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Page 44
[Illustration: Fig. 60.--Lunar Crater "Copernicus," after Secchi.]
The selenography of one side of the moon is much better known to
us than the geography of the earth. Our maps of the moon are far
more perfect than those of the earth; and the photographs of lunar
objects by Messrs. Draper and De la Rue are wonderfully perfect,
[Page 157] and the drawings of Padre Secchi equally so (Fig. 60).
The least change recognizable from the earth must be speedily
detected. There are frequently reports of discoveries of volcanoes
on the moon, but they prove to be illusions. The moon will probably
look the same to observers a thousand years hence as it does to-day.
This little orb, that is only 1/81 of the mass of the earth, has
twenty-eight mountains that are higher than Mont Blanc, that "monarch
of mountains," in Europe.
_Eclipses._
[Illustration: Fig. 61.--Eclipses; Shadows of Earth and Moon.]
It is evident that if the plane of the moon's orbit were to correspond
with that of the earth, as they all lie in the plane of the page
(Fig. 61), the moon must pass between the centres of the earth
and sun, and exactly behind the earth at every revolution. Such
successive and total darkenings would greatly derange all affairs
dependent on light. It is easily avoided. Venus does [Page 158] not
cross the disk of the sun at every revolution, because of the
inclination of the plane of its orbit to that of the earth (see Fig.
41, p. 107). So the plane of the orbit of the moon is inclined to
the orbit of the earth 5� 8' 39"; hence the full-moon is often above
or below the earth's shadow, and the earth is below or above the
moon's shadow at new moon. It is as if the moon's orbit were pulled
up one-quarter of an inch from the page behind the earth, and
depressed as much below it between the earth and the sun. The point
where the orbit of the moon penetrates the plane of the ecliptic is
called a node. If a new moon occur when the line of intersection of
the planes of orbits points to the sun, the sun must be eclipsed; if
the full-moon occur, the moon must be eclipsed. In any other
position the sun or moon will only be partially hidden, or no
eclipse will occur.
If the new moon be near the earth it will completely obscure the
sun. A dime covers it if held close to the eye. It may be so far
from the earth as to only partially hide the sun; and, if it cover
the centre, leave a ring of sunlight on every side. This is called
an annular eclipse. Two such eclipses will occur this year (1879).
If the full-moon passes near the earth, or is at perigee, it finds
the cone of shadow cast by the earth larger, and hence the eclipse
is greater; if it is far from the earth, or near apogee, the earth's
shadow is smaller, and the eclipse less, or is escaped altogether.
There is a certain periodicity in eclipses. Whenever the sun, moon,
and earth are in a line, as in the total eclipse of July 29th,
1878, they will be in the same position after the earth has made
about eighteen revolutions, [Page 159] and the moon two hundred and
sixteen--that is, eighteen years after. This period, however, is
disregarded by astronomers, and each eclipse calculated by itself to
the accuracy of a second.
How terrible is the fear of ignorance and superstition when the sun
or moon appear to be in the process of destruction! how delightful
are the joys of knowledge when its prophesies in regard to the
heavenly bodies are being fulfilled!
MARS.
The god or war; Its sign [Symbol], spear and shield.
MEAN DISTANCE FROM THE SUN, 141,000,000 MILES. DIAMETER, 4211 MILES.
REVOLUTION, AXIAL, 24H. 37M. 22.7S.; ORBITAL, 686.98 DAYS. VELOCITY
PER MINUTE, 899 MILES. SATELLITES, TWO.
[Illustration: Fig. 62.--Apparent Size of Mars at Mean and Extreme
Distances.]
At intervals, on an average of two years one month and nineteen
days, we find rising, as the sun goes down, the reddest star in
the heavens. Its brightness is exceedingly variable; sometimes
it scintillates, and sometimes it shines with a steady light. Its
marked peculiarities demand a close study. We find it to be Mars,
the fiery god of war. Its orbit is far from circular. At perihelion
it is 128,000,000 miles from the sun, and at aphelion 154,000,000;
hence its mean distance is about 141,000,000. So great a change
in its distance from the sun easily accounts for the change in
its brilliancy. Now, if Mars and the earth revolved in circular
orbits, the one 141,000,000 miles from the sun, and the other
92,000,000, they would approach at conjunction within 49,000,000
miles of each other, and at opposition be 233,000,000 miles apart.
But Mars at perihelion may be only 128,000,000 miles from the sun,
and earth at [Page 160] aphelion may be 94,000,000 miles from the
sun. They are, then, but 34,000,000 miles apart. This favorable
opportunity occurs about once in seventy-nine years. At its last
occurrence, in 1877, Mars introduced to us his two satellites, that
had never before been seen by man. In consequence of this greatly
varying distance, the apparent size of Mars differs very much (Fig.
62). Take a favorable time when the planet is near, also as near
overhead as it ever comes, so as to have as little atmosphere as
possible to penetrate, and study the planet. The first thing that
strikes the observer is a dazzling spot of white near the pole which
happens to be toward him, or at both poles when the planet is so
situated that they can be seen. When the north pole is turned toward
the sun the size of the spot sensibly diminishes, and the spot at
the south pole enlarges, and _vice versa_. Clearly they are
ice-fields. Hence Mars has water, and air to carry it, and heat to
melt ice. It is winter at the south pole when Mars is farthest from
the sun; therefore the ice-fields are larger than at the north pole.
It is summer at the south pole when Mars is nearest the sun. Hence
its ice-fields grow smaller [Page 161] than those of the north pole
in its summer. This carrying of water from pole to pole, and melting
of ice over such large areas, might give rise to uncomfortable
currents in ocean and air; but very likely an inhabitant of earth
might be transported to the surface of Mars, and be no more
surprised at what he observed there than if he went to some point of
the earth to him unknown. Day and night would be nearly of the same
length; winter would linger longer in the lap of spring; summer
would be one hundred and eighty-one days long; but as the seas are
more intermingled with the land, and the divisions of land have less
of continental magnitude, it may be conjectured that Mars might be a
comfortable place of residence to beings like men. Perhaps the
greatest surprise to the earthly visitor would be to find himself
weighing only four-tenths as much as usual, able to leap twice as
high, and lift considerable bowlders.
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