SUN
, Sol, Θ, in Astronomy, the great luminary which enlightens the world, and by his presence constitutes day.
The Sun, which was reckoned among the planets in the infancy of astronomy, should rather be counted among the fixed stars. He only appears brighter and larger than they do, because we keep constantly near the Sun; whereas we are immensely farther from the stars. But a spectator, placed as near to any star as we are to the Sun, would probably see that star a body as large and as bright as the Sun appears to us; and, on the other hand, a spectator as far distant from the Sun as we are from the stars, would see the Sun as small as we see a star, divested of all his circumvolving planets; and he would reckon it one of the stars in numbering them.
According to the Pythagorean and Copernican hypothesis, which is now generally received, and has been demonstrated to be the true system, the Sun is the common centre of all the planetary and cometary system; around which all the planets and comets, and our earth among the rest, revolve, in different periods, according to their different distances from the Sun.
But the Sun, though thus eased of that prodigious motion by which the Ancients imagined he revolved daily round our earth, yet is he not a perfectly quiescent body. For, from the phenomena of his maculæ or spots, it evidently appears, that he has a rotation round his axis, like that of the earth by which our natural day is measured, but only slower. For, some of these spots have made their first appearance near the edge or margin of the Sun, from thence they have seemed gradually to pass over the Sun's face to the opposite edge, then disappear; and hence, after an absence of about 14 days, they have reappeared in their first place, and have taken the same course over again; finishing their entire circuit in 27 days 12h 20m; which is hence inferred to be the period of the Sun's rotation round his axis: and therefore the periodical time of the Sun's revolution to a fixed star is 25d 15h 16m; because in 27d 12h 20m of the month of May, when the observations were made, the earth describes an angle about the Sun's centre of 26° 22′, and therefore as the angular motion. 360° + 26°22′ : 360° :: 27d 12h 20m : 25d 15h 16m. This motion of the spots is from west to east: whence we conclude the motion of the Sun, to which the other is owing, to be from east to west.
Beside this motion round his axis, the Sun, on account of the various attractions of the surrounding planets, is agitated by a small motion round the centre of gravity of the system.—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 made such a general motion not improbable. See Stars.
As for the apparent annual motion of the Sun round the earth; it is easily shewn, by astronomers, that the real annual motion of the earth, about the Sun, will cause such an appearance. A spectator in the Sun would see the earth move from west to east, for the same reason as we see the Sun move from east to west: and all the phenomena resulting from this annual motion in whichsoever of the bodies it be, will appear the same | from either. And hence arises that apparent motion of the Sun, by which he is seen to advance insensibly towards the eastern stars; in so much that, if any star, near the ecliptic, rise at any time with the Sun; after a few days the Sun will be got more to the east of the star, and the star will rise and set before him.
Those who have maintained that the substance of the Sun is fire, argue in the following manner: The Sun shines, and his rays, collected hy concave mirrors, or convex lenses, do 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 a duplicate ratio of the 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, where they were equally dense. The Sun's rays therefore, 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 should hinder but that the Sun may be a globe of fire, like our fires, invested with flame: and, supposing that the maculæ are formed out of the solar exhalations, they infer that the Sun is not pure fire; but that there are heterogeneous parts mixed along with it.
Philosophers have been much divided in opinion with respect to the nature of fire, light, and heat, and the causes that produce them: and they have given very different accounts of the agency of the Sun, with which, whether we consider them as substances or qualities, they are intimately connected, and on which they seem primarily to depend. Some, among whom we may reckon Sir Isaac Newton, consider the rays of light as composed of small particles, which are emitted from shining bodies, and move with uniform velocities in uniform mediums, but with variable velocities in mediums of variable densities. These particles, say they, act upon the minute constituent parts of bodies, not by impact, but at some indefinitely small distance; they attract and are attracted; and in being reflected or refracted, they excite a vibratory motion in the component particles. This motion increases the distance between the particles, and thus occasions an augmentation of bulk, or an expansion in every dimension, which is the most certain characteristic of fire. This expansion, which is the beginning of a disunion of the parts, being increased by the increasing magnitude of the vibrations proceeding from the continued agency of light, it may easily be apprehended, that the particles will at length vibrate beyond their sphere of mutual attraction, and thus the texture of the body will be altered or destroyed; from solid it may become fluid, as in melted gold; or from being fluid, it may be dispersed in vapour, as in boiling water.
Others, as Boerhaave, represent fire as a substance sui generis, unalterable in its nature, and incapable of being produced or destroyed; naturally existing in equal quantities in all places, imperceptible to our senses, and only discoverable by its effects, when, by various causes, it is collected for a time into a less space than that which it would otherwise occupy. The matter of this fire is not in any wise supposed to be derived from the Sun: the solar rays, whether direct or reflected, are of use only as they impel the particles of fire in parallel directions: that parallelism being destroyed, by intercepting the solar rays, the fire instantly assumes its natural state of uniform diffusion. According to this explication, which attributes heat to the matter of fire, when driven in parallel directions, a much greater degree must be given it when the quantity, so collected, is amassed into a focus; and yet the focus of the largest speculum does not heat the air or medium in which it is is found, but only bodies of densities different from that medium.
