with respect to the star. The moon may only just touch the star, forming a kind of occultation called an appulse. Or she may pass in such a manner that her centre passes across the star. In this case the occultation may occupy more than an hour. But, whatever be the duration of the occultation, at the expiration of the time, the star reappears from behind the western edge of the moon.

An observation of this kind, which is one of the most interesting of any which astronomy offers, and can be observed with any common telescope, or in the case of a bright star or a planet may be seen with the naked eye, shews very plainly the motion of the moon in her orbit. The following is a list of the occultations of stars by the moon, visible at Greenwich in the month of July, 1844.

There will not be any visible occultation of a planet by the moon in the year 1844: the nearest approach will be that of Venus, September 8, at 13h. 18m.

It will be observed, that the times are here reckoned astronomically from noon; so that, for instance, the immersion of v Capricorni, calculated for July 1, 15h. 39m., will occur at 3h. 39m. in the morning of July 2, civil time.

In order to comprehend the phenomena which the moon affords, it is necessary to form a clear conception of the moon's apparent path in the heavens.

This may be done by observing, from night to night, the principal stars near which the moon passes, and marking the places of the moon thus found upon a map of the stars, or upon a celestial globe. It will be found, that the path of the moon is not the equator; for, if it were, the moon when on the meridian would always be at the same altitude

This will only be a near approach. The emersion of Aquarii will not be visible.

above the horizon of a given place: neither is it the ecliptic; for a very rude representation of the moon's apparent path will shew, that the moon is sometimes to the north of the ecliptic and sometimes to the south of it; but never deviating from it more than about 5° 18', or five degrees and one third.

The following table, abridged from the Nautical Almanac, shews the place of the moon, with reference to the ecliptic, at noon on each day of the month of June, 1844; in which it will be seen, that the moon set out nearly from the ecliptic, and returned to it again. It must be remembered, that the longitude of a heavenly body is its angular distance from the first point of Aries, measured from west to east along the ecliptic; and its latitude is its angular distance north or south of the ecliptic.

If now we trace the course of the moon, as referred to the ecliptic, by means of this table, we shall obtain the following results:—

1. The moon cut the ecliptic a short time (about 8 hours) before noon of June 1st; her longitude then being about 254,° and the moon's motion being northward from the ecliptic. This intersection of the orbit of the moon with the ecliptic is called the ascending node.

2. The moon's northern latitude, or distance northwards from the NO VII-VOL. I.

3 E

ecliptic, continued to increase till June 7, when it became 5° 18′; after which time it diminished, until June 14th. On that day, about 14 hours after noon, the moon again cut the ecliptic, her motion then being southward from the ecliptic, and her longitude about 73° 50'. This second intersection of the orbit of the moon with the ecliptic is called the descending node.

3. After the moon had passed the descending node, her southern latitude increased until June 22, when it was 5° 16', and then decreased until June 21, on which day, at about 2 hours after noon, she again cut the ecliptic, passing towards the north, or came to her ascending node, her longitude at the time being about 253°.

4. The moon cut the ecliptic the second time, at her ascending node, at a point about 1° west of the place at which she cut it the first time. This is expressed by saying that the line of nodes, or line joining the ascending and descending node, regresses upon the ecliptic.

This regression does not depend upon the particular lunation, although its quantity is not always the same: the mean annual regression is 19°34'. It becomes more sensible, if we compare the different positions of the moon's node, after a considerable interval of time. For instance, in the corresponding part of the year 1843, the longitude of the moon's ascending node, which it entered June 12d. 15h. 2m. was about 273°. 10': whereas in 1844, June 28, 2h., the longitude of the moon's ascending node is about 254°: the node having thus regressed more than 19°.

The period in which the node regresses through 360°, or comes back to the same position upon the ecliptic, is about 18 years and 7 months. This is a very important era in many lunar phenomena, and especially in those of eclipses of the sun and moon.

5. The moon returns to the same longitude in about 27d. 7h. 43m. This period is slightly different from the period in which the moon completes a revolution among the fixed stars, since the first point of Aries has a motion from precession.

