OCCULTATION. This word, which might serve to design any eclipse of one heavenly body by another, and even the effect of cloud or fog, is particularly applied to the eclipse of a fixed star by the moon.
It has been seen [MooN] that the spiral course of the moon's real orbit must bring her at some time or other in the course of a revolution of the nodes (181 years) at or near to every star situated within about 5° of the ecliptic either way. There are consequently continual oceul tattoo, of stars by the moon; such of which as eau be Imola useful in finding the longitude are given ycnrly in (he' Nautical Almanac.' If the moon were (like the star) so distant that two spectators at nearly opposite points of the earth would not be sensibly removed from each other in space when their distance from each other was compared with their common distance from the moon, then all observers, where. some situated, would me the moon begin to hide a starat the same instant. They would all then be able to note by their different clocks the absolute instant of the same phenomenon ; and [Loxorreet] their differences of clock-time at the absolute instant would give their difference. of longitude. But the proximity of the moon makes it possible that one observer (at Al nifty see no occultation at the time when such a phenomenon his osscunsel to another (at a); and wakes it certain that two observers cannot see the beginning of an occultation at the Jame instant of absolute time. Both therefore must deduce from their own observe( times of commencement, and from their knowledge of the moon's place and motions, the times, at their severs' places, at which a spectator situated at the earth's centre would see the star touch the moon's limb ; and this being done, they are in the same position as if the moon had been too distant to require each a process. If, in this work, we had given the details of astronomical calculations, we should not have 'separated that required in reducing an occultation to the earth's centre from the similar ease which occurs in an eclipse of the sun or a transit of Venus: the principles employed in all are identical, though the first proems is rendered more simple than the second or thinl, by the very great distance of the star, which may be considered as marking the same point of the heavens stall places. For
the value of observed occultations in finding the longitude, see the article LONGITUDE.
Here we might close this article, if it were not for a singular cir cumstance which sometimes occurs ; and which cannot be traced either to the character of the telescope employed, of the observer, of the rather during the observation, or of the particular star under occulta tion. When the moon approaches the star, instead of an instant of contact, followed by disappearance of the star, the Latter for a few seconds is frequently thrown upon the moon, as if it were the nearer body, and were going to cross the moon's eurface,an Venus does that of the run during a transit. While this is taking place, which sometimes lasts until the star has fairly left the moon's limb or border, the star preserves its light, though it frequently undergoes a change of colour. Aldebaran, whose natural colour is inclining to red, has been seen to present this phenomenon much more frequently than any other star ; but e Tauri, et and 1 Mamba, Spica Virginia, Regulus, y Libras, 132 Tauri, at Caneri, 49 Librie, A Aquarii, 219 Aquarii, 187 Sagittarii, y Tauri, p Leonis, p Oeminorutn, 8 Cancri, 8 Pisciuni, ke., have been seen more or lees to come upon the moon's limb. See a detailed list of quotations from the different observers in a piper by 31r. South, On the Occultation of 8 Piecium by the Moen,' in the thinl volume of the Memoirs of the Astronomical Society.' It is to be noted that many of these appearances did not exhibit the whole of the phenomenon, but made the star hang for some iteconds upon the moon's limb, instead of immediately dimppearing.