Harvest moon and hunter's moon: the full moons of autumn
Why around the autumn equinox the full moon rises at almost the same time for several evenings, and where the traditional names of the full moons come from.
Of all the full moons of the year, the one closest to the autumn equinox has received a special name in European tradition. It is neither larger nor brighter than the others, but it has a real astronomical peculiarity, which for centuries was of practical use to those working in the fields until late in the evening.
Why the autumn moon rises early
On average the Moon rises about fifty minutes later each day than the day before. Around the autumn equinox, however, at middle latitudes of the northern hemisphere, the full moon and those of the nearby days rise with a much smaller delay, which in Italy can fall to less than half an hour. For several consecutive evenings, therefore, an almost full moon appears on the eastern horizon shortly after sunset.
The reason is geometrical. At that time of year the part of the ecliptic where the full moon lies, towards Pisces and Aries, makes a particularly shallow angle with the eastern horizon at rising. The Moon advances about thirteen degrees a day along the ecliptic, but with such a low inclination it sinks only a little below the horizon, and so its rising shifts only slightly.
A farmer's name
This extra light in the early hours of the night was precious when the harvest was finished by hand. Hence the English name harvest moon, for the full moon nearest the autumn equinox, in September or early October. The following full moon is called the hunter's moon, because it traditionally accompanied the hunting season after the end of the farm work.
Names of months and moons
Many peoples have named the moons or the months after seasonal work and phenomena. Bede the Venerable, in his treatise De temporum ratione of 725, lists the names of the months of the ancient Anglo-Saxons, who reckoned time by the Moon: September was Halegmonath, the month of sacred rites, and November Blodmonath, the month of blood, for the cattle sacrificed before winter.
North American almanacs popularised in the twentieth century names such as wolf moon, snow moon, worm moon or buck moon, attributing them to the traditions of various Native peoples and of European settlers. These names, now widespread in Europe too, in fact come from heterogeneous sources and do not form a single ancient calendar, even though some reflect customs that are genuinely documented.
A symbol of abundance
In tradition the harvest moon has been associated with abundance and with the completion of the agricultural year. In East Asia the full moon of the eighth lunar month, which usually falls between September and October, is celebrated with the Mid-Autumn Festival, during which round moon-shaped cakes are offered and the legend of Chang'e is told, the woman who according to myth rose to the Moon after drinking the elixir of immortality.
The coincidence between an astronomical fact and a human need explains well how these names were born. The harvest moon is not a poetic invention but the result of a celestial geometry that farmers had noticed long before astronomers described it, and which still today offers a series of bright evenings in autumn.