Page:Collier's New Encyclopedia v. 09.djvu/207

LEFT SUN 165 Strisr, PARALLAX OF THE radiating power. The meteoric theory attempts to account for the keeping up of its supply of energy by the fall of meteoric matter into the sun. A body falling into the sun from any consider- able distance will generate by the sudden arrest of its energy of translation an enormous amount of heat, 6,000 times as much as would be generated by its com- plete combustion if it were a mass of pure carbon. From the fact that meteors are constantly striking the earth we know that they must be all the time fall- ing into the sun, but nevertheless the greater part of the meteoric matter in the vicinity of the sun must circulate round it as the comets do. The most careful estimates seem to indicate that only a very small fraction of the sun's radiant energy can come from the fall of meteors into it. The only sufficient theory left is that of Helmholtz — the contraction hypothesis. Without going fully into the explanation of this it may be briefly stated that, supposing the bulk of sun to be mainly gaseous, a con- traction of about 250 feet per year in its diameter would supply all its present rate of radiation. At this rate it would take nearly 10,000 years to diminish its ap- parent diameter by a single second of arc, and it is doubtful if this amount could be certainly determined with our present means of measurement of this di- ameter. Age and Duration. — E verything points to the conclusion that the present condition of the sun is mainly gaseous, and its future supply of heat depends on that condition. The contraction can only keep up its temperature so long as it is principally in a gaseous state. As soon as any large part of its bulk liquefies (only the thin shell of photospheric clouds is now supposed to be in a liquid condition), it will begin to cool off and Its temperature will fall. This means the beginning of the end for life on the earth. The best estimates place this time as not more than 5,000,000 to 10,- 000,000 years off at the longest. The past history of the sun is involved in about the same obscurity as the nebular hypothesis. Certain data can not be furnished with exactness. Knowing the mass of the sun, we can compute how much heat has been generated in its con- densation from infinite space or from any assigned dimensions; but as to the rate at which this heat has been radiated in the past ages, and the rate at which con- traction has taken place, nothing definite can be stated. It may be considered that the age of the solar system is entirely unknown. SUN, CITY OF THE, Baalbek, for- merly center of sun-worship. SUN, ECLIPSES OF THE, caused by the moon coming between the earth and the sun, may be either partial, total, or annular. In a partial eclipse the ob- server is situated in the penumbra of the moon's shadow, and only a part of the sun's light is cut off on one side. In a total eclipse the observer is in the umbra of the moon's shadow, and all the light of the sun is cut off except that from the prominences and corona surround- ing the sun. In an annular eclipse the disk of the moon is wholly projected on that of the sun, but is not large enough to cover it completely, so that a ring of sunlight is left all round the moon. The apparent diameters of the sun and moon are so nearly equal that their variations with their varying distances make a cen- tral eclipse sometimes total and some- times annular. In fact, the same eclipse may be total for some parts of the earth where the sun and moon are near the meridian at the time, and annular for other parts where they are low down in the E. or the W. at the time of central phase. In the annular eclipse the ob- server, instead of being in the cone of the moon's shadow, is in the prolongation of that cone, beyond its apex, in the other nap of the cone. At present the only scientific importance of partial and annular eclipses is the use that may be made of them for determining the rela- tive positions of the sun and moon, and thus correcting the elements of the ter- restrial and lunar orbits. But the fleet- ing minutes of every total eclipse are now utilized so far as possible to study the sun's surroundings, especially the mysterious corona, which is so faint that it is only visible when the bright light of the photosphere is cut off (see SuN). Geometrically considered, a solar eclipse is only an occultation of the sun by the moon, like that of any other heavenly body by the moon in its motion across the heavens. SUN, PARALLAX OF THE, or SOLAR PARALLAX, the angle sub- tended by the radius of the earth at a distance equal to that of the earth from the sun. More exactly, it is the angle subtended by the equatorial radius of the earth at the mean distance of the sun, though this would be called, when speak- ing exactly, the mean equatorial hori- zontal parallax of the sun. It is, as nearly as we know it today, almost 8.8", with an uncertainty of probably not more than 0.01" or 0.02" different from this in either direction. This corresponds to a distance of the sun a little less than