SOT 471 tories. The volume of the sun exceeds the earth's nearly 1,253,000 times. His mean den- sity is almost exactly equal to one fourth of the earth's, so that his mass exceeds hers about 316,000 times. Gravity at the visible boun- dary of the solar globe exceeds gravity at the earth's surface about 27' 1 times ; and a body dropped from rest near the sun's surface would fall through 436 ft. in the first second, and have acquired a velocity of 872 ft. a second, or about 10 m. a minute. The sun's mass exceeds the combined mass of all the planets about 750 times. His mean distance from the earth has been estimated at about 91,430,000 m. ; though we may expect that the results obtained during the late transit of Venus (December, 1874) and to be obtained during the coming transit (December, 1882) will lead to some cor- rection of this estimate. It already appears probable that the sun's estimated mean dis- tance' must be increased to about 92,000,000 m. The greatest and least distances of the sun from the earth (assuming his mean distance to be 91,430,000 m.) are respectively 92,963,000 and 89,897,000 in. ; and his apparent diameter varies from 31' 31-8" to 32' 36'4" as he passes from his greatest to his least distance. The sun has an apparent motion among the stars from west to east along the great circle called the ecliptic (see ECLIPTIC), making a complete circuit of the heavens in 365 days, 6 hours, 9 minutes, and 9'6 seconds, though the passage from vernal equinox to vernal equinox (first point of Aries) occupies only 365d. 5h. 48in. 48'6s., because of the precession of the equi- noxes. (See PRECESSION.) These two periods are called respectively the sidereal year and the tropical year. There is one other astronomi- cal year (besides the civil, Julian, and lunar years) known as the anomalistic year, being the interval separating successive passages of the perigee of the solar path, viewed geomet- rically; its length amouuts to 365d. 6h. 13m. 49 - 3s. The apparent motion of the sun is not uniform in the ecliptic, owing to the eccen- tricity of the earth's orbit ; it is greatest about Dec. 31 to Jan. 1, when he moves through 1 1' 9-9" in 24h., and least about June 30-July 1, when he only moves through 57' 11'5" in 24h. The sun has also three real motions : 1, an axial rotation, the nature of which will presently be described ; 2, a motion about the centre of gravity of the whole solar system, but in consequence of the great superiority of his mass over that of all the other bodies this centre of inertia is always within the sun's vol- ume; 3, a progressive motion in space toward the direction of the constellation Hercules, the rate of which has been estimated at 150,000,- 000 m. per annum, but on evidence exceeding- ly questionable. The fact of solar motion to- ward Hercules is as nearly certain as possible, but the rate of this motion is not known. Recent researches suggest that it is far greater than the rate just mentioned, great though that rate may appear. Examined with a tele- scope, the sun's surface, which appears very nearly uniform to the naked eye, is seen to be brightest near the centre, and to grow pro- gressively darker toward the circumference. It is also marked by various irregularities, spots, faculas, mottling, besides other appear- ances requiring greater telescopic power for their detection. The spots on the sun were independently discovered by Galileo, Fabri- cius, Scheiner, and Harriot. It was soon per- ceived that they move in such a way as to indicate that they are real surface markings, not bodies passing between the earth and the sun, and that therefore the sun's rotation can be measured by observing them. It was found that the sun rotates in a period of about 25-J- days; and as the spots do not at all times pass on straight lines across the sun's face, but sometimes on a course slightly bowed up- ward and at others on a course slightly bowed downward, it was seen that the sun's axis of rotation is not quite upright as referred to the plane of the ecliptic, but slightly inclined. The following elements of the sun's rotation belong to the astronomy of recent times, having been deduced from results obtained by Carrington and Sporer, reduced to the year 1869 : ELEMENTS. Carrington. Sporer. Longitude of node of solar equator Inclination of solar equator 73 57' T 15 74 87' 6 67 Mean diurnal rotation 14 18 14 2T Mean rotation period 25-88d. 25-284d. It will be perceived that a mean rotation is in- dicated. Carrington's observations have shown that spots in different solar latitudes travel at different rates, varying in fact from a daily rotation through about 12$ in lat. 50 (nearly the highest in which spots have been observed) to a daily rotation through nearly 14 at the solar equator (where, however, spots are very rarely seen). Carrington gives the following formula for the rotation in different solar lati- tudes : daily rotation = 14 25'- (2 45') sin.5 lat. ; but this formula is purely empirical. The curious point about this variation in the rate of turning is that, taking two parts of the visi- ble solar surface in the same longitude, but one in lat 45 (say), the other on the equator, the latter will advance further and further in longi- tude from the former, gaining daily about two degrees, so that in the course of about 180 days it will have gained a complete revolution. That is to say, the sun's equator makes about two revolutions more per annum than the regions in 45 north and south solar latitude. The spots on the sun have usually a dark central region called the umbra, within which is a still darker part called the nucleus, while around this there is a fringe of fainter shade than the umbra, called the penumbra. Although the umbra and nucleus appear dark, however, it is not to be supposed that they are really dark ; in fact, Prof. Langley of Pittsburgh has succeeded in examining the light from the nucleus alone,
