SUN

our chief luminary and the ruling body of the planetary system, is @ radiant globe 866,400 m. in diameter, at a mean distance of 92,900,000 m., and subtending an angle, as viewed by us, Of 32’. It has a superficial area 11,900 times, a volume 1,300,000 times, those of the earth, but is only 331,000 times more ma ive. Its mean density is, accordingly, 0°255 the terrestrial, or 14 taking water as the unit, while gravity at its surface is of 27°6 times its terrestrial power. The sun rotates on an axis inclined 7° 15’ to the ecliptic in @ period lengthening systematically with increase of solar latitude, from about 25 days at the equator to 274% days in lat. 45°, as determined by observation of sun-spots; and spectroscopic measures of velocity show that the retardation continues at least up to lat. 75°, This vorticose movement evidently excludes a solid consistence to the body affected by it, and probably indicates virtual gaseity to the core. Besides, the enormous output of light and heat from the photosphere can only be maintained by a rapid interchange of material between the surface and the interior. The illuminative efficacy of sunlight on the earth surpa es thirty times that of an are light of 2,000 candle-power at a distance of one metre (Young). The accompanying heat-emi ions would suffice to melt every minute a shell of ice 64 ft. thick enclosing the sun; they represent the energy of 130,000 h.p. in continuous action on each square metre of the solar surface, The earth intercepts only y0.d00.000 Of this copious flood. The photospheric temp. derived by Stefan’s law from the solar constant is 6,000° to 7,000° C. Photometric comparison of different parts of the spectrum gives 5,300° C., and there is some reason to believe that it varies. Its perpetuation from age to age is explained on dynamical principles by the slow advance of contraction. An annual shrinkage of 300 ft. in the sun’s diameter would, it is estimated, supply the thermal expenditure, but not for an unlimited time. In five million years the sun would, by a continuance of the indicated proce , have become eight times denser than it is now, and would then almost certainly be incapacitated for vivid radiation. Its extinction may, however, be indefinitely postponed by unknown or barely suspected modes of action, such as the disintegration within its substance of elements akin to radium. Outside the photosphere there is the gaseous ‘reversing layer’ of the Chromosphere, of average thickne of about 5,000 m., and giving a bright line spectrum. Outside the chromosphere, again, | is the gaseous envelope of the Corona, extending to a great distance, and of | very small density. Its spectrum shows, | in addition to others, a brilliant green line of unknown origin. The chemical constitution of the is disclosed by the nature of its spectrum. About forty-one substances have recognized as solar ingredients by spectral lines. The sun moves as a among the stars, but with only half their average velocity. Together wit i. its entire system, it is transported at the rate of 12 m. a second a point on the sphere located by the authorities in R.A. 277°, dec. The nature of the orbit described unknown, but it appears to lie near plane of the Milky Way.