222

Herschel. This planet, called also Georgium Sidus, of the Georgian Planet, and Uranus, which had escaped the notice of all ancient astronomers, and even by the moderns was considered only as a fixed star, was, in 1781, by the celebrated astronomer whose name it bears, proved to be a moveable body, in all respects similar to those already described. This planet, although of great size, its diameter exceeding thirty-five thousand miles, performs his course at a distance so great, that it is seldom he can be discovered by the unassisted eye. His distance from the sun is computed to be about eighteen hundred millions of miles, or nearly two hundred times the distance of this earth from the sun. The period in which Herchel performs his diurnal revolution on his axis, has not yet been properly determined; but his annual revolution extends to no less than 30,637 of our days, or nearly 84 of our years.

Herschel is accompanied by six moons, or satellites, to be seen only by the use of the most powerful telescopes: of their magnitudes, distances from that planet, and periodical revolutions, little has yet been ascertained; but in one particular these satellites vary from the practice of all the other heavenly bodies, in performing their revolutions from east

to west.

The bodies described as performing these revolutions round the sun, are termed primary planets, and the others revolving round these are called secondary planets or satel lites, as constantly attending the primary.

It has already been said, that the earth is accompanied

by

by one satellite, which we call the moon,revolving at the distance of 60 semi-diameters of the earth, equal to 240,000 miles; her diameter is about 2,180 miles, somewhat more than the fourth part of that of the earth; the extent of her surface must, therefore, be nearly one-thirteenth part of the surface of the earth. Were the earth immoveable in its place in the heavens, the moon, by her ordinary motion, would perform a complete revolution from any given point of her orbit, to the same again, in about 27 days 7 hours 43 minutes 11 seconds: but, as during this period, the earth has been in progressive motion, in its annual orbit round the sun, it will require 29 days 12 hours 44 minutes 3 seconds, to bring the moon to a point corresponding to that from which her revolution was computed; and this last period is what we call a month.

The plane of the path described by the moon, or of her orbit, does not coincide with the plane of the earth's path, or of the ecliptic, being inclined to it in an angle of 5 degrees 8 minutes 48.9 seconds; and as she performs twelve revolutions round the earth in less time than the earth performs one round the sun, she must to us appear to follow a very irregular course through the heavens. The most remark. able circumstances, however, relating to the moon, are the continually varying appearances (or phases, according to the Greek term usually employed) she presents in the course of a revolution. As the moon, like our carth, and all the other planets, is an opaque, that is, a dark solid body, yielding no light of itself, but only illuminated by the rays of the sun, no part of her body can be seen, but when it is enlightened by those rays. Let us then suppose the moon, in the course of her revolutions round us, to be directly in the line between us and the sun; in this case it is evident no part of the illuminated body can be visible on the earth: next let the moon advance a little in her course, from east to west, the sun enlightening still one-half of her surface, a

small

sinall portion of it will begin to be perceptible to the westward of the Sun as he sets, in the form of an arch, or crescent, as it is called, broadest in the middle, and terminating above and below in sharp points. The illuminated part bending outwards towards the sun, on the moon's western edge. When she has arrived so far on her journey that line from the earth would at the moon form a right angle with another line from the sun, one half of the illuminated side will be visible to us, presenting the appearance of a semi-circle or half moon, and she is then said to have accomplished her first quarter. From this point, still proceeding from west to east, she will discover to us more and more of the illuminated hemisphere, bounded by two arches, and is then said to be gibbous or hump-backed. When' she has performed one half of her revolution, and comes to he directly in opposition to the sun, on a line drawn from him through the earth to her, we are then in the centre of the sun's rays falling on her body, and discover the whole illuminated hemisphere: the moon is then said to be full, or in her second quarter. Continuing her progress, the moon passing to the eastward of the line joining the earth and the sun, we no longer can perceive the whole enlightened part of her surface, but she again assumes a gibbous appearance, as in the portion of her course from the half to the full moon, with this difference only, that the illumin-' ated parts, equally turned towards the sun as before, occupy her castern edge. In this manner she advanoes to a point, where lines from her to the sun and to the earth' would form a right angle at her body, when one half only of the enlightened portion being visible to us, she again presents the appearance of a half moon, and has accomplished the third quarter of her revolution. From this stage, gradually approaching the line joining the sun and the earth, she assumes the appearance of a crescent, as in the beginning of her course, but in a reversed position; and at last com

ing to be directly between us and the sun, the illuminated portion of her surface entirely disappears, and she ceases to be visible to us. This point of conjunction of the moon with the sun is what is called the new moon.

Did the moon revolve in the plane connecting the sun and the earth, as often as she came into conjunction with the sun, she would necessarily come between him and the earth, intercepting a portion of his rays, corresponding to

her

apparent magnitude, or to the angle she subtends at the eye of an observer on the earth: this interception of the light and heat of the sun from extending to the earth, is what is termed an Eclipse of the Sun, from a Greek word signifying want or defect. Again, on the supposition that the earth and moon moved in the same plane passing through the sun, when the moon had performed one half of her monthly revolution, and came to be directly in opposition. to the sun, the earth, situated precisely between these bodies, would intercept the sun's rays from falling on the moon, and thus produce an eclipse of that luminary with respect to her, or what we commonly, but incorrectly, call an eclipse of the moon. But, as was already observed, the plane of the moon's path round the earth, crossing the plane of the earth's path round the Sun, at an angle of 5 degrees 8 minutes 48.9 seconds, eclipses can only happen when the line joining the sun and the earth meets the moon at or very near the two points where her orbit intersects that of the earth. In all other positions of these three bodies, the moon will be out of the line connecting the sun and the earth, so that neither can she interrupt his rays from falling on the earth, nor can the earth interrupt his rays from falling on the moon.

The earth and the moon being both opaque bodies, neither affording nor transmitting any light but what they receive from the sun, when the earth comes in between the

VOL. II.

2 G

sun

sun and the moon, she ought to be entirely deprived of light, and consequently invisible to us: this, however, is not the case,and for this reason, that the rays of the sun, in passing through our atmosphere, are refracted or bent in. wards towards the body of the earth, and following this new direction, fall upon the face of the moon, which thus becomes faintly visible. It is also a common thing in frosty or other very clear weather, when the moon is a few days old, and only a small part of her enlightened face is turned towards us, for the part unenlightened to be also perceptible, of a faint brown complexion: this again is the effect of the sun's light reflected to her from the earth, which performs to the moon in that respect the same office as the moon performs to the earth, but with much greater effect, for the surface of the earth being about thirteeen times as great as that of the moon, the earth-light reflected to the moon must be about thirteen times as great as the moonlight reflected on the earth.

From the moon's presenting to us the same appearance, in whatever part of her orbit she may be, it follows that her rotation on her axis must be performed in precisely the same time as her revolution round the earth; or, that she turns on her axis once in the course of one month. Hence it follows that only one-half of the moon's surface can receive any benefit from the sun's rays, as reflected from the earth, or from what an inhabitant of the moon would call earthlight, and that those who dwell on the opposite side of the moon, when turned away from the sun, can have no other light than such as we possess from the stars in a moonless night. By this peculiarity of the moon's rotation and revolution in the same time it also happens that only the inhabitants of one-half of the moon have ever seen our globe; and to these it must be a journey of no small interest to travel over a part of their own globe, in order to have a view