comes thicker. The elliptical boundary, Cac, becomes less curved; the outer boundary, CAC, constantly remaining a semicircle, and the line Cc, joining the cusps being always at right angles to a line drawn in the direction of the sun from the centre of the moon's disk.

When the moon is a little more than seven days old, it reaches such a position, that a line drawn from the earth to the moon is at right angles to a line drawn from the sun to the moon. Hence the circle upon the moon's disk which divides the light and dark hemispheres has its edge directed to the earth, and appears as a straight line joining the cusps, Cc: and the moon's disk appears half illuminated.

The moon still proceeding eastward in her orbit exposes towards the earth more and more of her illuminated hemisphere. The elliptical boundary between the light and dark portion of her disk is now turned towards the east of the line joining the cusps, and the appearance is that of a gibbous moon. The inner boundary of the light part of the disk approaches more and more nearly to the half circle CBc, until, at the instant of full moon, the coincidence is complete, and the whole is illuminated.

After this, the edge of the dark hemisphere of the moon comes upon the western part of her disk; and the phases just described recur, in an inverted order. Thus, then, the phases of the moon depend upon the relative position of the sun and moon with respect to the earth. The manner in which they are produced can easily be illustrated by holding at some little distance from the eye a ball, half of which is black and the other half white, and turning it slowly round. At any time when the sun and moon are above the horizon, a still more con

vincing method is to place in the light of the sun a ball, in such a manner that it is in the same direction, with respect to the eye, that the moon is, The illuminated part of the ball will be found to assume precisely the same appearance as the disk of the moon.

As the moon reflects her light to the earth, so the earth in turn reflects light to the moon; and, if seen from the moon, would present phases precisely similar to those which the moon presents as seen from the earth, with this difference, that the dark part of the moon's disk at any time corresponds to the light part of the disk which the earth presents to the moon. For instance, at the time when the moon appears a thin crescent of light, with its convexity turned towards the west, the earth, as seen from the moon, would appear a luminous circle slightly defective upon its eastern limb.

The diameter of the earth as seen from the moon is about three times and a half as great as the diameter of the moon seen from the earth; and consequently its visible surface is about thirteen times as great as that of the moon. Hence, at the period near the new moon, the part of the moon which is not enlightened by the sun is still receiving from the earth nearly as much light as we should receive from thirteen full moons. This light is again reflected to the earth rendering the obscure parts of the moon visible, and sometimes even shewing some of her prominent spots.

It is very remarkable, that the moon always turns the same face to the earth.

Even to the unassisted eye, it is evident that the spots on the moon are always seen in the same place. But this is rendered still more sensible, when the moon is viewed by means of a telescope. The surface is then found to be covered with numerous irregularities, which enable us to identify particular parts; and it is found, that, at whatever period of a lunation those spots are observed, they occupy very nearly the same part of her visible disk. For instance, a well recognized spot near the eastern edge, is always seen in that position, provided that portion of the moon is enlightened; another, near the centre, retains it relative place; and the same is true of all points which can be recognized. It follows from this, that the moon turns upon her own axis in the same time in which she completes a revolution round the earth, and that her axis is nearly perpendicular to the plane of her orbit. This will at once be understood, by considering, that, if the moon had no rotation about her own axis, all the parts of her surface would be presented to the earth in succession, in consequence

of her revolving round the earth; just as a globe, carried round a table presents all its sides successively towards the middle of the table. And if the moon revolved about her axis, in any time different from that in which she revolves about the earth, the spots upon her disk would change their position, sometimes appearing on one side of the disk, passing across it, and disappearing at the other side, in the same manner as is observed in the spots of the sun.

The peculiar cause of this remarkable adaptation is not well understood; but there is reason to think, that the same adjustment prevails in the other secondary planets on the solar system. Observations on periodical changes of brightness, in the satellites of Jupiter and Saturn, have led to the conclusion, that those bodies also revolve about their axes in the same time in which they complete a revolution about their respective primary planets.

One consequence of the correspondence of these two motions is, that from one half of the moon's surface the enlightened portion of the earth is always visible; so that that part of the moon, is continually enlightened, either by the sun or by the light reflected from the earth while the other hemisphere has periods of alternate light and darkness, recurring in about fourteen days.

