THE MOON, the satellite of the earth, is the nearest to us of all the
heavenly bodies, being at a mean distance of 240,000 miles. Its diameter
is 2,153 miles and, its density being little more than half that of the
earth, the force of gravity at its surface is very much less than that
at the surface of the earth. A body which weighs a pound here would only
weigh about two and one-half ounces if taken to the moon.
[Illustration: =THE SYSTEM OF MARS AND ITS MOONS CONTRASTED WITH THAT OF
THE EARTH AND MOON=
In this diagram the markings on the earth and Mars are to scale, the
orbits of the planets are seen in perspective and the measurements are
according to Prof. Percival Lowell.]
THE MOON’S ORBIT. Her path is approximately an ellipse with the earth in
one focus. Its apparent motion in the sky is from west to east, but she
moves much faster than the sun, taking about twenty-seven days eight
hours to travel all round the earth. The time between two successive new
moons (synodic period or lunation) is twenty-nine and one-half days. The
reason of the difference is that the sun moves slowly in his annual
course through the stars in the same direction as the moon, which
therefore in its revolution round the earth has to overtake him when it
returns. The moon rotates on its axis in the same time as it performs a
revolution in its orbit; hence the same half is always turned toward us.
When the moon in her orbit lies between the sun and the earth, she is
said to be in _conjunction_ with the sun; when the earth is between the
moon and the sun, the moon is said to be in _opposition_ to the sun. At
either of the two points midway from conjunction and opposition, i. e.
90° from conjunction or opposition, the moon is said to be in
_quadrature_.
THE PHASES OF THE MOON. Except at opposition--i. e. when the earth is
between the moon and sun--the whole of the moon’s disc does not appear
bright to us, and the amount of the bright surface seen by us is found
to depend on the relative positions of moon and sun. Half of the moon is
always illuminated by the sun; but when it is in conjunction between the
earth and sun the whole of the bright surface is on the side away from
us; so that the moon is invisible. As it moves farther from the line
joining earth and sun, a small portion of the bright side comes into
view as a narrow crescent. This increases till half the disc is
illuminated, when the lines joining earth and moon and earth and sun are
at right angles. From this time the moon loses its crescent shape and
becomes convex on both sides, or gibbous (Lat. _gibbus_, a hump)--the
maximum brightness, or full moon, occurring when sun and moon are on
opposite sides of the earth. After this the moon becomes gibbous, then
crescent, and vanishes before the time of new moon.
It is worthy of note that the moon is higher in the heavens and longer
above the horizon in the winter than in summer. This is owing to the
plane of its orbit being at night high towards the south in winter and
low in summer, as is the ecliptic. The moon’s orbit, like that of other
planets, is elliptical, but irregular. When nearest to the earth, she is
said to be in _perigee_; when at the greatest distance, in _apogee_.
[Illustration: =DIAGRAM SHOWING HOW THE MOON’S PHASES ARE CAUSED=
In the above diagram, the earth is in the center, and the circle ACFH
the orbit of the moon. Since the inclination of the plane of the moon’s
orbit to the plane of the ecliptic is only a few degrees, we may neglect
it in this case, and suppose the two planes to coincide. Let the sun lie
in the direction ES. Since the distance of the sun from the earth is
about three hundred and eighty-seven times the distance of the moon from
the earth, the lines ES, HS, BS, etc., drawn to the sun from different
points of the moon’s orbit, may be considered to be sensibly parallel.
Let us first suppose the moon to be in conjunction with the sun at the
point A. Here only the dark portion of the moon is turned towards the
earth, and the moon is therefore invisible. This is called new moon. As
the moon moves on towards B, the enlightened part begins to be visible,
and when it reaches C, half the enlightened part is visible, and the
moon is at its first quarter. When the moon is at F, in opposition to
the sun, all the illuminated part is turned towards the earth, and the
moon is full. The moon wanes after leaving F, passes through its last
quarter at H, and finally becomes again invisible at A.]
