surface of the whole disk. I only became aware of my
error when the opaque cloud, beginning to dissolve at the approach of
the Martial summer, allowed the real polar spot to be seen through its
vapors, as through a mist at first, and afterwards with great
distinctness. In this particular case, the snow-spot was considerably
smaller than the cloudy cap which covered it, and it is to be remarked
that it was not situated at the centre of this cloudy cap, but was east
of that centre; a fact which may account for the so-called polar spots
not being always observed on exactly opposite sides of the disk. From my
observations of 1877, 1878 and 1880, it appears that at the approach of
the autumnal equinox of the southern hemisphere of Mars, large, opaque
masses, like cumulus clouds in form, began to gather in the polar
regions of that hemisphere, and continued through autumn and winter,
dissolving only at the approach of spring. These clouds, which varied in
form and extent, were very unsteady at first, but as the winter drew
nearer they enlarged and became more permanent, covering large surfaces
for months at a time.
That the large white spots under consideration are real clouds in the
atmosphere of Mars, and are not due to a fall of snow, is proved by the
fact that these spots covered both seas and continents with equal
facility, even in the equatorial regions of the planet. Snow, of course,
could not cover the seas of Mars, unless these were all frozen over,
even in the equatorial zones; therefore, if the dark spots of Mars are
assumed to be due to water, these large white spots cannot well be
ascribed to snow.
The real polar spots of Mars seem to be in relief on the surface of the
planet, since the southern spot often appeared slightly shaded on the
side opposite to the Sun during my observations in 1877. In certain
cases, when they are on or very near the limb, they have been observed,
both by others and by myself, to project from the disk slightly.