Agrippa, who observed in Bithynia, records that in the twelfth year of Domitian [92 CE], on the seventh of Metroos according to the calendar of that region, at the beginning of the third hour of the night, the Moon occulted the rearmost, southern part of the Pleiades with its southern horn. That moment is in the 840th year from Nabonassar, Tybi [V] 2/3 in the Egyptian calendar [92, Nov 29/30], 4 seasonal hours before midnight, or 5 equinoctial hours (since the Sun was approximately in Sagittarius 6°). Therefore, reduced to the meridian of Alexandria, the observation occurred 5⅟₃ equinoctial hours before midnight, or 5¾ with respect to mean solar days. At this moment, the positions of the centre of the Moon were as follows:
| True longitude: | Taurus 3;7° | True latitude: | 4⅚° north of the ecliptic |
| Apparent longitude: | Taurus 3;15° | Apparent latitude: | 4° north of the ecliptic |
(for the culminating point was two-thirds through Pisces). Therefore, at that time the rearmost section of the Pleiades was, in longitude 33¼° towards the rear from the spring equinox, and [in latitude] 3⅔° north of the ecliptic.
These calculations by Ptolemy and the reported observation by Agrippa accord quite well with modern calculation as verified with Stellarium, although there are some discrepancies. Ptolemy states that Agrippa says that the time was the beginning of the third hour of the night, that is two hours after sunset, although it is not clear if he was using equinoctial hours or seasonal hours. Modern calculation shows that sunset in Nicaea, Bithynia was at 16:33, about an hour and a half before the middle of the occultation.
At the stated date (November 29th, 92 CE), the Moon was in waxing gibbous phase (91% illuminated) and the two main stars of the Pleiades that were occulted were 19 Tau and 20 Tau in the southeastern sky. The occultation lasted from 17:18 to 18:25 local time (that is, about 6 equinoctial hours before midnight). However, 19 Tau and 20 Tau constitute the northern part of the Pleiades, not the southern part as Agrippa states. At mid-occultation, approximately 17:57 (Ptolemy: 17:45) the actual coordinates of the Moon were:
| Coordinate System | Longitude | Latitude | |
|---|---|---|---|
| Ecliptic | λ = 33° 05' | β = 4° 24' | (Ptolemy: 33°15', 4°00') |
| Equatorial | α = 1h 57m | δ = 16° 47' | |
| Horizontal | A = 101° 28' | a = 38° 52' |
For a discussion of the discrepancies, see Toomer's note 70 in his Almagest translation.
Lunar Crater: Agrippa (diameter 44 km)
Last updated 26/04/20
No comments:
Post a Comment