SURFACE OF THE MOON. The moon is an opaque, cold globe, covered with
mountains, extinct volcanoes, and plains. She has neither water nor
atmosphere, and always presents the same surface to the earth in
consequence of rotating on her axis in the same time as she revolves
round the earth. Moonlight is only reflected sunlight, the illuminated
hemisphere being always turned towards the sun.
The face of the moon has been studied and mapped on a large scale. Its
chief features are three in number: (1) the numerous _volcanic
craters_, such as Tycho and Copernicus, which are mostly named after
distinguished men of science; (2) the wide, dark plains which are known
as _seas_, because they were formerly thought to consist of water; (3)
the curious systems of _bright streaks_, which radiate from many of
these craters, of which the most remarkable extend in all directions
from the great crater Tycho, near the moon’s south pole, and are
conspicuous even to the naked eye at the time of full moon.
THE MOON AND THE TIDES. The moon has long been known to have an effect
upon the tides, and may perhaps influence the winds. It is of enormous
importance to navigators for the determination of longitude, and hence
its movements have been investigated with the greatest care and
precision.
[Illustration: =HOW THE MOON FORMS “TIDES” IN THE CRUST OF THE EARTH=
By reason of its power of attraction, it is well recognized that the
Moon exercises a greater influence on the side of the earth which is
nearest to it. In consequence the earth is subject to a stress or pull
that tends to lengthen it out toward the moon, and then to recede as the
earth turns away on its axis.]
THE PLANET MARS. Nearest to the earth, with the single exception of
Venus, resembles the earth more closely than any other of the planets,
and is most favorably situated for our observation of all the heavenly
bodies, except the moon. It is a globe rather more than half the size of
the earth. When Mars comes nearest to the earth its distance from us is
about 35,000,000 miles. At these favorable moments its brightness is
about equal to Jupiter, and only surpassed by that of Venus. Mars has a
very pronounced red color, which is supposed to be due to the prevalence
of a rock like our red sandstone on its surface, or possibly to the
color of its vegetation.
Its density is much less--about three-quarters that of the earth; so a
pound weight placed on its surface would not weigh much more than six
ounces, and a ponderous elephant would, if there, be able to jump about
with the agility of a fawn.
The heat and light which Mars receives from the sun, therefore, vary
enormously, and so cause a difference in the lengths of winter and
summer in his north and south hemispheres, the seasons in the north
hemisphere being far more temperate than those in the south. Viewed with
the telescope, large dark green spots are seen, the rest of the surface
being of a ruddy tint, except at the two poles, where two white spots
are observed and considered to be due to large masses of snow and ice.
It has been supposed that the greenish spots are oceans, and the ruddy
parts land. The spectroscope has shown that watery vapor is present in
Mars’ atmosphere, and appearances like huge rain-clouds sometimes
obscure a part of the planet for a considerable period. Physical
processes seem to go on there much the same as on our planet; hence many
believe that Mars is inhabited and forms, in fact, a miniature picture
of the earth.
JUPITER. By far the largest of the planets is second in brilliancy to
Venus, unlike which, however, it is a “superior” planet, having its
orbit outside that of the earth. It is about five times as brilliant as
Sirius, the brightest of the fixed stars.
The planet is a beautiful object when viewed with a telescope; it is
probable that the markings are entirely due to its atmosphere, and that
the actual surface of the planet is rarely visible. Jupiter has hardly
yet cooled from the condition of incandescence, and it is only slightly
solidified. It possesses eight satellites, four of which were discovered
by Galileo when he applied the telescope first to the investigation of
the heavens. By means of these satellites the first observations of the
velocity of light were made. A fifth was discovered in 1892 at the Lick
Observatory.
SATURN was recognized as a planet by the ancients, and was the outside
member of the solar system as known by them. His diameters at the
equator and poles differ considerably, the protuberance at the equator
giving him there a diameter of 74,000 miles, while at the poles it is
only 68,000. In size Saturn is the largest of the planets except
Jupiter, being in fact seven hundred times larger than our earth, but
his density is so small that he would be able to float on water far more
easily than an iceberg. From this it follows that he cannot consist of
solid or liquid matter, and in fact we can only view a mass of clouds
intensely heated within, the whole being probably a planet in the early
stage of development--younger even than Jupiter.
