Harvest Moon

The Harvest Moon is the full moon falling nearest the September equinox. That makes it the one traditional name defined astronomically rather than by the calendar, and it is why the title moves: usually it belongs to September’s full moon, but roughly one year in four or five it lands on October’s instead. That is what the rule requires — the title crosses into October only when the full moon nearest the equinox falls in the first week of the month, which is about seven days out of every 29.5. In those years September keeps its older name, the Corn Moon, from the maize harvest.

What makes the Harvest Moon genuinely unusual is worth understanding, because it is a real astronomical effect and not a piece of folklore. On an average night the moon rises about 50 minutes later than it did the night before. Around the September equinox, the angle the moon’s orbital path makes with the horizon is at its shallowest for northern observers, and that delay shrinks to as little as 25 or 30 minutes.

The practical consequence is several consecutive evenings with a bright, nearly full moon rising around sunset — no long gap of darkness between the sun going down and the moon coming up. Before artificial light, that was the difference between stopping at dusk and working the harvest into the night. The name is a description of a labour schedule.

Also known as

• Corn Moon
• Barley Moon
• Fruit Moon

The 1778 record

Travelling among the Dakota in 1766 and 1767, Captain Jonathan Carver wrote down all twelve month names in a single passage. Published in 1778, it predates the almanac list by roughly a century and a half and is the oldest independent check on these names.