30 



SCIENCE. 



[N. S. Vol. II. No. 28. 



illustrate. Eecalling that the distance round 

 the Earth's equator is about 24,000 miles, 

 ten times this gives the distance of the 

 Moon, which is practically inconceivable; 

 but the Sun is 390 times more remote. As 

 the two bodies are about the same in appar- 

 ent size, it follows that the Sun's actual 

 diameter is about 390 (accurately 400) 

 times greater than the Moon's. 



The available methods of ascertaining 

 the Sun's distance, more than a dozen in 

 number, may be divided into three classes: 

 (1) hy geometry or trigonometry; (2) by 

 gravitational effects of Sun, Moon and plan- 

 ets; (3) by the velocity of transmission of 

 light. The first includes transits of Venus, 

 and near approaches of the Earth to Mars, 

 or to small planets exterior thereto, at 

 which times the distances of these bodies 

 from the Earth are not difficult to measure. 

 Adopting, with Professor Young, the num- 

 ber 100 as indicating a method which would 

 insure absolute accui'acj', this class of de- 

 terminations will range all the way from 

 20 to 90. The second class of methods, too 

 mathematical for explanation here, depends 

 on the Earth's mass, and their present value 

 may be expressed as 40 to 70 ; but the pe- 

 culiar nature of one of them (utilizing the 

 disturbances which the Earth produces in 

 the motion of Venus and Mars) offers an 

 accuracy continually increasing, so that 200 

 years hence it alone will have settled the 

 Sun's distance with a precision entitled to 

 the number 95. But the best methods now 

 available are embraced in the third class, 

 which employ the velocity of light (deter- 

 mined by actual physical experiment) , and 

 their present worth is about 80 or 90. The 

 problem of the Sun's distance is one of the 

 noblest ever gi-appled bj^ the mind of man ; 

 and no one of the numerous elements with 

 which it is complexly interwoven can yet 

 be said to have been determined with the 

 highest attainable precision. 



An admirable summary of investigation 



of the Sun's distance is given by Dr. Gill as 

 an introduction to IMi's. Gill's Six Montlis in 

 Ascension (London, 1880), an account of an 

 expedition to that island three years pre- 

 viously. The value of the Sun's parallax, 

 8". 848 ± 0".013, determined by Professor 

 Newcomb (^Washington Observations, 1865), 

 and now become classic, is adopted in all the 

 national astronomical ephemerides except 

 the French, which adheres to a larger value 

 of Le Verrier. Independent determinations 

 of this constant below given show the meas- 

 ure of modern precision in this important 

 field of research ; and the relations of the 

 values to each other will be apparent on re- 

 calling that the addition of 0".01 to the 

 Sun's parallax is equivalent to diminishing 

 his distance about 105,000 miles : 



(1880) Todd Velocity of Light 8.808 ± 0.006 



(1881) PuisEux Contact and Micrometer 



observations, Transit of 



Venus, 1874 8.8 



(1881) Todd American Photographs, 



Transit of Venus, 1874 8.8S3 + 0.034 

 (1885) Newcomb Velocity of Light 8.794 



(1885) Obrecht French Photographs, 



Transit of Venus, 1874 8.81 ± 0.06 

 (1887) Cruls ■ Brazilian Observations, 



Transit of Venus, 1S82 8.808 



(1887) E. J. Stone British Contact-Observa- 



tions, Transit of Venus, 



1882 8.832 ± 0.024 



(1888) Haekness American Photographs 



Transit of Venus, 1882 8.842 + 0.012 



(1889) H.iEKNESS Planetary Masses 8.795 + 0.016 



(1890) Battermann Lunar Occultations 8.794 ±0.016 

 (1S90) Newcojib Re-discussion Transits of 



Venus, 1761 and 1709 8.79 ± 0.034 

 (1892) AuwERS German Heliometer Ob- 



servations, Transits of 

 Venus, 1874 and 1882 8.880 + 0.022 



(1892) Gill \ Opposition / (12) Victoria 8.809 



(1893) Gill V of Small ) (SO) Sappho 8.811 



(1894) Gill & Elkin ) Planets ( ( 7) Iris 8.S25 ± 0.008 



Also several other values of this important 

 constant have been derived, and there is an 

 increasing tendency to cluster round the 

 figure 8".81. 



Professor Harkuess published in 1891 a 

 laborious paper entitled The Solar Parallax 

 and its Related Constants (Washington Ob- 

 servations, 1885), in which this quantity is 

 treated, not as an independent constant, but 

 as " entangled with the lunar parallax, the 



