SUN

(Ger. sonne.) In Astronomy, the central body of our system, about which all the planets and comets revolve, and by which their motions are regulated and controlled. In Physics, the sun is the source of light and heat; and therefore the primary cause of all the motions aijd changes effected on the surface of the earth by those great agents of nature. Apparent Habitude of the Sun. — The sun presents to the naked eye the appearance of a luminous circular disc, subtending an angle of rather more than half a degree. But on measuring accurately the diameter of the disc by means of a micrometer, it is found to be not always the same, but subject to an annual variation. In fact, as the earth describes an ellipse, of which the sun occupies one of the foci, its distance from the sun is constantly changing, and the variation of the sun's apparent diameter is a nece ary consequence of the change of distance. "When the earth is in its perihelion, or point of its orbit nearest to the sun, the sun's apparent diameter is 32' 35-6"; and when the earth is at its aphelion, or most distant point, the apparent diameter is 31' 31-0". The mean apparent diameter, or diameter at the sun's mean distance, is 32' 2-9". Distance of the Sun. — The sun's true distance is found from his horizontal parallax. This is so small a quantity that it would scarcely be po ible to determine it in the usual way by observations made on opposite sides of the earth; but the astronomical phenomena of the transits of the planet Venus over the sun's disc afford the means of determining it with the utmost precision. From such phenomena the parallax has been found to be Only 8-6 "; that is to say, if an observer could be placed at the centre of the sun, the earth's semidiameter would be seen by him under an angle of 8-6". From this it follows that the mean distance of the sun from the earth must be at least equal to 24,047 times the earth's radius: and as the radius of the earth is nearly 4000 miles, it follows that the sun's true distance from the earth must be about 96,000,000 miles. See Planet. In order to obtain a more distinct idea of this enormous distance, we may compute the time in which it would be pa ed by some" of the swiftest motions with which we are acquainted. A 24 lb. cannon ball, fired with a charge of 8 lbs. of gunpowder, is projected with a velocity of about IGOO feet in a second. Supposing it were to continue to move towards the sun witli the same uniform velocity, it would require ten years to reach his surface. A body travelling with the velocity of sound would require about five years to pa from the sun to the earth. Light itself, which travels with the astonishing velocity of 192,.')00 miles in a second, only reaches the earth eight minutes and eighteen seconds after leaving the sun's surface. Knowing the distance and apparent diameter of the sun, it is easy to compute its real dimensions. A body which subtends an arc of 32' 3" of a circle, whose radius is 96,000,000 miles, must have a real diameter of 892,000 miles; which, therefore, is the diameter of the sun. Or, since the true diameter of the sun must have to the diameter of the earth the same ratio which the sun's apparent diameter as seen from the earth has to the earth's apparent diameter as seen from the sun, that is, a ratio of 32' 3" to 8-6", or of 111-8 to 1, it follows that the sun's diameter must be nearly 112 times the diameter of the earth. Hence the volume of the sun must be 1,397,415 (the cube of 111"8) times, or, in round numbers, 1,400,000 times the volume of the earth. The mean distance of the moon being rather le than 60 times the earth's radius, if the sun's centre were placed at the centre of the earth its surface would be at twice the distance of the moon's orbit; and the volume of a sphere whose radius is equal to that of the moon's orbit would only be an eighth part of the volume of the sun. The sun's volume is 500 times greater than the volumes of all the planets taken together. Ma and Density of the Sun The magnitude and distance of the sun are determined by direct observation; his ma as compared with that of the earth is deduced from the law of universal gravitation, and the theory of central forces. The earth revolves round the sun at a given distance, and in a given time; and from these data the centrifugal force becomes known. But the force which counteracts this centrifugal force, and compels the earth to move in its elliptic orbit, and prevents it from fly. ing off in a tangent to that orbit, is the sun's attraction. Hence the solar attraction is equal to the centrifugal force. But the earth's attraction at its surface is a known force, being that which causes a heavy body to fall through IG-j^ feet in a second of time; and consequently, from the known law 1193 diminution proportionally to the inverse square of the distance, the earth's attractive force at any distance from the surface also becomes known. Now, on comparing