EROS, in astronomy, one of the minor planets, discovered photographically by Witt in 1898, at the Urania Observatory, Berlin. The orbits of most of the other known asteroids lie wholly beyond that of Mars; but Eros ap proaches much nearer to the sun, and at times it may be within 13,500,000 miles of the earth. It is this fact which gives the planetoid its great astronomical interest The relative dimen sions of the solar sy-stem are known with high precision, and if any dimension can be accu rately measured in miles, all the other dimen sions become known at once, in terms of the same unit. It is apparently possible to determine the parallax of Eros (and hence its distance from the earth in miles) with relatively high precision and a correspondingly accurate deter mination of the absolute dimensions of the solar system in general will result. As Eros ap proaches the earth more closely than any other heavenly body except the moon, its parallax is relatively large; and the fact that its diameter does not exceed 20 miles, so that it appears in the telescope as a mere point of light without a sensible disk, indicates that extremely precise micrometric measures of its position on the heavens may be had. Astronomers are keenly alive to the possibilities offered by this seem ingly insignificant little planet, and at every favorable opposition Eros will be studied with exceeding care.
The planet itself is known to be a little world, nearly round, which revolves about the sun in a period of 643 days. A very remark
able fact about it is that it is found to vary periodically in brightness; when brightest it is more than three times as bright as when faint est, the period of a complete variation being somewhat more than five hours. It was sup gested that the apparently single planet is, in fact, two planets, so close together that they ap pear to us as one, the time of their revolution about their common centre of gravity being twice the period of the apparent variation in brightness. More careful photometric study, i however, renders it certain that the planet is single, having one side much brighter than the other, and that its variation in brightness is due to its axial rotation.
An asteroid very similar to Eros was dis covered by Wolf on 4 Feb. 1918, and although the orbit of this new body is far larger than that of Eros, the eccentricity is so great (0.553), that when nearest to the earth it is but little more than 17 millions of miles from us. Thus this asteroid and Eros come nearer to us than any other planets of the solar system. It hap pens that the time of nearest approach for both of these bodies is toward the beginning of the year 1931. It is probable that from observa tions made at this time the distance of the sun will be ascertained with an accuracy far trans cending that available at present.