. So the plane of the orbit of the moon is inclined to
the orbit of the earth 5 deg. 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