harvest moon

Most full moons have folklore names - Hay Moon, Fruit Moon, etc. - essentially forming a twelve month calendar for an agricultural world. Tonight's full moon, the Harvest Moon, also has an astronomical significance.

The Moon rises roughly 50 minutes later every day. However, this is an average figure, and near the vernal and autumnal equinoxes the actual time difference is furthest from the average. In Spring, as seen from mid-northern latitudes, the Moon can rise as much as 90 minutes later each night, but in the Autumn the figure can drop to as little as 14 minutes. In the southern hemisphere the situation is reversed.

A full moon, because it is directly on the opposite side of the Earth to the Sun, always rises at about sunset and sets around sunrise. At the time of the vernal equinox, roughly 22^nd September, the period of darkness between sunset and moonrise remains small for up to a week after the full moon. Conveniently, this happens at exactly the time when farmers are working round the clock to harvest their crops - a huge boon in the days before electric lights - so the full moon closest to the equinox become known as the Harvest Moon. The precise date varies between the second week in September and the first week in October.

Visible features are as for the fourteen-fifteen day moon.