The Total Eclipse of the Moon, January 28

Abstract

A TOTAL eclipse of the Moon offers some special advantages for the exact determination of the diameter and distance of our satellite. Observations of the bright limbs are exposed to considerable errors from the effect of irradiation, and liable to be affected by personal habit in the observer. The method of occultationshas, under ordinary circumstances, proved scarcely more successful, owing chiefly to the fact that immersion and emersion so seldom take place under similar conditions. But in a total eclipse of the Moon, the disappearances and reappearances occur at limbs under similar illumination, and since the diminution of the Moon's light allows much fainter stars to be seen close to the Moon than can usually be observed, a much greater number of observations can be made than under ordinary conditions, and the effects of local irregularities of the Moon's circumference can be eliminated by observations made at a great number of points. If, then, as many Observatories as possible would combine to observe the occultations of the small stars passed over by the Moon during its eclipse, the labours of a few hours would give materials for a better determination of its diameter and parallax than could otherwise be obtained from the observations of many years. In view of these advantages, and noting too how hitherto they had been neglected by astronomers, Dr. Döllen, of Pulkowa, published a paper in the Astronomische Nachrichten, No. 2615, previous to the eclipse of October 4, 1884, in which he gave a catalogue of 116 stars which would be occulted during that eclipse, and begged for the co-operation of as many observers as possible. Unfortunately, the weather in many places was very unfavourable, and even where the sky was clear an unforeseen hindrance to observation was experienced in the unusual faintness of the eclipsed Moon. The part of the sky, too, through which it was passing was bare of stars above the 9th and 10th magnitudes. Still the results were sufficiently successful to encourage Prof. Struve and Dr. Dollen to repeat the attempt, especially as under several aspects the approaching eclipse of January 28 presents more favourable conditions than that of October 4, 1884: the magnitude of the eclipse will be somewhat larger, and the duration of the total phase a few minutes longer. Accordingly, Dr. Döllen has drawn up a catalogue of 300 stars which will be occulted, whilst Prof. Struve has computed by a graphical method the times of disappearance and reappearance, and the position-angles of the occulted stars, for 120 Observatories, which he has invited to cooperate with him in the work of observation. The experience gained during the 1884 eclipse has led Dr. Döllen to include only those stars occulted during the total phase or immediately, before and after, but he has thought it well to give stars down to the 11th magnitude.