the centrifugal force due to the earth's motion in its orbit with the force of terrestrial gravity on a body at the distance of the sun, it is found that the former exceeds the latter in the ratio of 354936 to ]. But when the distance is the same, the attractive forces of two bodies are directly as their ma es or quantities of ponderable matter; and hence the sun's ma is 354936 times greater than that of the earth. It is also found to be about 800 times greater than the aggregate of the ma es of all the planets and satellites. The density of a body is directly as its ma , and inversely as its volume. If, therefore, we call the density of the earth 1, the ratio of the density of the sun to that of the earth will be as 354936 -r 1397415 to 1, or as "0254 to 1; whence the sun's density is about one fourth of that of the earth. In order to compare the force of solar gravity at the sun's surface with that of terrestrial gravity at the earth's surface, we must recollect that the attraction of a body on an exterior point is directly as the ma of the attracting body, and mversely as the square of the distance of the point from its centre. Hence, since the sun's ma is 354936 times that of the earth, and the radius (or distance of the surface from the centre) 1118 times that of the earth, if A denote the force of solar gravity at the sun's surface, and B that of terrestrial gravity at the earth's surface, then. 354936 J_ ^ ■ (111-8)2 • (1)2' or A is to B as 27-9 (nearly) to 1. A body, therefore, which at the earth's surface weighs one pound, would weigh 27-9 pounds if carried to the surface of the sun. " An ordinary man would not only be unable to sustain his own weight on the sun, but would be literally crushed to atoms under the load." {Herscliel's Astronotny.) Rotation of the Sun. — The sun's surface, when viewed through the telescope, is frequently diversified with a number of dark patches or spots, which being observed from day to day are found not to remain in the same place, but to move acro the surface, without, however, changing their relative positions. These phenomena are accounted for by supposing the sun to have a rotatory motion about an axis from west to east. From a comparison of the best observations Delambre found the period of rotation to be about 25 days; and that the sun's equator is inclined to the ecliptic in an angle of about 7° 20', and intersects it in a line which makes an angle of 80° 21' with the line of the equinoxes. The observations of the spots are, however, subject to great uncertainty; but the fact of rotation from west to east, or in the same direction.as the earth and all the other planets, is established. Spots of the Sun — The solar spots, to which allusion has just been made as furnishing the elements of the sun's rotation, present very remarkable appearances, and have been the subject of numerous theories respecting the nature and physical constitution of the sun. They began to attract attention soon aftpr the discoverj^ of the telescope, and their phenomena are described with sufficient accuracy by Hevelius and Scheiner. In form they are exceedingly irregular; and, when watched. attentively for some time, are observed to enlarge and contract, and to change perpetually their form and outline. Frequently they disappear without approaching the edge of the disc, and break out suddenly in places where none were seen before. Sometimes a spot has been seen to break up into several parts, and the fragments to separate from each other, as if acted upon by an explosive force. The nucleus of a spot is perfectly black, and sharply defined; and is surrounded by an umbra, or border of a fainter shade. In the neighbourhood of large spots, or in places where they are numerous, bright streaks, or portions more luminous than the general surface of the sun, called facu Ice, are frequently seen. Among these faculae spots are often observed to break out; and when this does not happen they are generally succeeded by spots, larger in proportion to the brightne of the antecedent faculae. The region of the spots is generally confined to the equatorial parts of the sun, within about 30° of the equator; but occasionally they are observed over the whole disc. The magnitude of the spots, and the scale on which their movements are performed, are not the least remarkable circumstances connected with the phenomena. At the distance of the sun, a line which subtends an angle of one second is equal to about 460 miles, and a circle of that diameter has an area of about 166,000 miles; and this is the smallest space which can be distinctly discerned on the sun's disc. But a spot was observed by Mayer having a diameter equal to l-20th of the diameter of the sun, or upwards of 96", and consequently (supposing it to be circular) covering an area of 1520 millions of square miles, — upwards of 30 times the whole surface of the earth. Numerous hypotheses have been formed respecting [s. 1206]