cerning the places of the signs of the zodiac, not to be reconciled in any other manner.

A complete explanation of the cause which occasions the precession of the equinoxes, would require the aid of the most abstruse mathematics, and therefore cannot be properly introduced here. The cause itself, may, however, be stated in a few words.

It has already been explained, that the revolution of the earth round its axis, has caused an excess of matter to be accumulated at the equator, and hence, that the equatorial, is greater than the polar diameter, by 34 miles. Now the attraction of the sun, and moon, on this accumulated matter at the equator, has the effect of slowly turning the earth about the axis of the ecliptic, and thus causing the precession of the equinoxes.

The Moon.

While the earth revolves round the sun, the moon revolves round the earth, completing her revolutions once in 27 days, 7 hours, and 43 minutes, and at the distance of 240,000 miles from the earth. The period of the moon's change, that is, from new moon to new moon again, is 29 days, 12 hours, and 44 minutes.

The time of the moon's revolution round the earth is called her periodical month; and the time from change to change is called her synodical month. If the earth had no annual motion, these two periods would be equal, but because the earth, goes forward in her orbit, while the moon goes round the earth the moon must go as much further, from change to change, to make these periods equal, as the earth goes for ward during that time, which is more than the twelfth part of her orbit, there being more than twelve lunar periods in the year.

These two revolutions may be familiarly illustrated by the motions of the hour and minute hands of a watch. Let us suppose the 12 hours marked on the dial plate of a watch to represent the 12 signs of the zodiac through which the sun seems to pass in his yearly revolution, while the hour hand of

What is the cause of the precession of the equinoxes? What is the period of the moon's revolution found the earth? What is the period from new moon to new moon again? What are these two periods called? Why are not the periodical and synodical months equal? How are these two revolutions of the moon illustrated by the two hands of a watch?

the watch represents the sun, and the minute hand the moon. Then, as the hour hand goes around the dial plate once in 12 hours, so the sun apparently goes around the zodiac once in 12 months; and as the minute hand makes 12 revolutions to one of the hour hand, so the moon makes 12 revolutions to one of the sun. But the moon, or minute hand, must go more than once round, from any point on the circle, where it last came in conjunction with the sun, or hour hand, to overtake it again, since the hour hand will have moved forward of the place where it was last overtaken, and consequently the next conjunction must be forward of the place where the last happened. During an hour, the hour hard describes the twelfth part of the circle, but the minute hand has not only to go round the whole circle in an hour, but also such a portion of it, as the hour hand has moved forward since they last met. Thus at 12 o'clock, the hands are in conjunction; the next conjunction is 5 minutes 27 seconds past 1 o'clock; the next, 10 min. 54 sec. past II o'clock; the third, 16 min. 21 sec. past III; the 4th, 21 min. 49 sec. past IV; the 5th, 27 min. 10 sec. past V; the 6th, 32 min. 43 sec. past VI; the 7th, 38 min. 10 sec. past VII; the 8th, 43 min. 38 sec. past VIII; the 9th, 49 min. 5 sec. past IX; the 10th, 54 min. 32 sec. past X; and the next conjunction is at XII.

Now although the moon passes around the earth in 27 days 7 hours and 43 minutes, yet her change does not take place at the end of this period, because her changes are not occasioned by her revolutions alone, but by her coming periodically into the same position in respect to the sun. At her change, she is in conjunction with the sun, when she is not seen at all, and at this time astronomers call it new moon, though generally, we say it is new moon two days afterwards, when a small part of her face is to be seen. The reason why there is not a new moon at the end of 27 days, will be obvious, from the motions of the hands of a watch; for we see that more than a revolution of the minute hand is required to bring it again in the same position with the hour hand, by about the twelfth part of the circle.

The same principle is true in respect to the moon; for as

Mention the time of several conjunctions between the two hands of a watch. Why do not the moon's changes takes place at the periods of her revolution around the earth? How much longer does it take the moon to come again in conjunction with the sun, than it does to perform her periodical revolution?

the earth advances in its orbit, it takes the moon 2 days 5 hours and 1 minute longer to come again in conjunction with the sun, than it does to make her monthly revolution round the earth; and this 2 days, 5 hours and 1 minute being added to 27 days 7 hours and 43 minutes, the time of the periodical revolution, makes 29 days 12 hours and 44 minutes, the period of her synodical revolution.

The moon always presents the same side, or face, towards the earth, and hence it is evident that she turns on her axis but once, while she is performing one revolution round the earth, so that the inhabitants of the moon have but one day, and one night, in the course of a lunar month.

