|Hipparchos of Nicäa († ca
The most important astronomer of his time was the first to describe the apparent motion of the stars due to the Earth's precession: The star Spica had drifted by 2° since his predecessor Timocharis had described its occultation by the moon. This had happened in the 36th year of the Alexander era of Callippus.
Meanwhile this would go back about 200 years...
In fact 2° precession equal only 144 years. Hipparchus would therefore have dated the epoch 54 years too early:
When it comes to the orientation on the course of the moon, an error of 54 years easily goes unnoticed, since after this period of time solar and lunar eclipses repeat at almost the same place and time of day - something Hipparch himself had discovered and named 'Exeligmos'. The error led to a triplication of Alexander's epoch:
A. The Alexander Era after Callippos:
In the 32nd year of the 3rd Callippus cycle, the 178th year after Alexander's death, Hipparchus had observed the autumnal equinox. If the years were counted correctly, the 'year 1' was 183 years ago. (Aristarch had observed the summer solstice in the 50th year of Callippus, the 44th after Alexander's death → †Alx 6.CP1)
B. The alternative Alexander Era of Hipparchos:
The year 50.CP.3 minus 200 years minus 35 (i.e. 1.CP.1 -54 years)
Timocharis' observations referred to the Alexander Epoch, which was established by Kallippos at the solstice at new moon before the conquest of Babylon. As Hipparchus concluded, about 200 years would have passed since then.
Timocharis' report states, the moon covered Spica at the beginning of the third hour after sunset in Alexandria [~17:40ut] on the 15th Elaphebolion|5th Tybi in the 36th year of Callippos (i.e. February 9th 60ce).
With a lunar diameter of 0.4°, moving within a band of ±5.1° from the ecliptic, the chance of obscuring spica within any given month is about 1:25. Observation on Luna XV of a given month and within a given hour further reduces the chance by 1:8500 (i.e. 1:29.5 x12 x24). Furthermore: The visual placement of the contact at the moon's limb may be accurate within ±2' (i.e. 1/12 of the moon's diameter): Timocharis' observation was definitely unique throughout antiquity!
C. The Seleucid Era [Epoch 311bc]:
Since this epoch is not compatible with Alexander's biography, it was attributed to Alexander's General: Seleukos Nicator the Great was said to have conquered and 're-founded' Edessa 19 years (i.e. one lunar cycle) after Alexander, then also Gaza and Babylon, before he started another campaign to India. After all, a chronology of Alexander IV that was irrelevant to him would have taken over?
According to Jewish tradition, the Seleucid era began in the sixth year of Alexander the Great's reign [i.e. trad. 330bc]. Alexander was linked to the Jewish chronology by a tax exemption in the Sabbatical year - likewise Callippus' era by the beginning of the year for the Jews, which was linked to the course of the moon; obsolescence by 54 years would therefore have been contradictory. However, 19 years later (54j-19j=35j=5x7j), the cycles of the moon's course and the Sabbatical year coincided with Hipparcho's calculation. Precession of the celestial bodies 35 years before Timocharis would in fact have been ½° (which could possibly be verified by lore), and the precession calculated by Hipparchus for 54 years likewise resulted in ½°.