648 POPULAR SCIENCE MONTHLY. 



tographic telescope, it is probable that Eros can Le photographed there 

 after it has been lost sight of at other observatories. At least, the first 

 determination of its position at the recent opposition was made from 

 a photograph obtained there by Dr. Stewart. The interest shown by 

 these two institutions is equaled by that of many other observatories 

 in Europe and the United States. The chief object of these labors is 

 the determination of the solar parallax, which is the angle subtended 

 at the sun by the earth's radius, and which is a measure of his dis- 

 tance. The methods which are in use for the solution of this problem 

 may be divided into three groups, geometrical, gravitational and phys- 

 ical. The present investigation belongs to the first of these. The 

 uatural and direct method for measuring the sun's distance would be 

 to select two stations on the earth, whose distance apart must be 

 known, and from them measure the angle which that distance sub- 

 tends at the sun itself. If the distance is the earth's radius the meas- 

 ured angle is the solar parallax. In fact, however, this apparently easy 

 and direct method has now no value whatever, since the angle con- 

 cerned is too small to give the best results, and also the sun is a very 

 difficult object on which to make measurements of precision. Some 

 other, nearer and more suitable object must be sought, and, in quest 

 of the most exact results possible, astronomers have observed Venus, 

 when in transit across the sun's face, Mars near opposition and various 

 asteroids. Of these different geometrical methods, observations of 

 the asteroids appear to have furnished the best results, so that the 

 discovery of Eros comes at a most fortunate time to give astronomers 

 an opportunity of testing this method under the most favorable con- 

 ditions. It must be remembered, however, that the recent opposition 

 of Eros was not an especially favorable one, and it is not certain that 

 better results will be obtained at this time than have been secured 

 in recent years by Dr. Gill at the Cape of Good Hope, in cooperation 

 with Dr. Elkins, of Yale, and others. That work depended upon 

 heliometric observations of the asteroids Iris, Victoria and Sappho, 

 whose least distances from the earth are 0.84, 0.82 and 0.84 astronom- 

 ical units. At the recent opposition the distance of Eros was little more 

 than a third as great, and this in itself gives Eros an enormous advan- 

 tage. It has been feared, however, that the faintness and rapid motion 

 of Eros would prevent observations of the highest precision, which 

 might be sufficient to balance the advantage which its nearness gave. 

 Probably the difficulties on these accounts have not proved so great 

 as was at first feared. Even if the present determination yields no bet- 

 ter results than have been obtained before, it will make a very valuable 

 check on previous determinations, and bring out the best methods to be 

 pursued at some later and more favorable opposition. In this connec- 

 tion it may be of interest to recall that Halley, who first pointed out the 



