Page:Popular Science Monthly Volume 58.djvu/656

648 telescope, it is probable that Eros can be 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 distance. The methods which are in use for the solution of this problem may be divided into three groups, geometrical, gravitational and physical. The present investigation belongs to the first of these. The natural 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 subtends at the sun itself. If the distance is the earth's radius the measured angle is the solar parallax. In fact, however, this apparently easy and direct method has now no value whatever, since the angle concerned 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 conditions. 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 astronomical 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 advantage. 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 better 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 connection it may be of interest to recall that Halley, who first pointed out the