Collier's New Encyclopedia (1921)/Sun, Parallax of the

SUN, PARALLAX OF THE, or SOLAR PARALLAX, the angle subtended by the radius of the earth at a distance equal to that of the earth from the sun. More exactly, it is the angle subtended by the equatorial radius of the earth at the mean distance of the sun, though this would be called, when speaking exactly, the mean equatorial horizontal parallax of the sun. It is, as nearly as we know it today, almost 8.8", with an uncertainty of probably not more than 0.01" or 0.02" different from this in either direction. This corresponds to a distance of the sun a little less than

93,000,000 miles. More time, labor, and money have been expended in the determination of the polar parallax than of any other astronomical constant, on account of its importance in giving us an accurate base line with which to gauge the solar system and then strike out into the depths of stellar space. Before the last transit of Venus it was thought that careful observation would give an exceedingly accurate value of this constant; but these hopes were disappointed, and it is known that we have at hand at any time more accurate methods of determining the distance of the sun than from any possible observation of a transit of Venus. The most accurate direct method is likely to be the direct measurement of the parallax of the nearer and brighter asteroids by the heliometer.