ASTRONOMY. 



and sky like our own ; yet, if we reflect 

 a moment, we shall perceive that this 

 is perfectly true. As we are preserved 

 in our situations by the power of attrac- 

 tion, which draws us towards the centre 

 of the earth, we call that direction down 

 which tends to the centre, and the con- 

 trary. We mentioned before, that the 

 earth has two motions, the one a diurnal 

 motion round its own axis, the other an 

 annual motion round the sun. It is the 

 former which causes light and darkness, 

 day and night ; for when one side of the 

 earth is turned towards the sun, it re- 

 ceives his rays, and is illuminated, caus- 

 ing 1 day ; on the contrary, when one side 

 of the earth is turned from the sun, we 

 are in darkness, and then we have night. 

 We see, therefore, by how much more 

 simple means this change is effected, 

 than they imagined, who supposed that 

 the earth was fixed, and that the immense 

 globe of the sun \vas whirled round the 

 earth with the amazing velocity that 

 would be necessary. Twilight is owing 

 to the refraction of the rays of light by 

 our atmosphere, through which they pass, 

 and which, by bending them, occasion 

 some to arrive at a part of the earth that 

 could not receive any direct rays from 

 the sun. It is the annual motion of the 

 earth round the sun which occasions the 

 diversity of seasons. To understand this, 

 we must observe, what has been already 

 mentioned, that the axis of the earth is 

 inclined to the plane of its orbit 23, 

 and it keeps always parallel to itself; 

 that is, it is always directed to the same 

 star. Let fig. 5, Plate II, represent the 

 earth in different parts of its elliptic or- 

 bit. In the spring, the circle which se- 

 parates the light from the dark side of 

 the globe, called the terminator, passes 

 through the poles n, s, as appears in the 

 position A. The earth, then, in its diur- 

 nal rotation about its axis, has every part 

 of its surface as long in light as in shade; 

 therefore the days are equal to the nights 

 all over the world, the sun being at that 

 time vertical to the equatorial parts of 

 the earth. As the earth proceeds in its 

 orbit, and comes into the position B, the 

 sun becomes vertical to those parts of the 

 earth under the tropic, and the inhabi- 

 tants of the northern hemisphere will en- 

 joy summer, on account of the solar rays 

 falling more perpendicularly upon them ; 

 they will also have their days longer than 

 their nights, in proportion as they are 

 more distant from the equator ; and those 

 within the polar circle, as will be perceiv- 

 ed by the figure, will bave constant day- 



light. At the same time the inhabitants 

 of the southern hemisphere have winter, 

 their days being shorter than their nights, 

 in proportion as they are farther from 

 the equator ; and the inhabitants of the 

 polar regions will have constant night. 

 The earth then continues its course to the 

 position C, when the terminator again 

 passes through the poles, and the days 

 and nights are equal. After this the 

 earth advances to the position D, at which 

 time the inhabitants of the northern he- 

 misphere have winter, and their days are 

 shorter than their nights. The positions 

 B and D are the solstitial points, and A 

 and C the equinoctial points ; they are 

 not equidistant from each other, because 

 the sun is not in the centre, but in the fo- 

 cus of the ellipsis. In summer, when the 

 earth is at B, the sun is farther from it 

 than in the winter, when the earth is at 

 D; and in fact, the diameter of the sun 

 appears longer in winter than in summer. 

 The difference of heat is not owing to the 

 sun's being nearer to us, or more remote, 

 but to the degree of obliquity with which 

 its rays strike any part of the earth. 



The Moon is, next to the sun, the most 

 remarkable of the celestial objects. Its 

 form is spherical, like that of the earth 

 round which it revolves, and by which it 

 is carried round the sun. Its orbit is also 

 elliptical, having the earth in one of the 

 foci of the ellipsis. The moon always 

 keeps the same side towards the earth, 

 showing only at one time a little more of 

 one side, and at another time a little more 

 of the other side. When the moon is 

 viewed through a good telescope, its sur- 

 face appears covered with ridges, moun- 

 tains, pits, and cavities of great variety. 

 Some parts of its surface also reflect less 

 light than the rest. It has been conjec- 

 tured that the part which reflects the 

 least light is water, and the brightest part 

 land. The heights of the lunar moun- 

 tains were formerly supposed to be much 

 greater than those of our earth ; but Dr. 

 Herschel has demonstrated that very few 

 are more than half a mile high, and the 

 highest little more than a mile. Several 

 volcanos, or burning mountains, have 

 been discovered in it. It has been doubt- 

 ed whether the moon has an atmosphere 

 like ours, but the latest observations ap- 

 pear to prove that it has. The moon is 

 seen by means of the light which comes 

 to it from the sun being reflected from it. 

 Its changes or phases depend upon its 

 situation relatively to the earth and the 

 sun. When the moon is in opposition to 

 the sun } the enlightened side is turned 



