EROS, an asteroid of exceptional interest owing to its close approaches to the earth. In 1898 the number of known asteroids had mounted up to over four hundred, and as there was consider able monotony in their appearance and in the nature of their orbits many astronomers were beginning to question whether the time spent in searching for them and in calculating their movements was worth while. Then came a discovery, which might never have been made had the search been abandoned, that gave full compen sation for all the time thus spent. On Aug. 13, 1898, Dr. G. Witt, at the Urania Observatory, Berlin, discovered photographically a faint asteroid of the 1 1 th magnitude that was retrograding at the unprecedented rate of half a degree per day; this unusual motion caused it to be widely observed, and after a short interval Dr. Berberich of the Berlin Rechen-Institut published elements which proved to be very near the truth. The new body was designated at first 1898 DQ, but subsequently received the permanent number 433 and the name Eros (masculine names are given only to aster oids with very exceptional orbits, feminine ones being the rule).
Its period of revolution is 643 days, as compared with 687 for Mars. It was already known that a few asteroids come nearer to the sun at their perihelia than Mars at its aphelion ; but Eros's mean distance is considerably less than that of Mars. As its ec centricity is 2/9, its least distance from the sun is only 113 astronomical units, so that it can approach the earth within 14 million miles, a much smaller distance than that of any other planetary body except the moon. The distance is rendered less by the fact that the point where Eros crosses the plane of the earth's orbit (descending node) is distant only 2 4 ° from the perihelion point; since the orbit is inclined some 'or to that of the earth, the least distance would be twice as great if the node were 9o° away from the perihelion.
Unfortunately very near approaches of Eros to the earth are rare; they occur only when the planet passes perihelion about the date Jan. 22, of any year. It was unlucky that the conditions were the most favourable possible in Jan. 4 i years before the dis covery; indeed subsequent examination of the immense stores of photographic plates exposed at Harvard College Observatory re vealed 17 images of Eros at the opposition of 1893-94, and four images on plates taken at Arequipa, Peru, at the opposition of 1896. These images were used to give a better determination of the orbit, but the opportunity was missed of obtaining observa tions for the solar parallax.
The synodic period of Eros (that is, the average interval be tween one opposition and the next) is 8451 days, or two years and 115 days. Three synodic periods fall short of seven years by about three weeks ; it follows that there was an approach to favourable conditions at the opposition at the end of 19oo; and a parallax campaign was carried on at that time at the leading observatories of the northern hemisphere, the southern hemisphere being ex cluded owing to the high north declination of Eros (see article PARALLAX). Another very favourable opposition occurred in 1931. A convenient measure of the value of an opposition for parallax purposes is given by the number of days that elapses be tween the planet's passage through perihelion and the earth's pas sage through the longitude of that perihelion.
The following figures are from Monthly Notices of the Royal Astronomical Society for Feb. Igo' ; it gives the interval in days, the minus sign indicating that the planet comes first to the longi tude, the plus sign that the earth comes first: 1894, 0.4o days; 1901, +17.42; 1923-24, +7.95; ; 17.13; I .54; 1982, -1-14.05 These are all the oppositions up to the year 2000, for which the interval is less than 24 days. Dr. Witt's later value for 1931 is 5.91 days, so that the table needs a correction of -}-I a days at that point. The correction in 1975 may be about twice as great, which would change the sign of the interval without greatly alter ing its magnitude. It will be seen that 81 years brings back a recur rence of conditions, this being equal to 46 revolutions of Eros.
It will be seen that the 1931 apparition is the best one till (though 1938 is not much inferior to it) and extensive preparations are already being made to utilize it to the utmost ; these include accurate observations of many stars near the planet's track, to serve as points of reference. A valuable by-product of the parallax investigation is a determination of the mass of the moon; the moon causes the earth to describe a small circle the radius of which is about 3,00o miles in a monthly period; this motion produces a wave in the apparent motion of Eros, with the same period. A peculiarity in the apparent motion of Eros at a perihelion opposi tion is that, unlike most of the superior planets, it does not retro grade, since its linear velocity, even when multiplied by the cosine of its inclination, exceeds that of the earth. Eros was unique in this respect when discovered, but Albert, Alinda, and Ganymede have since been found to share the peculiarity.
