very nearly so; and in the horizon of the observer, if viewed from any part of the edge of the disc.

(147.) The moon, being much nearer the earth than any other of the heavenly bodies, must pass between it and those of them, which lie near the plane of the moon's orbit. When it passes between the sun and earth, the sun is said to be eclipsed; (a phenomenon soon to be explained), when between the earth and a star, the star is said to be occulted, or hidden. This is sometimes a very interesting phenomenon, particularly, when the occultation of a large star occurs in the new or old of the moon. In the former case, as the invisible part of the moon passes between us and the star, the latter appears to vanish instantaneously in the blue sky; and in the latter, the convex side of the cresent, being in the direction of the moon's motion, a star re-appears from behind the invisible part of the disc, with equally surprising suddenness.

CHAPTER VIII.

OF ECLIPSES.

Form of the Earth's Shadow-Lunar Eclipse-Not monthlyLunar ecliptic Limits-Duration of lunar Eclipse-Moon not invisible during an Eclipse-Solar Eclipse-Solar ecliptic Limits, greater than lunar-More solar than lunar EclipsesMore lunar than solar Eclipses visible at any one PlaceDuration of solar Eclipse-Chaldean Period.

(148.) As the earth receives its light from the sun, it must constantly cast a shadow in the direction op. posite this luminary. In this we are involved the

Does the side of the moon towards the earth, always enjoy earthshine during the absence of the sun? Is the opposite side of the moon always destitute of earth shine? Why? See art. 139. When is a star said to be occulted? When is this phenomenon particularly interesting? In what direction does the earth's shadow continually lie?

moment the sun sinks below our horizon, and its darkness would at that moment equal the shades of midnight, were it not partially illuminated by light reflected from the atmosphere. One half of the earth's surface is embraced in its shadow; of course, the circumference of the latter, where its outline rests upon the earth, equals that of the earth.

(149.) The form of the shadow of a globular body, depends upon its size, as compared with that of the illuminating object. Were the sun and an opaque globe of the same size, the shadow of the latter would be indefinitely prolonged, and its circumference invariable; but if the sun be the larger body, the circumference of the shadow will continually decrease till it terminates in a point, called its vertex, and its form

FIGURE 32.

will be that of a cone, as represented in fig. 32. A sugar loaf is cone-shaped.

(150.) The earth's shadow is of the form last described, and its centre as seen from the earth, always occupies that point in the ecliptic exactly opposite the centre of the sun. Its mean length is 864,000 miles, more than 31 times the distance of the moon from the earth; and its apparent diameter at the mean distance of the moon, is 82 minutes, more If, then, the than 24 times that of the moon. moon revolved around the earth in the plane of

How often are we involved in this shadow? What does the greatest circumference of the shadow equal? Suppose an opaque body were the same size of the sun, of what form would its shadow be? What if the sun be the larger body? Of what form is the earth's shadow? Towards what point in the ecliptic is the vertex of the earth's shadow constantly directed? What is the mean length of the earth's shadow? How many times is the moon's distance contained in this amount? How much greater is the apparent diameter of the earth's shadow than that of the moon? At the mean distance of the latter, what would be the conseqence of this, if the moon revolved around the earth in the plane of the ecliptic? Why will the moon become invisible when it passes through the earth's shadow?

9

the ecliptic, it would pass through this shadow and become invisible, every time it passed through that point in the ecliptic exactly opposite the sun; or at every full moon. Such phenomena do take place, but not every month, and are called lunar eclipses. In fig. 33, S represents the sun; A the earth; ABC its shadow; and M the moon. If the moon passes en

tirely within the shadow, the eclipse (which signifies failure of light), is called total, and partial if only a part of the disc enters.

(151.) Were not the amount of the inclination of the moon's orbit to the plane of the ecliptic, greater than the apparent radius of the earth's shadow, together with that of the moon, an eclipse might take place every month; but the moon's orbit (art. 130), is inclined 5° 17', while the greatest apparent radius of the earth's shadow, at the distance of the moon, is only 45' 52", and the greatest apparent radius of the moon when that of the earth's shadow is greatest, 16′ 5′′ : their sum then is (45′52′′+16' 5"=) 61' 57", which

Does such a phenomenon ever take place? What is it called? Will you explain fig. 33? When is an eclipse total? When partial? Under what circumstances might an eclipse take place every month? What is the greatest apparent radius of the earth's shadow, at the distance of the moon? What that of the moon, at the time when the earth's shadow is the greatest?

subtracted from 5° 17′, leaves nearly 4o, which is the angular distance between the edge of the moon and that of the earth's shadow, when the moon fulls at its

FIGURE 34.

a

E

greatest distance from the ecliptic. Fig. 34 will illustrate this: let EC represent an arc of the ecliptic; A E, an arc of the moon's orbit; enan arc of a secondary containing 5° 17' and measuring the greatest angular distance of E C, AE; c the earth's shadow; ec its

radius; and don that of the moon; the angular distance of d from cis neariy 4°. (152.) It is evident from fig. 29, that when the moon is in that point of its orbit most distant from the ecliptic, it is also at its greatest distance from either node; of course, as it approaches a node, it also approaches the ecliptic. It is found that if the moon full within 11° 25′ of a node, an eclipse may occur; this quantity is therefore called the lunar ecliptic limit; and if within about 90 of a node, an eclipse must take place. Were the apparent radius of the earth's shadow and that of the moon invariable amounts, the moon would always suffer an eclipse within a certain angular distance of a node; but both these amounts are variable: the former, because the extreme lines A B, CB fig. 33, of the earth's shadow, converge more rapidly when the earth is in perigee or nearest the sun, than when in apogee, and thus render its apparent radius at any given distance from the earth less; and

What is the difference of the sum of these amounts, and the amount of the inclination of the moon's orbit to the plane of the ecliptic? What does this amount constitute? What is evident from fig. 29? Within what angular distance of a node, may a lunar eclipse take place? What is this distance called? Within what angular distance of a node must an eclipse take place? Under what circumstances would an eclipse invariably occur within a certain distance of a node? Why is not the first of these amounts invariable?

the latter for the reason given in art. 129. In fig. 35,

E C may represent the moon's orbit, A B the ecliptic, Na node, and DF the lunar ecliptic limits, which points are distant from a a, in the moon's orbit, nearly

62'.

(153.) The longest lunar eclipses occur when the moon fulls in a node. These are called central eclipses, for in such cases, the moon's centre, at the middle of the eclipse, coincides with the centre of the shadow; of course, it describes an apparent diameter of the shadow in passing through it. As this at a mean, equals 82' and that of the moon equals only 32' the moon must describe (82-32=) 50' or nearly a degree after its complete immersion, before any part of it can emerge from the shadow; and the moon, during the

Why is not the second amount invariable? Will you explain fig. 35? When are lunar eclipses longest? Why are these called central eclipses? During a central eclipse, what is the angular distance which the moon must describe, after its complete immersion within the earth's shadow, before its edge can emerge from the shadow? Will you explain fig. 36?