THE

The Sun is the great source of light, heat, and animation to all those bodies; and to the influence of which, combined with their sidereal and diurnal revolutions, they owe the successive alternations of summer and winter, day and night.

2. The Sun is the largest body yet known in the universe; its mean diameter being 887,000 American miles, or about 112 times the mean diameter of the Earth; and its size 1,406,550 times that of the Earth; but its mass or quantity of matter is only 32,960 times greater, and its density about that of the Earth.

A body which weighs one pound at the surface of the Earth, would, if removed to the surface of the Sun, weigh 27 pounds, 14 ounces and 15 drachms, and bodies would fall there with a velocity of 334 feet 8 inches in the first second of time.

3. The apparent diameter of the Sun, as seen from the Earth, undergoes a periodical variation. It is greatest when the Earth is in its perihelion, which is about December the 31st, at which time it is 32′ 35.6"; and it is least, when the Earth is in its aphelion, which is about July the 1st, at which time it is 31' 31". Its mean apparent diameter is therefore 32' 3.3"

The greatest equation of the Sun's centre is 1° 55′ 27.7", which diminishes at the rate of 16.9′′ in a century. The Sun's horizontal parallax, as determined by the transit of Venus, is 83". See the chapter on Parallax, &c. in a subsequent part of this work.

4. The Sun is surrounded with an atmosphere of great extent; its height, according to Dr. Herschel, is not less than 1843, nor greater than 2765 miles.

This atmosphere, Dr. Herschel thinks, consists of elastic fluids that are more or less lucid and transparent, and of which the lucid ones furnish all the bodies in the solar system with light; and he supposes that the density of the luminous solar clouds need not be greater than that of our

Aurora Borealis, to produce the effect with which we are acquainted.

5. The Sun is frequently obscured by spots, some of which have been observed so large as to exceed the Earth five or six times in diameter. Sometimes, though rarely, the Sun has appeared pure and without spots, for several years together.

The number, position, and magnitude of the solar spots, are very variable; they are often very numerous, and of considerable extent. Some imagine they may become so numerous as to hide the whole face of the Sun, or at least the greater part of it; and to this they ascribe what Plutarch mentions, viz. that in the first year of the reign of Augustus, the Sun's light was so faint and obscure, that one might look steadily at it with the naked eye. To which Kepler adds, that in 1547, the Sun appeared reddish, as when viewed through a thick mist; and hence he conjectures that the spots in the sun are a kind of dark smoke, or clouds floating on his surface.

The solar spots, in general, consist of a dark space, or umbra, of an irregular form; they are almost always surrounded by a penumbra, which is enclosed in a cloud of light, more brilliant than the rest of the Sun, and in the midst of which the spots are seen to form and disappear. All this, according to La Place, indicates that at the surface of this enormous fire, vivid effervescences take place, of which our volcanoes form but a feeble representation. But whatever be the nature of the solar spots, they have made us acquainted with a remarkable phenomenon, that is, the rotation of the Sun.

Amidst all their variations we can discover regular motions, which are exactly the same as the corresponding points of the surface of the Sun, if we suppose it to have a motion of rotation on an axis, almost perpendicular to the ecliptic, in the direction of its apparent annual motion round the earth.

6. The continued observation of these spots shows that the Sun revolves on its axis in 25 days, 10 hours; that its figure is not truly spherical, but an oblate spheroid like the earth; and that the

solar equator is inclined 7° 30′ to the plane of the ecliptic.

For, some of these spots have made their first appearance near the eastern edge, from thence they have seemed gradually to pass over the Sun's disc to the opposite edge, then disappear; and hence, after an absence of about 14 days, they have re-appeared in their first place, and have taken the same course over again; finishing their entire circuit in 27 days, 12 hours and 20 minutes, which is hence inferred to be the period of the Sun's rotation round his axis; and therefore the periodical time to a fixed star, usually called the sidereal revolution, is 25d. 15h. 16m.; because, in 27d. 12h. 20m. of the month of May, when the observation was made, the Earth describes an angle about the Sun's centre of 260 22', and therefore as the angular motion 360°+260 22 or 3860 22: 360°:: 27d. 12h. 20m.: 25d. 15h. 16m. As the solar spots appear to move on the Sun's disc, from the eastern to the western edge, whence we may conclude the motion of the Sun, to which the other is owing, to be from west to east, or in the same direction, with respect to the order of the signs, as the diurnal rotation of the Earth. The more correct period of the Sun's rotation is now stated at 25d. 10h. as in the above article.

7. The Sun, together with the planets, moves round the common centre of gravity of the solar system, which is nearly in the centre of the Sun.

