opposite to those of the Moon seen from the Earth. And it is reflected light from a gibbous primary which renders the dark part of this satellite visible when horned, directly after, or immediately before the change. Moving in its orbit to the east, the Moon comes to the meridian about 50 minutes later each evening.

CHAPTER XII.

ECLIPSES.

WHERE their causes are unknown, superstitious terrors, perhaps, always attend eclipses. Eclipse, in Astronomy, means a temporary deprivation of light, and is rarely used, but when applied to the Sun and Moon.

In describing eclipses of these luminaries, their discs are di vided into twelve parts, called digits. The proportion of a lumi nary obscured, in a partial eclipse, i designated by the number of digits said to be eclipsed. If 8 digits are eclipsed, then twothirds of the luminary will be obscured.

LUNAR ECLIPSES.

AN eclipse of the Moon is occasioned by the interposition of the Earth, between the Sun and Moon, and consequently, it must occur when the Moon is in opposition to the Sun, or at the full Moon; as seen in Plate iv. Figure 1.

If the plane of the Moon's orbit coincided with the Ecliptic, there would be a lunar eclipse at every full Moon, and a solar eclipse at every change.

How has superstition regarded Eclipses? What is an Eclipse? Into how many parts are the discs of luminaries divided? What are they called?

What occasions an eclipse of the Moon? What are the Moon's nodes ?

But the orbit of the Moon, makes an angle with the ecliptic of 530. It will therefore appear in one half of its orbit, north of the apparent annual path of the Sun among the stars, and the other half of its orbit, south of the Ecliptic. When it crosses the Ecliptic, as it must do twice in every revolution, these are its nodes.

When a full Moon occurs at a greater distance than 120 from one of its nodes, no lunar eclipse can reach any part of the Earth. If within 120 of a node at the full, either a partial or total eclipse will take place. If total, it would be visible over a whole hemisphere, if partial, it might not be. In a partial lunar eclipse, but a part of the Moon's disc passes through the shadow of the Earth; in a total eclipse the whole. Partial eclipses may last but a short period; total lunar eclipses will continue from less than three hours, to nearly four hours. In a total eclipse, the Moon is visible and of a copper color.

The shadow of the Earth is of a conical shape, because the Sun is vastly larger than the planet; and the orbits of both the Earth and Moon, are ellipses, and not circles; therefore the width of the shadow of the Earth, where the Moon passes through it in eclipses, will vary very considerably. The longest eclipse of the lesser light will occur, when the Sun is most remote from the Earth, and the Moon is nearest; the shortest, when the Moon is remotest, and the Sun nearest. An eclipse of the Moon, always commences on its eastern limb or edge.

How near to a node must the Moon be, to occasion an eclipse? m total lunar eclipses, is the Moon visible? What is its color? How long may such eclipse continue ?

What reasons are assigned for the variation in the duration. of lunar eclipses? When will they be longest ?-when shortest ? On which limb of the Moon do they commence ?

At Alexandria, in Egypt, more than 2000 years ago, the Moon was seen eclipsed when rising; while the Sun was still above the horizon. When the refractive properties of the atmosphere are discussed, the cause of this interesting phenomenon will be assigned. But tables of the Moon's present place, would not admit of that eclipse having commenced at Alexandria, until the Moon was nearly two hours above the horizon. But the phenomenon was so peculiar, how could the observer be mistaken? The true solution is this, that the Moon is nearer to the Earth now, than it then was; but La Place has demonstrated, that the orbit of the Earth is less elliptical now, than then, but that the variations in orbits and distances are all within known limits.

The average number of total lunar eclipses in a century, is about 30. The average number of eclipses of both luminaries in a year, is about four. There may be seven eclipses in a year, and there may be but two. If but two, they will both be of the Sun.

SOLAR ECLIPSES.

These are occasioned by the Moon's passing between the Earth and the Sun; and will be easily understood by consulting Plate iv. Figure 2.

Eclipses of the sun may be either partial, annular, or total. Partial eclipses of the Sun will occur

What phenomenon occurred at Alexandria? How does that prove the Moon nearer to the Earth Now, than then? What has La Place discovered? What is the average number of total lunar eclipses in a century? What the average number of eclipses in a year? What the extremes?

What occasions a solar eclipse? When will partial solar eclipses occur?