Lunar Occultations

Lunar occultations are easily observable phenomena explicit in some ancient observations and implicit in some myths. They are therefore of archaeoastronomical interest. The Moon can occasionally be observed to pass near (sometimes called an appulse—but see §2.1) or in front of (an occultation) a star or planet. All the bright planets, Uranus, and all the stars within the major standstill declination range of the Moon, ±~28.5°, are regularly occulted by the Moon. The first magnitude stars Aldebaran (a Tau) at 8 = +16°5, Antares (a Sco) at 8 = -26°5, Regulus (a Leo) at 8 = 12°0, and Spica (a Vir) at 8 = -11.0, (cf. Table 3.1) are occulted more than once every 18.6 years. Pederson (1987) points out the importance of such events for ancient astronomy. The Alexandrian astronomer Timocharis observed the occultation of Spica (a Vir) in 294 b.c. and a conjunction of Spica with the Moon in 283 b.c.; subsequently, Hipparchos established the position of Spica relative to the autumnal equinox during two lunar eclipses in 146 and 135 b.c. From Hipparchos's lunar theory, he would have been able to find the longitudes of Spica from the earlier observations and, with these data, find the shift in Spica's position relative to the equinoxes. In this way, he could have found the precession of the equinoxes; this is, in fact, the way Ptolemy claimed to have done it and demonstrated how it was done (cf. Toomer 1984, pp. 327ff). Aristotle (de Caelo, 2,12) recorded his own observation of the occultation of Mars— watching it disappear behind the east limb of the Moon and reappear beyond the west limb. His interest was purely cos-mological (no times or dates are given): A demonstration that the Moon is closer than is Mars. Stephenson and Clark (1978) recovered this information: They state that the sole Mars occultation visible in Athens during the appropriate time interval occurred on May 4, 357 b.c. A RedShift simulation confirms that an occultation of Mars did occur on this date and could have been viewed in Athens.

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