M. de Luc (Lettres Physiques) is of opinion, that the solar rays are the principal cause of heat; but that they heat such bodies only as do not allow them a free passage. In this remark he agrees with Newton; but then he differs totally from him, as well as from Boerhaave, concerning the nature of the rays of the Sun. He does not admit the emanation of any luminous corpuscles from the Sun, or other self-shining substances, but supposes all space to be filled with an ether of great elasticity and small density, and that light consists in the vibrations of this ether, as sound consists in the vibrations of the air. “Upon Newton's supposition, says an excellent writer, the cause by which the particles of light, and the corpuscles constituting other bodies are mutually attracted and repelled, is uncertain. The reason of the uniform diffusion of fire, of its vibration, and repercussion, as stated in Boerhaave's opinion, is equally inexplicable. And in the last mentioned hypothesis, we may add to the other difficulties attending the supposition of an universal ether, the want of a first mover to make the Sun vibrate. Of these several opinions concerning elementary fire, it may be said, as Cicero remarked upon the opinions of philosophers concerning the nature of the soul: Harum sententiarum quæ vera sit, Deus aliquis viderit; quæ verisimillima, magna questio est.” Watson's Chem. Ess. vol. 1, pa. 164.
As to the Figure of the Sun; this, like the planets, is not perfectly globular, but spheroidical, being higher about the equator than at the poles. The reason of which is this: the Sun has a motion about his own axis; and therefore the solar matter will have an endeavour to recede from the axis, and that with the greater force as their distances from it, or the circles they move in, are greater: but the equator is the greatest circle; and the rest, towards the poles, continually decrease; therefore the solar matter, though at first in a spherical form, will endeavour to recede from the centre of the equator farther than from the centres of the parallels. Consequently, since the gravity, by which it is retained in its place, is supposed to be uniform throughout the whole Sun, it will really recede from the centre more at | the equator, than at any of the parallels; and hence the Sun's diameter will be greater through the equator, than through the poles; that is, the Sun's figure is not perfectly spherical, but spheroidical.
Several particulars of the Sun, related by Newton, in his Principia, are as follow:
1. That the density of the Sun's heat, which is proportional to his light, is 7 times as great at Mercury as with us; and therefore our water there would be all carried off, and boil away: for he found by experiments of the thermometer, that a heat but 7 times greater than that of the Sun beams in summer, will serve to make water boil.
2. That the quantity of matter in the Sun is to that in Jupiter, nearly as 1100 to 1; and that the distance of that planet from the Sun, is in the same ratio to the Sun's semidiameter.
3. That the matter in the Sun is to that in Saturn, as 2360 to 1; and the distance of Saturn from the Sun is in a ratio but little less than that of the Sun's semidiameter. And hence, that the common centre of gravity of the Sun and Jupiter is nearly in the superficies of the Sun; of the Sun and Saturn, a little within it.
4. And by the same mode of calculation it will be found, that the common centre of gravity of all the planets, cannot be more than the length of the solar diameter distant from the centre of the Sun. This common centre of gravity he proves is at rest; and therefore though the Sun, by reason of the various positions of the planets, may be moved every way, yet it cannot recede far from the common centre of gravity, and this, he thinks, ought to be accounted the centre of our world. Book 3, prop. 12.
5. By means of the solar spots it hath been discovered, that the Sun revolves round his own axis, without moving considerably out of his place, in about 25 days, and that the axis of this motion is inclined to the ecliptic in an angle of 87° 30′ nearly. The Sun's apparent diameter being sensibly longer in December than in June, the Sun must be proportionably nearer to the earth in winter than in Summer; in the former of which seasons therefore will be the perihelion, in the latter the aphelion: and this is also confirmed by the earth's motion being quicker in December than in June, as it is by about 1/15 part. For since the earth always describes equal areas in equal times, whenever it moves swifter, it must needs be nearer to the Sun: and for this reason there are about 8 days more from the sun's vernal equinox to the autumnal, than from the autumnal to the vernal.
6. That the Sun's diameter is equal to 100 diameters of the earth; and therefore the body of the Sun must be 1000000 times greater than that of the earth.—Mr. Azout assures us, that he observed, by a very exact method, the Sun's diameter to be no less than 21′ 45″ in his apogee, and not greater than 32′ 45″ in his perigee.
7. According to Newton, in his theory of the moon, the mean apparent diameter of the Sun is 32′ 12″.— The Sun's horizontal parallax is now fixed at 8″5/10.
8. If you divide 360 degrees (the whole ecliptic) by the quantity of the solar year, it will give 59′ 8″ &c, which therefore is the medium quantity of the Sun's daily motion: and if this 59′ 8″ be divided by 24, you have the Sun's horary motion equal to 2′ 28″: and if this last be divided by 60, it will give his motion in a minute, &c. And in this way are the tables of the Sun's mean motion constructed, as placed in books of Astronomical tables and calculations.
