To determine the distance of the sun from the earth has always been an inte resting problem to astronomers, and they have tried every method which astronomy or geometry possesses, in order to resolve it. The amplest and most natural is that which mathematicians employ to measure distant terrestrial objects. From the two extremities of a base, whose length is known, the angles which the visual rays from the object, whose distance is to be measured, make with the base, are mea sured by means of a quadrant ; their sum subtracted from 180° gives the angle which these raysform at the object where they intersect. This angle is called the parallax, and when it is once known, it is easy, by means of trigonometry, to ascer tain the distance of the object. Let A B, in fig. 4, be the given base, and C the ob ject whose distance we wish to ascertain. The angles C A B and C B A, formed by the rays C A and C B with the base, may be ascertained by observation ; and their sum subtracted from 180° leaves the an gle A C B, which is the parallax of the object C. It gives us the apparent size of the base A B, as seen from C. When this method is applied to the sun, it is ne cessary to have the largest possible base. Let us suppose two observers on the same meridian, observing at the same in stant the meridian altitude of the centre of the sun, and his distance from the same pole. The difference of the two distan ces observed will be the angle under which the line which separates the obser vers will be seen from the centre of the sun. The position of the observers gives this line in parts of the earth's radius. Hence it is easy to determine, by obser vation, the angle at which the semidi ameter of the earth would be seen from the centre of the sun. This angle is the sun's parallax. But it is too small to be determined with precision by that me thod. We can only conclude from it, that the sun's distance from the earth is at least equal to 10,000 diameters of the earth. Other methods have been disco vered for finding the parallax with much greater precision. It amounts very near ly to 8".8 : hence it follows, that the dis tance of the sunfrom the earth amounts to at least 23.405 semidiameters of the earth.
The sun was long considered, from its constant emanation of heat and light, as an immense globe of fire. When viewed through a telescope, several dark spots are visible on its surface, which are of various sizes and durations. From the motion of these spots, the sun ha its found to move round its axis, and ts axis is found to be inclined to the ecliptic. Various opinions have been formed re specting these spots ; they have been con sidered as opaque islands in the liquid igneous matter, and by some as pits or cavities in the body of the sun. In 1788, Mr. King published a Dissertation on the Sun, in which he advanced, that the real body of the sun is less than its apparent diameter ; that we never discern the real body of the sun itself, except when we behold its spots ; that the sun is inhabit ed as well as our earth, and is not ne cessarily subject to burning heat ; and that there is in reality no violent element ary heat existing in the rays of the sun themselves essentially, but that they pro duce heat only when they come into con tact with the planetary bodies. Several years after this, Mr. Herschel published his theory of the nature of the sun, which is briefly as follows: he considers the sun as a most magnificent habitable globe, surrounded by a double set of clouds. Those which are nearest its opaque body are less bright, and more closely connect ed together, than those of the upper stra tum, which form the luminous apparent globe we behold. This luminous exter nal matter is of a phosphoric nature, hav ing several accidental openings in it, through which we see the sun's body, or the more opaque clouds beneath. These openings form the spots that we see. Mercury. This planet being the nearest to the sun, and the least in magnitude, is very seldom visible. It never appears more than a few degrees from the sun's disc, and is generally lost in the splendour of the solar beams. On this account astro nomers have had few opportunities of making accurate observations upon it ; no spots have been observed upon it, con sequently the time of its rotation on its axis is not known. Being an inferior pla net, it consequently must show phases like the moon ; and it never appears quite full to us. It is seen sometimes passing over the sun's disc, which is called its transit.
Venus is the brightest and largest, to appearance, of all the planets, and is dis tinguished from the rest by her superio rity of lustre. It is generally called the morning or evening star, according as it precedes or follows the apparent course of the sun. Some have thought that they could discover spots upon its disc, but Herschel has not been able to see them ; consequently, the time of rotation round its axis is not decidedly known. Venus al
so appears with phases, and transits some times take place, which are of very great importance in astronomy.
The Earth, which we inhabit, is, as has been proved, a globular body ; it is not, however, a perfect sphere, but a spheroid, having its equatorial diameter longer than the polar diameter, or axis. It is conse quently flattest at the poles, and more protuberant at the equator. The diame ter at the equator is 7893 English miles ; that at the pole is 7928 miles. The sur face of the earth is much diversified with mountains and wallies, land and water. The highest mountains in it are the An des, in South America, some of which are about four miles in perpendicular altitude. About two-thirds of the globe is covered with water. In consequence of the earth's being a globe, people standing upon op posite sides of it must have their feet to wards each other. When in this situation, they are called antipodes to each other. Hence it appears that there is no real up or down ; for what is up to one country is down to another. It must seem strange to those who are ignorant of the shape of the earth, to suppose, that if we could bore a hole downwards, deep enough, we should come to the other side of the world, where we should find a surface 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 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 was 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 23P, 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, a, 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 have constant clay 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.