SUN, TIIE, the great luminary upon which not only our well-being but our ence depends, has been from the earliest ages a source of wonder Ad admiration, and its worship was probably the very first form of idolatry. See SIIN-WORSHIP.
When the true system of the universe became known, one of the first labors of astronomers was to ascertain the distance and size of the sun, and these have been known for some time with tolerable precision; but until lately the most vague and unsatisfactory theories regarding its chemical and physical constitution have continued to prevail.
Within the last few years, however, our knowledge of its chemical and physical con stitution has increased with a rapidity probably unequaled in any other branch of science. Our knowledge regarding the sun is best arranged under three heads: viz., The general relations of the sun to our globe; the sun's chemical constitution; and its physical constitution.
Relations of the Sun.to the Earth, as the Source of Light and order to appreciate the grandeur of the scale on which solar activity is carried on, it is only necessary to kuow a few facts relative to the sun, which are best expressed by numbers.
1. Distance of the Sun from the difficulty in ascertaing the parallax (q.v.) of the sun arises from the smallness of the base line as compared with the distance of the object. The distance of the observing stations must always be less than 8,000 m.; from this the parallax of the moon, which is only 30 times 8,000, can be observed directly with tolerable nearness. But when the distance is many thousands times the length of the base line, the triangle is " ill-conditioned" or unfavorable to accuracy, and the problem must be approached indirectly. The first attempt to measure the distance of the sun was that made by the Greek astronomer Aristarchus in the third c. B.C., who made it only about one-twentieth of what we now know it to be. Even the great astronomer Kepler in the seventeenth c. could only say that the distance must be at least between 13 and 14 millions of miles. Subsequent estimates—for, owing to the imperfection of the methods and instruments, they were little better than estimates—rose to 80 mil lions. At last, in 1716, the English astronomer Halley proposed a method of employing
the transits of Venus. Accordingly, the transits of 1761, and 1769, were observed in a variety of places; but the results at first deduced were discordant and unsatisfactory, until in 1824 the German astronomer Encke " discussed " the observations of 1769, and arrived at a distance of about millions of miles; and this number held its place iu books of astronomy for a good many years. In the mean time, in the absence of transits, other methods, become possible through the growing perfection of astronomical instru ments, were tried, and most of them concurred in pointing to a value nearly 3+ millions less than that above stated; so that 91,500,000 came to be accepted as the approximate distance of the sun, until the transit of 1874 should settle it more definitely.
A transit can occur only when the planet is in or near one of her nodes at the time of inferior conjunction, so as to be in a line between the earth and the sun. The coin cidence of these two conditions follows a rather complex law. There are usually two transits within eight years of one another, and then a lapse of 105 or 122 years, when another couple of transits occur, with eight years between them. The transit of 1874 will be followed by one in 1882, and there will not be another until June, 2,004.
The way in which a transit is turned to account may be understood by the help of the accompanying diagram, where E represents the earth; V, Venus; and S the sun. It is to be premised that the relative distances of the planets from the sun are well known. Their periodic times can be observed with accuracy, and from these by Kepler's (q.v.) law we can deduce the proportions of the distances, but not the distances themselves. It is thus known that if the distance of the earth from the sun is taken as 100, that of Venus is 72. In the fig. then, AV is 28, or about one-third of Va or Vb.