TELESCOPIC INVESTIGATION General Characteristics.—The sun presents to the telescopic view a dazzling white circular disc, with a sharp edge, whose diameter subtends an angle of about half a degree. Slight but systematic variations of the angular diameter reveal the that the distance from the earth to the sun is not constant, and that the earth's orbit is an ellipse with the sun at one of the foci. Several methods have been employed to measure the various distances of the sun from the earth ; it will suffice to describe one of them. Early in the I7th century Johann Kepler, by the method of trial and error, discovered rules from which an accurate plan of the solar system could be drawn, although the scale could not be given. It then became necessary only to measure at some instant the distance between any two of the bodies represented in the plan in order to fix the scale for the whole. A minor planet named Eros at certain times in its career comes within 14 million miles of the earth, and if its direction be then measured simul taneously from two places at a known distance apart on the earth the distance of the planet can be calculated from the angle between the directions. A scale is thus obtained from which the distance of the sun from the earth at any time follows im mediately. The mean value throughout the year (from which the actual distance never departs much) is found to be about 92,900, 00o miles. This is generally expressed by saying that the solar parallax is 8".8o (i.e., 8.8o seconds of arc), the solar parallax being the angle the earth's equatorial radius subtends at the distance of the sun.
The sun's linear diameter follows at once from this measure ment and the observed angular diameter. It is approximately 864,00o miles—far transcending the powers of imagination, but small as stellar diameters go; the sun is, in fact, a dwarf star. Its mass is calculated from the force with which, according to Newton's law of gravitation, it attracts the earth, whose mass is separately determined (see EARTH). The result-2 X i.e., 2 X tons, or 33 2,000 times the mass of the earth—is about the average for stellar masses. Mass and diameter are sufficient data for the estimation of the mean density of the sun, viz.—I.41 times that of water. This figure, of course, affords no information about the density in any particular region or the rate of variation of density from the surface inwards.
Close examination of the night sky over a prolonged period shows that around the constellation Hercules the stars appear, on the whole, to be getting farther apart, whereas, in the opposite direction, they appear to be gathering together. This is interpreted as an indication that the sun, with its attendant satellites, is mov ing towards a point in the former region.
Surface Features.—These general characteristics must now be supplemented by more intimate knowledge. When the surface of the sun is carefully examined it is found to present a grained structure ; the appearance has been likened to that of rice grains and even willow leaves. The brightness of the disc in other respects also is not uniform ; the region near the circumference, or limb, is appreciably less bright than the centre of the disc—a fact which appears most obviously in photographs. More striking violations of uniformity are caused by sunspots, which are fre quently visible, sometimes even to the unaided eye. A sunspot generally consists of a central, apparently black, umbra, sur rounded by a less dark penumbra (see Plate). It should be said at once that each of these regions is in reality exceedingly bright, and appears dark only by contrast with the still brighter solar disc. Spots often cluster together in groups, but perhaps even more significant is their characteristic of associating in pairs. They move steadily across the face of the sun in such a manner as to leave no doubt that the motion is due to a rotation of the sun about an axis. The sun, therefore, like the earth, is approxi mately a sphere, and its bright visible surface is accordingly known as the photosphere. A remarkable feature of the sun's rotation is that it does not take place at the same rate in all latitudes; i.e., the sun does not rotate as a rigid body; the nearer to the equator the faster is the rotation. Sunspots very rarely appear outside two zones of the photosphere, bounded by circles of latitude about 5° and 4o° N. and S. respectively. Within each of these regions R. C. Carrington found, as the result of a large number of observations, that the mean rotation period was 25.38 days, while near the equator the period was only 24.5 days.