THE SOLAR PARALLAX. 



51. INTRODUCTORY AND HISTORICAL. 



The motives which prompted the observations forming the basis of this discussion have been 

 so fully developed by the astronomer under whose direction the expedition was conducted, and 

 the method of investigation contemplated has been so thoroughly explained, that no farther 

 consideration of these subjects seems required or appropriate. 



The measure of the sun's distance has been well called by the Astronomer Royal of England 

 the noblest problem in astronomy. This distance, known or unknown, is, and must ever be, 

 the standard length in which every linear measure of a celestial object beyond the moon is 

 directly or indirectly expressed ; whether it be the distance of a satellite, a comet, or a fixed 

 star ; the dimensions of a planet, or the gauge of a nebula. It is the astronomical unit ; and 

 every stellar distance is only known as a proportional one, until this unit is established. It is, 

 therefore, manifestly the duty of astronomers to flinch from no labor which gives a remote 

 prospect of increasing the precision of our measurement of this fundamental quantity. The 

 materials now presented for the discussion are the fruit of a national expedition, instituted at 

 the advice, and with the approval, of the nation's ablest astronomers. To their attainment 

 three years were dedicated by the zeal of the leader in the enterprise and his unwearied assistants, 

 and astronomers in other lands have contributed. No pains ought to be spared in deducing 

 from them all that they can be made to yield in furtherance of the end for which they were 

 designed. This consideration has been kept steadily in view, and is a sufficient reply to any 

 criticism which may attribute over-refinement to the numerical computations, or too great 

 minuteness to the combinations of the materials under discussion. 



When Keppler, after extended study of Tycho's observations, arrived not later than 1620, at 

 the conviction* that the solar parallax could not exceed 1', he attributed to the sun a distance 

 more than three times greater than philosophers had previously supposedf, although his own 

 limit was but one-seventh part as remote as we now know that it should have been. He had, 

 in 1609, in his bookj on the motion of Mars, called it difficult to fix the distance more exactly 

 than between TOO and 2,000 semi-diameters of the earth, (corresponding to a parallax between 

 1' 45" and 4' 55",) and in his Ephemerides for 1617 and 1618, he had supposed the parallax 

 to be 2' 29", according to Tycho Brahe, who deduced it from observations of the moon.|| Peter 

 Criiger, Keppler's intimate friend, upbraided him for removing the sun " to such a huge dis- 

 tance,"T which would destroy the value of all Tycho's tables, after he had himself adopted 

 the Tychonian value in the Ephemeris a few years before ; but Keppler replied** that he had 

 studied the subject with care, and did not hesitate to reduce Tycho's parallax by 1' 40", or two- 



Epitome Astronomies Copermcaruf, pp. 478-480, 486-490. 



fKicciOLi, Almag&lum Novwn, I., 107. TYCHO BBAHE, Progymn., pp. 97,415,463. 



%De Motibus Stella Mortis, p. 71. 



KICCIOLI, Almag. Nov. I, 108. HANSCH, Epulote ad J. KEPPLEBUM, p. 473. 



||GA8SENDi, TYCHONIS Vita, p. 102. 



^| IBID. pp. 473-4. CRUDER to KEPPLER, 1624, July 15. 



"Letter to PETE CHUOER, 1624, September 9. HANSCH Epitt. p. 455. 