The most remarkable characteristic of Saturn, which makes him an object
of such interest in the sky, is his possession of a luminous ring. The
ring is only luminous on account of its reflection of the sun’s light;
hence is invisible to us when, for instance, we are endeavoring to look
at the ring from below while the sun is shining above. It also sometimes
happens that the plane of the rings passes through the sun or through
the center of the earth, in which case only the thin edge of the rings
can be seen. The ring is divided into two parts, the inner being the
wider, while another faint division appears to divide the outer part
into two smaller rings. In 1850 another ring was discovered; this is
quite different from the outer rings, being dark, and generally known as
the dusky ring of Saturn. The outer ones, though far from solid, can
receive a shadow of Saturn, and themselves cast one on his disc. The
rings are not continuous masses of matter, but consist of countless
myriads of tiny satellites, so close together that to the observer they
appear as one body. The planet has eight satellites which seldom pass
behind or in front of the planet’s disc, and therefore are not objects
of great interest.
URANUS is the next planet beyond Saturn. His mass is about fifteen times
as much as that of the earth, an amount which makes him more than
outweigh Mercury, Venus, the Earth, and Mars combined. All astronomers
do not agree in their estimation of these numbers, Uranus being too far
away for measurements to be more than approximate. Gravity on his
surface is only three-quarters of what it is here. Uranus has four
satellites, and possibly faint rings like those which encircle Saturn.
NEPTUNE is farthest from the sun, the distance between the two bodies
being about 2,750,000,000 miles. At this immense distance it will,
according to Kepler’s laws, take a long time to travel once around its
orbit, and this time has been found to be one hundred and sixty-five of
our years. Although it is ninety-seven times as large as the earth, yet,
on account of its enormous distance from us it can only just be seen,
even with a powerful telescope. Neptune possesses one satellite, which
moves around the planet in rather less than six days.
MERCURY is the smallest planet, except the planetoids, in the solar
system, and the one nearest the sun. It is never seen for more than two
hours before sunrise or after sunset, and is not always visible then;
but when it does appear, it is extremely brilliant. Even when it is most
distant the sun appears four and a half times as big to it as it does to
us, and when the two are at their nearest, this small planet gets ten
times as much light and heat as we do. It is, however, so small and
difficult to observe, that comparatively little is known of it.
VENUS appears to us as the most brilliant of all the planets, sometimes
heralding the sun’s approach in the morning and sometimes following him
at night. Hence she has been called the “morning” and the “evening”
star; and the ancient Greeks, believing her to be two bodies, and not
one, called her Hesperus (Vesper) when she appeared at night, but
Phosphorus when she preceded the dawn, this last name having been
translated in the Latin, Lucifer. We know very little of the actual
surface of Venus, for her envelope of clouds remains constantly in front
of us to baffle curiosity, and never lifts to give us a glimpse of the
planet beneath. These clouds send on to us the light they borrow from
the sun, and shine to us with a brilliant silvery lustre interrupted
here and there with shadowy markings of short duration. But when Venus
shines to us in crescent-form, certain spots near the ends of the horns
can be seen more definitely, and the effects of light and shadow round
these points suggest that they are lofty peaks, reaching above the
clouds.
THE MINOR PLANETS OR ASTEROIDS. The space between Mars and Jupiter is
occupied by a strange and numerous swarm of _minor planets_ or
_asteroids_. The first of these singular bodies was discovered by an
Italian astronomer, Piazzi, on the first night of the nineteenth
century. Three others were discovered within the course of the next
seven years, and the number now known is upward of 600, most of which
have been recognized by the record of their motion on photographs of the
sky. The four asteroids first discovered, Ceres, Pallas, Juno, and
Vesta, are naturally the largest, ranging in diameter from four hundred
to one hundred and eighteen miles.
Vesta, though not the largest, is considerably the brightest of the
minor planets, and is occasionally visible to the naked eye. None of the
other asteroids has a diameter so great as one hundred miles, and
probably the majority of them are only ten or twenty miles in diameter.