NEW MEASUREMENTS OF DISTANCE OF SUN. 107 



visual measurement of distance from star to star; the observations 

 extended over months instead of hours ; the}^ could be pursued without 

 any of the disquietino- feelings that a temporary breakdown would 

 ruin everything; and they were brought to a successful end in a i)aral- 

 lax of 8 -78 seconds. But one doubt was cast upon the result. "Was it 

 possible that the red color of Mars had influenced the measures syste- 

 matically ? It could not be denied that the effect of the dispersion of 

 the air, which gave the planet a blue fringe above that might be lost 

 in the blue sky, and a red fringe below that would be indistinguish- 

 able from the red planet itself, might have produced some effect; and 

 Sir David Gill resolved to try again, utilizing this time three minor 

 planets farther away than Mars, with less parallax therefore but with 

 disks so small that they were indistinguishable from stars. 



In 1888 and 1889 five observatories, the Cape in the Southern 

 HemisjDhere, and Yale, Gottingen, Leipzig, and the Eadcliffe Obser- 

 vatory at Oxford in the Northern Hemisphere, combined to observe 

 the planets Victoria, Iris, and Sappho with the heliometer. The 

 labor was immense. The observations proved to be so accurate that 

 they demanded the use in a great part of the work of eight figure 

 logarithms. When only a few years ago the whole work was 

 published in two enormous volumes of Annals of the Cape Obser- 

 vatory, it might well have been thought that here was the last word 

 of observation for many years. Yet Ave are now attacking the 

 i:)roblem with more energy than ever. 



About ten years ago the end of the century was in sight, and there 

 was a general impression abroad that it was time to set one's house 

 in order and to make a good start on the 1st of January, 1001. The 

 directors of the four nautical almanacs (the British, French, German, 

 and American) resolved to meet in Paris in 1896, and w^ith the help 

 of certain distinguished advisers to agree upon a uniform set of con- 

 stants to be adopted in all the Almanacs from the y&ar 1901. Among 

 these constants was the solar parallax. ^Ve may summarize the infor- 

 mation which was at the disposal of the conference thus: 



Solar purallax from — Seconds. 



Gill's heliometer, minor planets 8 • 802 



Constant of aberration of ligbt 8 ■ 7i)!) 



Parallactic inequality of moon 8-7!)l 



Mass of earth from motion of node of Venus alone S' 7()2 



Mass of earth from secular variation of four inner planets 8- ")<) 



Gill's heliometer measures of minor planets gave 8 -SO^ seconds, and 

 no other direct observational result could be compared with this; (lie 

 transits of Venus w^ere discredited even though some of the final 

 results had not, and have not even now, been pul)lisluMl. Tlu' most 

 recent determinations of the constant of the aberration of light gave 

 8 -799 seconds, the parallactic inequality of the moon, 8 -794 seconds. 