During the interval in which the moon leaves a certain position among the fixed stars, and returns to it again, the sun has moved towards the east, in his apparent orbit, through nearly 30°. Hence, the moon has to move through a considerable are before she comes to the same position, with respect to the sun; and her synodic period, (or period with respect to the sun,) upon which her phases depend, is, as we have seen, 29d. 12h. 44m.

Upon the whole, then, it appears that the path which the moon appears to describe in the heavens, about the earth, is not a circle, returning into itself, but a curve which cuts the ecliptic in points which are continually varying. Thus, if NCK represents the ecliptic; N the ascending node where the moon leaves the ecliptic, A the first point of Aries, and consequently the

C

A

M

N

n

K

arc AMKN the longitude of the ascending node, the moon comes to its descending node at M, which is not exactly opposite to N, and again comes to its ascending node at n, a point to the westward of N, the node having thus regressed through Nn.

The inclination of the moon's orbit to the ecliptic also continually varies but the amount of that inclination increases and decreases periodically, its greatest value being about 5° 18', and its least value about 5°.

Since the inclination of the ecliptic to the equator is about 23°28′, the moon may be about 28°.46' distant from the equator to the north or south; whereas the sun can never be more than about 23°.28' dis tant from the equator.

This accounts for the well known fact, that the moon, at its meridian passage, appears sometimes much higher than the sun ever does, and at other times appears much lower than the sun. The greatest variation of this kind occurs when the ascending node is in the first point of Aries. When the ascending node is in the first point of Libra, or to the west of it, the moon's orbit falls between the ecliptic and the equator; and the variation of the moon's declination is then less than that of the sun. This will be the position of the moon's nodes in the year 1848, and for 9 years after that time.

The phases of the moon, or the different appearances which the moon presents in different parts of her periodical revolution round the earth, are among the best known of her phenomena.

The general principle, upon which they depend, is this. If a line is drawn from the eye to the centre of the moon, a plane perpendicular to that line, passing through the centre of the moon, divides the moon into two hemispheres, that which is towards the earth, forming the part of the moon visible to the spectator. The boundary of that hemisphere as seen from the earth must be a circle: and if the whole of it were equally enlightened, the appearance of it would be a luminous circle, in fact, that of the full moon.

But the moon is not sensibly self-luminous. The light by which she shines is derived from the sun. That half of her surface which is turned towards the sun is enlightened: the other half is dark. Hence, the visible enlightened part of the moon will be that portion of her surface, which forms a part both of the hemisphere which is turned towards the sun, and of the hemisphere which is turned towards the earth.

At the full moon those two hemispheres are the same: for the sun and the earth are then in the same direction with reference to the moon. Hence the whole of the visible part of the moon is enlightened, and her appearance is that of a bright circle.

At the new moon, the enlightened and the visible hemispheres of the moon are exactly opposite to each other, since the moon is at that time between the sun and the earth. Hence, the part of the moon visible at the earth is entirely dark: and, if the moon is seen at all, it is only by passing across some part of the sun's disk, and thus causing an eclipse of the sun.

In all intermediate positions, between the new and the full moon, a portion only of the visible hemisphere is also enlightened, hence the apparent form of the moon will be a figure bounded on one side by a half circle, and on the other side by half of an ellipse, the figure which a circle appears to assume when seen obliquely.

sun.

This will be clearly comprehended, if we trace the gradual augmentation of the moon's apparent disk. Within a few hours after the new moon, the moon has moved sensibly to the eastward of the Hence, the eastern edge of the illuminated hemisphere of the moon begins to steal upon the western edge of the visible hemisphere, forming a fine crescent of silver light, the line joining the cusps, or pointed ends, being at right angles to a line drawn from the centre of the moon's disk towards the sun.

In this position, the ellipse, which bounds. the inner portion of the moon's crescent differs very little from the semicircle CAC, which bounds the outer portion. As the moon advances in her orbit, the edge of the enlightened hemisphere encroaches more and more upon the western point of the visible hemisphere. The crescent consequently be