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SUN. This magnificent luminary, the great source of light, heat, and life, appears to us a circular and resplendent disk; from which appearance, and the observation of the solar spots (described below), it follows that this body has a form nearly spherical, and turns round its axis once in about twentyfive and a half days, because a sphere only can appear to the eye like a circular disk in all positions. The true relation of the sun, not only to our earth, but to all the planets of our system, has been known since the discoveries of Kepler. The primary planets, accompanied by their moons, revolve about the sun in elliptical orbits, which have but little eccentricity the sun itself being situated in a focus common to all these ellipses. His mean distance from the earth, which has been finally determined, with tolerable accuracy, by the observation of his parallax (see the subsequent part of this article), amounts, in round numbers, to about 95,000,000 miles: the sun, therefore, is above 400 times farther distant from us than the moon; and a cannon.ball which moves 600 feet in each second, would require about 26 years to reach it. The apparent diameter of the sun is pretty nearly the same as that of the moon: it is somewhat above half a degree ; yet, according to the various points of the earth's orbit, from which we observe the same, varies somewhata, necessary consequence of the elliptical form of this orbit. The conclusions which we draw from the differences in the apparent magnitude of the sun as to the different distances of this body from the earth, agree perfectly with what we learn, respecting the same subject, from other sources ; so that this point may be considered as well settled. The mass of the sun, which exceeds that of all the planets together 800 times, is, in proportion to that of our earth, according to Piazzi,as 329,630 to 1; the diameter exceeds that of the earth 112 times, the surface 12,700 times, the solid contents 1,435,000 times. The earth appears, as Biot says, by this statement, a mere grain of sand, compared to the sun, which, again, in his turn, is but a point in infinite space. Respecting the physical structure of the sun, astronomers have entertained different opinions, from times immemorial. The hypothesis of Herschel is, that the sun is an opaque body, having on its surface mountains and valleys, like the earth, the whole surrounded by an atmosphere constantly filled with luminous clouds. These sometimes open in particular places, and allow the body of the sun to be seen ; hence the appearance of solar spots. This hypothesis seems to be preferable to that of Laplace (who imagines the sun to be a burning body), because it allows us to conceive that the sun is inhabited, which better agrees with the wise use made of space by a beneficent Omnipotence. Parallax of the Sun.Parallax and horizontal parallax have been explained in the article Parallax, The horizontal parallax of the sun has been known with greater accuracy since the transit of Venus over the sun's disk in 1761 and 1769. As the orbit of the earth includes that of Venus, the latter must sometimes appear between us and the sun. The durationof such a transit for the centre of the earth may be calculated; and on comparing this with the duration actually observed on the surface of the earth, the difference of the two results enables us to deduce the horizontal parallax of the sun, and hence the distance of the two luminaries from each other. In this way the mean horizontal parallax of the sun has been estimated by Durejour at 8' 8", and by Biot at 8' 7", which makes the mean distance of the sun from the earth amount to 23,439 times the o radius of the earth (which is about 4000 miles in length), or, in round numbers, 94,000,000 miles. If this horizontal parallax is taken but one tenth of a second smaller, we must add to this distance an amount equal to 215 times the radius of the earth, which explains the difference in the statements of the distance. This distance having been ascertained with tolerable accuracy, we possess the measure of our whole planetary system, as, according to the second law of Kepler (q. v.), the cubes of the mean distances of the planets from the sun are as the squares of the periods of their revolutions (which have long been known). Therefore the determining of this distance is of the highest importance. Respecting the transit of Venus, see Lalande's Astronomie, Enke's Distance of the Sun from the Earth, by the Transit of Venus in 1761, and the Transit of Venus in 1769 (in German). (See Transit.) Spots on the Sun.Spots of irregular form are often observed in the disk of the sun (q. v.), in greater or less number. They appear in the centre dark, and towards the margin have a whitishgray umbra, which, however, is often observed spreading over large surfaces, without, that black centre* They originate and disappear, sometimes quickly, and without apparent cause, in the middle of the disk; but more frequently are observed to rise on the eastern margin, and move towards the western, where, thirteen days after being first seen, they disappear, and again appear on the eastern margin after a little longer period. The spots appear to revolve round the sun in about twentyseven days. At particular seasons, they move over the sun in straight lines; at all other times, in lines more or less curved ; and the paths described by different spots observed at the same time are always parallel to each other, and always have their curvature and position determined by the season. They appear broadest when near the middle point of their passage. All this is satisfactorily explained, if the spots are considered to adhere to the sun, and the latter is considered to turn according to the order of the signs round its axis, which is inclined at an angle of 82^° to the ecliptic of the earth. The real duration of this rotation, as deduced from the apparent rotation of twentyseven days, is equal to twentyfive days. This difference is occasioned by the fact that the earth, from which this rotation is observed, is itself moving in the mean time. Herschel's opinion on the nature of these spots we have mentioned in the previous part of this article.