This small motion of the Sun round the centre of gravity is occasioned by the various attractions of the surrounding planets.

8. Besides the two real motions of the Sun already mentioned, the Sun has also two apparent motions; that is, the diurnal motion from east to west, and his annual motion in the ecliptic; but these apparent motions arise from the real motions of the earth on its axis, and in its orbit.

Whether the Sun and stars have any proper motion of their own in the immensity of space, however small, is not absolutely certain, though some very accurate observers have intimated conjectures of this kind, and have shown that such a general motion is not improbable. Dr. Herschel

conceives the Sun and the planets to have a general mỏtion, which carries the solar system towards the constellation Hercules.

As to the nature of the Sun.

For many ages the Sun was believed to be a globe of fire, and those who have maintained this hypothesis, argue in the collected rays, following manner. The Sun shines, and his by concave mirrors, or convex lenses, will burn, consume, and melt the most solid bodies, or else convert them into ashes or glass; therefore, as the force of the solar rays is diminished by their divergency, in the duplicate ratio of their distances reciprocally taken, it is evident that their force and effect are the same, when collected by a burning lens or mirror, as if we were at such distance from the sun, therefore, where they were equally dense. The Sun's rays, in the neighbourhood of the Sun, produce the same effects as might be expected from the most vehement fire; consequently, the Sun is of a fiery substance.

Hence it follows, that its surface is probably every where fluid; that being the condition of flame. Indeed, whether the whole body of the Sun be fluid, as some think, or solid as others, they do not presume to determine; but as there are no other marks, by which to distinguish fire from other bodies, but light, heat, a power of burning, consuming, melting, calcining, and vitrifying, they do not see what objection should be made to the hypothesis, that the Sun is a globe of fire like our fires, invested with a flame; and, supposing that the maculæ, or spots, are formed out of the solar exhalations, they infer that the Sun is not pure fire, but that there are heterogeneous parts mixed with it.

But, the majority of modern astronomers have rejected this opinion, and several of them have published very ingenious hypotheses on this curious subject. One of the most plausible and ingenious theories on this subject is given by Dr. Herschel, in the philosophical transactions of the Royal Society. He supposes the Sun has an atmosphere resembling that of the Earth, and that this atmosphere consists of various elastic fluids, some of which exhibit a shining brilliancy, while others are merely transparent. Whenever the luminous fluid is removed, the body of the Sun may be seen through those that are transparent. In like manner, an observer placed in the Moon will see the solid body of the Earth only in those places where the transparent fluids of our at

mosphere will permit him. In others, the opaque vapours will reflect the Sun's light, without permitting his solid body to be seen on the surface of our globe. In the same manner he illustrates the various appearances of spots in the Sun. Such appearances, he thinks, may be easily and satisfactorily explained, if it be allowed that the real solid body of the Sun itself is seen on these occasions, though we seldom see more than its shining atmosphere. Dr. Herschel apprehends that there are considerable inequalities in the surface of the Sun, and that there may be elevations not less than 500 or 600 miles in height; that a very high country, or chain of mountains, may oftener become visible by the removal of the obstructing fluid than the lower regions, on account of its not being so deeply covered by it. In the year 1799, he observed a spot on the Sun large enough to be discerned by the naked eye, for it extended more than fifty thousand miles. He also says, that he observed a large spot in 1783, which he followed up to the edge of the Sun's limb; that he plainly perceived it to be depressed below the surface of the Sun, and that it had very broad shelving sides. This appearance may be explained by a gentle and gradual removal of the shining fluid, which permits us to see the globe of the Sun. Dr. Herschel also says, that on the 26th of August, 1792, he examined the Sun with several powers, from 90 to 500, and that it evidently appeared that the black spots were the opaque ground, or body of the sun, and that the luminous part was an atmosphere, which being interrupted or broken, gave a glimpse of the Sun himself. He further adds, that with his seven feet reflector, which was in an excellent state of perfection, he could see the spots, as on former occasions, with the same telescope, much depressed below the surface of the luminous part.

On the 8th of September, 1792, he made a speculum, which he brought to a perfect figure on hone, without polish; this had the effect of stifling a great part of the Sun's rays; and on this account the object speculum admitted a great aperture, which enabled him to see with more comfort and less danger. He then discovered that the surface of the Sun was unequal, many parts of it being elevated, and others being depressed; but this inequality was in the shining surface only, for he thinks that the real body of the Sun is seldom seen otherwise than in its black spots.

As light is a transparent fluid, it may not be impossible that the Sun's real surface may be now and then perceived.