August 16, 1907] 



SCIENCE 



197 



that we should not arrive at any certainty 

 as to the true value of the solar parallax 

 from observations of transits of Venus, but 

 that the modern heliometer applied to the 

 measurement of angular distances between 

 stars and the star-like images of minor 

 planets would yield results of far higher 

 precision. 



The results of the observations of the 

 minor planets Iris, Victoria and Sappho at 

 their favorable oppositions in the years 

 1888 and 1889, which were made with the 

 cooperation of the chief heliometer and 

 meridian observatories, fully justified this 

 prediction.* The sun's distance is now 

 almost certainly known within one thou- 

 sandth part of its amount. The same 

 series of observations also yielded a very 

 reliable determination of the mass of the 

 moon. 



The more recently discovered planet 

 Eros, which in 1900 approached the earth 

 within one third of the mean distance of 

 the sun, afforded a most unexpected and 

 welcome opportunity for redetermining the 

 solar parallax — an opportunity which was 

 largely taken advantage of by the prin- 

 cipal observatories of the northern hemis- 

 phere. Unfortunately the high northern 

 declination of the planet prevented its ob- 

 servation at the Cape and other southern 

 observatories. So far as the results have 

 been reduced and published^ they give an 

 almost exact accordance with the value of 

 the solar parallax derived from the helio- 

 meter observations of the minor planets, 

 Iris, Victoria and Sappho in 1888 and 

 1889. 



But in 1931 Eros will approach the earth 

 within one sixth part of the sun's mean 

 distance, and the fault will rest with as- 



' Annals of the Cape Observatory, Vol. VI.. part 

 6, p. 29. 



^Monthly Notices R.A.S., Hinks, Vol. LXIV., 

 p. 725; Christie, Vol. LXVII., p. 382. 



tronomers of that day if they do not suc- 

 ceed in determining the solar parallax 

 within one ten-thousandth part of its 

 amount. 



To some of us who struggled so hard to 

 arrive at a tenth part of this accuracy 

 under the less favorable geometrical con- 

 ditions that were available before the dis- 

 covery of Eros, how enviable seems the 

 opportunity ! 



And yet, if we come to think of it 

 rightly, the true opportunity and the chief 

 responsibility is ours, for now and not 

 twenty years hence is the time to begin our 

 preparation; now is the time to study the 

 origin of those systematic errors which un- 

 doubtedly attach to some of our photo- 

 graphic processes; and then we ought to 

 construct telescopes specially designed for 

 the work. These telescopes should be ap- 

 plied to the charting of the stars near the 

 path which Eros will describe at its op- 

 position in 1931, and the resulting star- 

 coordinates derived from the plates photo- 

 graphed by the different telescopes should 

 be rigorously intercompared. Then, if all 

 the telescopes give identical results for the 

 star-places, we can be certain that they 

 will record without systematic error the 

 position of Eros. If they do not give iden- 

 tical results, the source of the errors must 

 be traced. 



The planet will describe such a long 

 path in the sky during the opposition of 

 1931 that it is already time to begin the 

 meridian observations which are necessary 

 to determine the places of the stars that 

 are to be used for determining the con- 

 stants of the plates. It is desirable, there- 

 fore, that some agreement should be come 

 to with respect to selection of these refer- 

 ence-stars, in order that all the principal 

 meridian observatories in the world may 

 take part in observing them. 



I venture to suggest that a Congress of 