An examination of the moon by means of a telescope makes us acquainted with the general nature of her surface. The most favour. able time for observing particular portions, is within three or four days of the time of new moon, since the sun's light then falls obliquely upon the parts which are upon the border of light and darkness, and thus renders the irregularities more apparent. The whole visible surface of the moon can be observed near the time of the full moon; but the light of the sun then fills all the inequalities and caverns alike, and renders the difference in their appearance less conspicuous.

It is not easy to convey an accurate notion of the appearance of the moon, without the aid of drawings. The surface is extremely irregular. In some parts, it is broken up into huge craters, which have every mark of a volcanic origin. In the middle of many of the excavated parts, rises a heap, somewhat in the same manner as the bottom of a glass bottle projects within it. The ridges connecting the elevated parts are extremely abrupt, so that it frequently happens that a brilliant fine line of light projects into the dark part of the moon, occasioned by the sun's light striking upon such a ridge, while the valleys are yet in shadow. And sometimes, from a similar cause,

a small luminous bead, like a star, appears to run out from the fine needle-like point of one of the cusps.

In those parts of the moon which are not so rugged, the surface is mottled with smaller inequalities, which appear to have an igneous origin. In fact, the large volcanic district in Auvergne, of it could be viewed from a great distance, would present completely a similar aspect. It is believed, that active volcanos have been recognized on the moon's surface. There are other portions, which present plains of vast extent, bounded by rugged edges. But there is no indication either of water, or of an atmosphere.

It follows, that the moon would be unfit for a residence for creatures endued with bodily faculties similar to our own.

The distance of the moon from the earth is about sixty radii of the earth, or about 240, 000 miles.

This is ascertained, by observing the apparent position of the moon, with reference to the stars, as seen from different known points of the earth's surface. For instance, suppose, what is very nearly true, that Stockholm and the Cape of Good Hope, indicated by S and C in the figure, are upon the same meridian, about 20° east of London, and that the moon were observed at the same instant on the meridian at each place. Then, since two

M

S

be seen from the centre of the earth. This difference of position is called parallax. For the same reason, the moon, as seen from the Cape of Good Hope, would appear farther to the north, than if it could be seen from the centre of the earth; and still farther to the north than when observed at Stockholm.

Meanwhile, the distance between any two points on the earth's sur face being quite inconsiderable with respect to that of the fixed stars, the position of a fixed star will be sensibly the same as seen from each station. Hence, if the distance of the moon northward or southward from the same star, be observed at the same time, at two such places on the

earth's surface, the difference will shew the change in the moon's position, arising from the change of the spectator's place, and will give the means of computing the distance of the moon from each station, and from the centre of the earth.

There is one phenomenon relating to the moon, which should here be mentioned; that of the harvest moon. It is observed, that at the full moon which occurs nearest to the autumnal equinox, the interval between the times of rising of the moon on successive nights, is less than at any other full moon. Thus, the average time of the moon's rising is about three quarters of an hour later, on each successive night. But at that period of the year, near the full moon, the interval will be little more than a quarter of an hour.

The reason is this. At the autumnal equinox, the sun is in the first point of Libra. Hence the moon, when full, and consequently opposite to the sun, will be in that part of her orbit which is nearest to the first point of Aries. Now it can easily be proved, and may be shewn by reference to a common globe, that the inclination of the ecliptic to the horizon is, in north latitudes, the least, when the first point of Aries is rising. In fact, the ecliptic at such a time will be perceived to lie along the horizon at a very small inclination. For instance, in north latitude 52°, the inclination of the equator to the horizon is 38°: and, since the inclination of the ecliptic to the horizon is sometimes greater and sometimes less than this, and as frequently greater as less, this may be taken as the mean inclination of the ecliptic to the horizon. But the extreme inclinations of the ecliptic to the horizon will be 23°. 28'., greater and less than this: the greatest being 61°.28', when the first point of Libra is rising, and the least 14°.32', when the first point of Aries is rising.

It follows, then, that, if the moon moved in the ecliptic, and described the same portion of that great circle every day, her course, with reference to the part of the horizon at which she rises, would coincide much more nearly with the horizon itself at that time than at any other; and consequently, that the daily change in the position of the moon in the ecliptic would cause the moon to rise farther from the south on every successive night, but would cause but little difference in the time of her rising.

We have already seen, that the moon's orbit is not the ecliptic, but nearly a circle inclined about 5° 18' to the ecliptic. But since the position of the nodes of this orbit on the ecliptic are continually vary