[9] Ancient Meaning of the Full Moon

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What did the full moon mean to the ordinary person in ancient times? We have one example of what it meant to the Jewish philosopher Philo who lived in Alexandria, Egypt and who wrote in the early first century. On page 17 of Philo_QE (section 9), in a context concerning Passover, Philo wrote, “For when it [the moon] has become full on the fourteenth (day), it becomes full of light in the perception of the people.” On page 401 of Philo_7 (Special Laws 2:155), in a context concerning the seventh month, Philo wrote, “The feast begins at the middle of the month, on the fifteenth day, when the moon is full, a day purposely chosen because then there is no darkness, but everything is continuously lighted up as the sun shines from morning to evening and the moon from evening to morning and while the stars give place to each other no shadow is cast upon their brightness.” We see here that Philo considers both the 14th and the 15th days of the month to be days of the full moon. Hence he does not consider the full moon to be an instant in time or only one day of the month, but a general period when the moon is quite circular. As an ordinary person he did not adopt the meaning for the full moon of advanced Greek astronomers as a mathematically predicted moment when a lunar eclipse would sometimes occur. Due to the elliptical orbit of the moon, this mathematical moment will vary by a few days in relation to the conjunction, and it will also vary by a few days in relation to the new crescent. The precision of mathematics was not Philo's approach to the meaning of the full moon.

Although Philo, a Jew who lived in Alexandria, Egypt, is a historical first century witness that the moon is full on the 14th and 15th days of the Jewish months, this is not a biblical argument that a biblical month is full on the 14th and 15th days of the month.

In the first century BCE Vitruvius wrote the views of the Greek astronomer and mathematician Aristarchus of Samos (c. 280 BCE) concerning the full moon. On page 264 Vitruvius (translated by Morgan) wrote, “On the fourteenth day, being diametrically across the whole extent of the firmament from the sun, she is at her full and rises when the sun is setting.” This is approximately the rule given above, namely the first evening in which the moon rises in the east after the sun sets in the west begins the day after the moment of the full moon. However, Philo of Alexandria took a looser concept of the full moon allowing both the 14th and 15th days of the month to be days of the full moon.