62 



THE SUN AND ITS OFFICE IN THE UNIVERSE. 



JReatl at a meeting of the Royal Spciety, Tasmania. 



By F. Abbott, F.R.A.S., &c. 



The Bim, to us, is not only the largest apparent star in the 

 universe, hat also the most brilliant, and that which exercises 

 over the earth the most dominant influences. It is from hiiu 

 that all the energies developed on the surface of the earth 

 incessantly flow, and are continually and successfully carried 

 on by the two hundred and thirty millionth part of the force 

 radiated, ^vhich is all the earth is able to receive of the sun's 

 rays. From him also, at epochs immensely distant from us, 

 the planets have been thrown out successively, at first in the 

 form of nebulous rings — agglomerations of matter which have 

 in the end become condensed, and now form the planets of our 

 system. 



Compared with the mass of the earth, the mass of the sun 

 is only about 355,000 times as great although its volume is 

 1,4C 0,000 times larger, which indicates a less density. The 

 matter of which the sun is composed is found to weigh but 

 little more, volume for volume, than a quarter of that of which 

 our own globe is formed. To express the weight of the sun in 

 tons, by figures, would be useless — the number ranks among 

 those which express nothing to the mind. 



The light and heat received by our earth from the sun being 

 taken as unity, the planet Mercury would be 6.673, and the 

 planet Neptune, 0.001. Mercury is, therefore, lighted and 

 heated nearly seven times as much as the earth ; and the 

 light and heat of the sun have an intensity 6673 times greater 

 on the planet Mercury than at the surface of Neptune, where 

 the apparent diameter of the solar disc is only Imin. 4sec. 

 To compare and know correctly the calorific and luminous 

 intensity of the sun at the surface of the different planets 

 of our system, we must be acquainted wdth the nature of their 

 atmospheres, and in what proportion the waves of light and 

 heat are absorbed, in passing through those gaseous envelopes. 

 Mercury, for instance, may have an atmosphere so dense that 

 the soil does not receive more of the sun's influence than the 

 soil of the earth. 



It was in August, 1612, that Galileo wrote, in the second 

 of his three celebrated letters, concerning the spots on the 

 sun. " For the time to come," he says, " there will remain 

 to physicists a field for speculations about the substance and 

 manner of production of such vast masses as are the solar- 

 spots, concerning which problem I would not venture to affirm 