The motion of Eros, as seen from Mars, would be very curious; each planet might witness a transit of the other across the sun, while at other times each would see the other in the polar constel lations. Owing to the large eccentricities of their orbits, each would in turn gain on the other in longitude. Their synodic period, or average interval between their conjunctions, is 27.6 years; but the unequal rates of motion would cause large variations in the intervals. The least distance of Mars from Eros is about 21 mil lion miles, half as great again as Eros's least distance from the earth. Owing, however, to the slow secular movements of nodes and perihelion points, there must have been times in the distant past when very close approaches of Eros and Mars were possible; and it has been conjectured that Mars was responsible for intro ducing Eros into its present orbit at such an encounter. While this is possible, Eros's small perihelion distance stands less in need of explanation since it has been found to be common to three other planets (Albert, etc.) .
The perturbations of Eros by the earth are very considerable. Prof. H. N. Russell published some computations about them in Astron. Journ. Nos. 483, 484. The largest term is 747" sin (7g 4g') where g, g' are the mean anomalies of Eros, and the earth. The apparent shift in the position of Eros is magnified by its proximity, and the total displacement from one extreme to the other may amount to nearly three degrees; this shift will in time give a very accurate value of the earth's mass in terms of that of the sun, and hence of the sun's distance. But the period of the term is 414 years, and the method will scarcely reach its full effi ciency till two or three of these periods have elapsed. However, in 1921 Herr Noteboom published a determination of the sun's parallax by this method, for which he found the small probable error of oo I ", his value being 8" 7 99. Prof. Russell also indicated the principal terms of the perturbations of Eros by Mars; the three largest, omitting one of very long period, have coefficients II"59, 22"81, I I"85, and periods 408 years, 78.o years and years. The period of the first is so close to that of the large earth term as to require a long interval to elapse before they can be separated.
The diameter of Eros is conjectured to be about 17 miles, as suming its albedo to be the same as that of Pallas. It appears somewhat brighter than the seventh magnitude at the most favour able oppositions, and somewhat fainter than the I I th at the most unfavourable ones; its mass would be 1/167,00o,0oo of the earth's, assuming its density equal to that of the moon. Its angular diam eter may be 4" in Jan. 1931, an amount that would be measurable with a large interferometer. Such measures might also decide whether the planet's outline is circular or irregular; there appears to be no reason to expect such small bodies to be spherical, since gravity would be weak compared with cohesion. The fact that the light of Eros shows irregular variability lends colour to the hy pothesis of non-sphericity. In Feb. I 90 1 the light-range exceeded a magnitude; the period being 5 hours 16 minutes; there were two unequal maxima and minima in this period, but three months later the light-variation almost disappeared. The variation was noticed again in 1903, but was absent in 1907. It is suggested in explanation that the axis of the planet is highly inclined; when the pole of rotation is central on the disc there would be no light variation. It may also be noted that if the axis of rotation were not a principal axis it would shift within the body.
Eros will be easily seen with binoculars at the beginning of 1931. The nearest approach to the earth, 161 million miles, is near the midnight between Jan. 3o and 31. (A. C. D. C.) EROSION OF LAND. The process of removal of portions of the earth's crust by natural agencies, of which the most im portant is water. The wastage of the sea coast, or coast erosion, is described in the article COAST PROTECTION AND RECLAMA TION ; it is brought about in the main by the action of the sea waves but also, in part, by the disintegration or degradation of cliffs by atmospheric agents such as rain and frost and by tidal scour. In rivers and estuaries the erosion of banks is caused by the scouring action of the river stream, particularly in times of flood and, in the case of estuaries, also by the tidal flow on the ebb tide when river and tide-water combine in their erosive action. (See RIVER ENGINEERING.) The land surface generally is subjected to a continuous process of erosion by the action of rain, melting snow, ice and frost, the resulting detritus and sediment being carried by small streams into the rivers and thence to the ocean (see GEOLOGY). In some arid and desert tracts wind has an important effect in bringing about the erosion of rocks by driving sand ; and the surface of sand dunes, where not held together and protected by vegetation, is subject to ero sion and change by the drifting of blown sand.