The importance of astronomical observatories among early civilizations, especially among early agricultural societies, came directly from the need of people to construct calendars on which they could incorporate their celestial observations to predict the position of the sun and the phases of the moon for crop planting and harvesting (Genesis 1:14-18, English Standard Version; Struik, 1987).
A harvest-time thanksgiving
Working late into the evening hours in a not-so-distant past, farmers in North America relied heavily on the light of the full moon nearest the fall equinox to help bring in their crops. This moon came to be called the Harvest Moon.
— The full moon of harvest —
Although each year the fall equinox settles on the 21st or 22nd of September, the full moon phase nearest this seasonal change ranges between mid-September and early October.
In fact, during this time of the year, moon rise occurs 20 minutes sooner each evening (or only 30 minutes later as opposed to the normal 50 minutes later each successive night). Collectively, this cycle spans three days before and after the official full moon calendar date, or roughly a week’s worth of time. Even more, moon rise each evening during that week is nearly simultaneous with sunset. And the cumulative amount of light (that is, the light given by the sun and all the light reflected by the moon) translates into quite a striking event for harvesters, especially in the days before flood lamps appeared on farm equipment.
With the choicest fruits of the sun and the rich yield of the months.
– Deuteronomy 33:14 (ESV)
Fall equinox occurred this year on Saturday, September 22nd, at 9:54 p.m. (EDT), but the official full moon of harvest – the full Harvest Moon – occurs tonight, Monday, September 24th, at 10:52 p.m. (EDT). This timestamp marks the date and time at which there is 100% completeness in the face of the moon.
So, shortly after enjoying tonight’s sunset at 7:22 p.m. (EDT), look towards the east and enjoy this year’s Harvest Moon, which is set to rise at 7:35 p.m. (EDT).
Struik, D.J. (1987). The beginnings.A concise history of mathematics (4th ed.). New York, NY: Dover Publications, Inc.