804 



Sl'N 



of Mars in 1862, observed by Stone and Winnecke, 

 justified their doubts, fixing the distance some- 

 where between 91 and 92) million miles. The 

 method employed so far resembled that of the 

 transits of Venua that it depended on 



the distance of a nearer object than the sun viz. 

 the planet Mars in opposition. From this, the 

 proportions of the planetary distances from tin- 

 aim IM-III;; accurately known, the solar distance w u> 

 easily calculated. 



Meanwhile, bv a most ingenious method, another 

 measure of this was obtained. Komer (1675), 

 Delambre (1792), and Glasenapp (1874) had ascer- 

 tained ( the hist with great accuracy ) by observa- 

 tion of Jupiter's Satellites (q.v.) that light takes 

 500-84 seconds to cross the earth's orbit from side 

 to side Kilasenapp's result). Also the amount 

 of the Aberration of Light (q.v.) had been care- 

 fully measured. If the velocity of light were 

 known these would afford a means of estimating 

 the sun's distance. This velocity was BMMOfM 

 by Fizeau and Foucault in 1862. The result con- 

 tinued the later and smaller estimate of solar dis- 

 tance given above. A rediscnssion of the transit 

 observations of 17(39 by Ponalky (1864) and Stone 

 ( 1868 ) also confirmed it 



The transit of Venus in 1874 was impatiently 

 awaited, as with modern instruments and methods 

 a final settlement of the question was anticipated. 

 But, although .-iliout eighty posts of observation 

 were provided all over the world and many ob- 

 servers carefully trained, little or no progress was 

 made. Atmospheric effects and photographic de- 

 fects left an uncertainty estimated by Professor 

 Harkness of Washington, D.C., at 1 J million miles. 



Dr Gill in 1877 observed a favourable opposition 

 of Mars, which gave a result of 93,080,000 miles. 

 Observations of minor Planets (q.v.) were also 

 utilised, and a number of expeditions sought a 

 value from the transit of 1882. Michelson of the 

 United States navy anew determined (in 1879) 

 the velocity of light, and Professor Harkness 

 used his value for it in another estimate. The 

 amount of accuracy obtainable at present in such 

 discussions may be judged by the various esti- 

 mates given by the best authorities as follows : 

 Professor Harkness, 92,365,000 miles ; Professor 

 Young, 92,885,000 ; Dr Ball, 93,000,000 ; Mr Stone, 

 92,000,000; M. Faye, 92,750,000. These various 

 values will explain the varying estimates of the 

 size, mass, density, &c, of the members of the 

 solar system, as the sun's distance enters as a 

 factor into all such calculations. The table at 

 the beginning of this article is based on a solar 

 parallax of 8"'794. In it the reader will find the 

 results as to the sun's size, mass, density, and 

 gravitational power of this conclusion as to his 

 distance. 



(2) The tun' i true motion in space is ascertained 

 from the comparison o'f observed stellar proper 

 motion* (see STARS). It is directed to a point on 

 the line joining the stars r and n Herculis. Its 

 velocity is T623 radii of the earth's orbit per 

 annum. 



(3) The investigation of the physical structure 

 and chemical constitution of the sun has been in 

 modern times most successful. A long series of 

 efforts by many workers has brought us to some- 

 thing like definite ideas as to its radiating power, 

 which is a fundamental factor in this investigation 

 (see H BAT). In Is:i7 Pouillet measured the amount 

 of solar radiation. His result was that I '76 calorie* 

 per minute were received on every square centi- 

 metre of our earth V surface. Much of the sun's 

 heat is absorbed ny the terrestrial atmosphere. 

 Hence Forbes ascended the Faulhorn in 1842 and 

 obtained there the greater value of 2'85 calories. 

 Violle on Mont lilauc in 1875 got 2 54. Professor 



J.iingley , iirohahly the most accurate olwerver. gives 

 very nearly .TOO. Computations of the sun's t-m- 

 iH-iiitme in degrees Cent, have varied from H few 

 bundled* to many millions. They are essentially 

 misleading, as the condition of matin in tli<- HID 

 is not vet known sullicientlv well to enable us to 

 e.-ilrnlate it* temjM-rature from its radiation. We 

 know, however, with certainty that the most re- 

 fractory Milisiances are valorised long before the 

 solar temperature is reached. Ami the sun's sin 

 face, seen bv I.anglev through the then smoke-laden 

 air of Pittsburgh, a|>|>eaicd fttUO times a* bright as 

 the molten metal in the fierce heat of a llesseim i 

 converter. At the temjieralurr indicated by thi^ 

 all known sul>staiices would exist as tenuous vapour, 

 were the pressure bearing on them that ot our 

 terrestrial atmosphere. lint in the interior of the 

 sun, under pressmes inconceivable to our minds. 

 such va|Miiirs would liehave vei v differently. I'nder 

 such conditions the usual distinctions between 

 solid, liijiiid, and gaseous forms of matter to w Inch 

 we are accustomed would lie obliterated. In fact, 

 how matter would liehave in such a state science 

 at present cannot tell. Of the sun's surface, how. 

 ever, we have learned much. According to the 

 researches of Professor Rowland of Johns Hopkins 

 I'niversity, lialtimore, in 1891, the following 

 elements are present there. The list is in order, 

 according to the number of spectral lines in the 

 elements identified in the solar S|iet-ti-um (q.v.), 

 iron coming first with more than 2000 lines i.lmti- 

 fied, potassium last with 1 only. Iron, nickel, 

 titanium, manganese, chromium, cobalt, carbon, 

 vanadium, zirconium, cerium, calcium, scandium, 

 neodymium, lanthanum, yttrium, niobium, molyb- 

 denum, palladium, magnesium, sodium, silicon, 

 strontium, barium, aluminium, cadmium, rhodium, 

 erbium, zinc, copper, silver, glucinum, germanium, 

 tin, lead, potassium, and piixxilily indium, osmium, 

 platinum, ruthenium, tantalum, thorium, tungsten, 

 uranium. 



These as vapours form a layer upon the solar 

 surface, which is in fact the solar atmosphere. 

 Immediately beneath this is the photosjrfiere, which 

 marks to the eye the boundary of the sun's disc, 

 Above this layer of vapours rise vast jets and clouds 

 called variously flames, prominences, or jirotu !>- 

 ances. Above these again is the bright and 

 curiously shaped solar corona, extending along the 

 ecliptic, as once seen, to a distance of twelve 

 solar diameters. 



The photosphere presenta to the telescope of low 

 power an apparently even surface. I'nder higher 

 powers its structure is seen to be complex. The 

 whole surface is granulated, resembling a gravel 

 heap seen from a little distance. These granules 

 have been decril>ed as like ' willow leaves ' and 

 'rice grains.' A multitude of minute dark points 

 or pores, black in comparison with the granules. 

 serve to emphasise their outline. This may be said 

 to be the normal condition of the photosphere. 

 There are always, however, some portions of tin- 

 surface which siiow an indistinctness of granula- 

 tion, sometimes so marked that they are named 

 'veiled snots.' Bands of this indistinctness in less 

 marked form spread over the whole photosphere as 

 a kind of network called by French observers the 

 riseau photospkfrique. They are continually in a 

 state of fluctuation, and are most probably due to 

 the currents of varying density in the solar at mo 

 sphere. The granules and spores are due to 

 intense convection currents, the tops of ascending 

 masses of va]xiiir glowing white with the heat 

 derived from the solar interior. These show us 

 'granules,' while the descending masses, having 

 radiated their energy, return to be again heated 

 below the surface, and in their descent show as the 

 comparatively dark 'pores.' The appearance of 



