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 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.]