One half of the moon is never in the dark, because when this half is not enlightened by the sun, a strong light is reflected to her from the earth, during the sun's absence. The other half of the moon enjoys alternately two weeks of the sun's light, and two weeks of total darkness.

The moon is a globe, like our earth, and, like the earth, shines only by the light reflected from the sun; therefore, while that half of her which is turned towards the sun is enlightened, the other half is in darkness. Did the moon shine by her own light, she would be constantly visible to us, for then, being an orb, and every part illuminated, we should see her constantly full and round, as we do the sun.

One of the most interesting circumstances to us, respecting the moon, is, the constant changes which she undergoes, in her passage around the earth. When she first appears, a day or two after her change, we can see only a small portion of her enlightened side, which is in the form of a crescent; and at this time she is commonly called new moon. From this period, she goes on increasing, or showing more and more of her face, every evening, until at last she becomes round, and her face fully illuminated. She then begins again to decrease, by apparently losing a small section of her face, and the next evening, another small section from the same part, and so on, decreasing a little every day, until she entirely disappears; and having been absent a day or two, re-appears, in the form of a crescent, or new moon, as before.

How is it proved that the moon makes but one revolution on her axis, as she passes around the earth? One half of the moon is never in the dark; explain why this is so. How long is the day and night at the other half? How is it shown that the moon shines only by reflected light?

When the moon disappears, she is said to be in conjunction, that is, she is in the same direction from us with the sun. When she is full, she is said to be in opposition, that is, she is in that part of the heavens opposite to the sun, as seen by us. The different appearances of the moon, from new to full, and from full to change, are owing to her presenting different portions of her enlightened surface towards us at different times. These appearances are called the phases ofthe moon,and are easily accounted for, and understood by the following figure. Fig. 205.

and A, B, C, D, Now when the sun, as at A, her

Let S, fig. 205, be the sun, E the earth, E, the moon in different parts of her orbit. moon changes, or is in conjunction with the dark side is turned towards the earth, and she is invisible, as represented at a. The sun always shines on one half of the moon, in every direction, as represented at A and B, on the inner circle; but we at the earth see only such portions of the enlightened half as are turned towards us. After her change, when she has moved from A to B, a small part of her illuminated side comes in sight, and she appears horned, as at b, and is then called the new moon. When she arrives at C, several days afterwards, one half of her disc is visible, and she appears as at c, her appearance being the same in both circles. At this point she is said to be in her first quarter, because she has passed through a quarter of her orbit, and is 90 degrees

When is the moon said to be in conjunction with the sun, and when in opposition to the sun? What are the phases of the moon? Describe fig. 205, and show how the moon passes from change to full, and from full to change.

At D, she

from the place of her conjunction with the sun. shows us still more of her enlightened side, and is then said to appear gibbous, as at d. When she comes to E, her whole enlightened side is turned towards the earth, and she appears in all the splendor of a full moon. During the other half of her revolution, she daily shows less and less of her illuminated side, until she again becomes invisible by her conjunction with the sun. Thus in passing from her conjunction a, to her full, e, the moon appears, every day to increase, while in going from her full to her conjunction again, she appears to us constantly to decrease, but as seen from the sun, she appears always full.

The earth, seen by the inhabitants of the moon, exhibits the same phases that the moon does to us, but in a contrary order. When the moon is in her conjunction, and consequently invisible to us, the earth appears full to the people of the moon, and when the moon is full to us, the earth is dark to them.

The earth appears thirteen times larger to the lunarians than the moon does to us. As the moon always keeps the same side towards the earth, and turns on her axis only as she moves round the earth, we never see her opposite side. Consequently, the lunarians who live on the opposite side to us never see the earth at all. To those who live on the middle of the side next to us, our earth is always visible, and directly over head, turning on its axis nearly thirty times as rapidly as the moon, for she turns only once in about thirty days. A lunar astronomer, who should happen to live directly oppo site to that side of the moon, which is next to us, would have to travel a quarter of the circumference of the moon, or about 1500 miles, to see our earth above the horizon, and if he had the curiosity to see such a glorious orb, in its full splendor over his head, he must travel 3000 miles. But if his curiosity equalled that of terrestrials, he would be amply compensated by beholding so glorious a nocturnal luminary, a moon thirteen times as large as ours.

That the earth shines upon the moon as she does upon us, is proved by the fact that the outline of her whole disc may

What is said concerning the phases of the earth, as seen from the moon? When does the earth appear full at the moon? When is the earth, in her change, to the people of the moon? Why do those who live on one side of the moon never see the earth? How is it known that the earth shines upon the moon, as the moon does upon us?