Note 527 — Chapter VI · LUNAR CALENDARS AND THE WEEK

The lunar or synodic month, determined by the synodic revolution of the moon, is the time between two successive conjunctions of that luminary with the sun, and may be measured from new moon to new moon or from full to full. It varies about thirteen hours by reason of eccentricities of the moon’s orbit and of that of the earth about the sun, but its mean value is 29 days, 12 hours, 44 minutes, and 3 seconds. The length of the “light month,” or period of the moon’s visibility, though commonly taken at three days, is a variable quantity. It is the usual practice to assume that the moon becomes visible on the first evening when she is more than thirty hours old at sunset. Her mean age when first seen is, therefore, 30 hours + 24/2 hours = 1 day, 18 hours. See J. K. Fotheringham, “On the Smallest Visible Phase of the Moon,” Monthly Notices of the Royal Astronomical Society, 1910, lxx, 527-531.