The other accredited method of determining the parallax is by observation- of the planetoid-. The method i- extremely simple in practice. and also very accurate. It is necessary merely to await a time when one of these little bodies is favorably situated for observation. and then to fix its pe-ition tele=copically with respect to the neighboring -tar-. Let this be done simultane ously at two observatories situated very f apart on the earth. and using. ef course. the same stars in both places. Then it is clear that the position determined for the planet will not be quite the same at the two observatories. The baseline is the straight line joining the two ob servatories. A= a result we obtain directly the distance of the planet from the earth. in terms of this base-line as a unit. But this is known in miles from existing geodetic survey- of the earth. Thus we arrive at a knowledge. in miles, of the planetoid's distance. But from the ordinary proc of astronomical oh-ervaiion. we know the =o-called element= of the rs orbit. (.=.ee ELEMENTS.) We can find from these. by a sim ple computation based on Kei ler's laws. the dis tance of the planetoid from the earth in term- of the latter's distance from the sun. Knowing then the distance between the earth and planetoil in miles from the special parallax observations. and in terms of the distance 'earth-sun' from theory, we can at once deduce the value of 'earth sun' in miles. This. combined with our knowl edge from geoclesy of the terrestrial radio- in miles. enables u- to empute the angle subtended by that radio, to an observer in the sun—in other the solar parallax.
The planetoid method was put in operation by no he-- than twenty-five observatories. acting in corTeration. By thus increasing the number of
observation stations the precisii n of the final result could not fail to be greatly enhanced. The entire was planned and managed by Gill. of the Cape of Good Hope n,-ervat,,ry. The takinz a principal part in the work. in addition to that at tl e t ape of Good pe, were those at Bamberg. Leipzig. and, in this country. Yale University. All these ob=ervati ries are provided with the n odern In li on eter 19.v.), the most precise al paratu= for measurement en the sky at i resent known to seicnee.
The parallax determination= through the con stant of aberration have been made princilally in c nnecti n with the study of latitude varia tion.
It may be of interest here to -how the importance or 'weight' of the various parallax meth T' e general work of high multi rity on this t pie i= the Fumitn-• ti k ir( t, by "'imon Newcomb 1Washingt n. . assiLmed the followinz ei • ' or relative d0g,rees of precision: NeN%comb already at that time in posses sion of preliminary results of Gill's researches, comD11111Wated to him in advance of publication. But after his writing several important series of aberration determinations were made public, and the results exhibited certain small discord ance, among themselves. which discredited some what the aberration method, and consequently enhanced the relative weight of the planetoid method. The latter method gained also from the discovery in 1598 of a new planetoid, much more favorably situated for parallax determinations than any previously known. See EROS.
The final result for the solar parallax at pres ent accepted by astronomers in general is S".80, corresponding to a distance of 92,759,000 miles between the earth and the sun.