Harvest moon(s)

Harvest moon(s)
  • Before calendars, people used the period between full moons, of which there are 12 a year, to count the days

Before people had calendars to help them measure the year, people used the period between full moons to count the days. As there are usually 12 full moons in a year, it was an easy, if not very accurate, way to mark the year. Various civilisations gave a different name to each of the full moons and some of these have stuck and are still used today.

Yesterday and today there will be a full moon, although to be accurate, the full moon occurred at 2.41 pm universal time yesterday afternoon. Usually the full moon that occurs closest to the Autumn Equinox is christened the Harvest Moon. For the first time in many years we have two full moons that can claim the title, as the last full moon occurred on 6 September and so both are more or less equal distance from the equinox that occurred on 21 September.

The Harvest Moon usually rises a little earlier than other full moons simply because it is so close to the equinox when the earth has 12 hours of day and 12 hours of night. This early rising traditionally helped farmers with extra light so that they could continue gathering crops later into the evening.

The Harvest Moon usually appears a red or orange in colour as it rises. This has nothing to do with the moon itself, it is because as the moon rises, its light has to pass through a denser part of our atmosphere which causes the light to glow red, the same way we sometimes see bright red sunrises and sunsets.

On average, the moon rises about 50 minutes later each day. But when a full moon happens close to the Autumn Equinox, the moon rises only about 30 to 35 minutes later for several days before and after the full Harvest Moon.

For very high northern latitudes, there's even less time between successive moon rises. It happens because the ecliptic, or the moon's path across the sky, makes a narrow angle with the evening horizon near the Autumn Equinox.

The difference between 50 minutes and 35 minutes may not seem like much but it means that, in the nights after the full Harvest Moon, you'll see the moon rising in the east relatively soon after sunset. The moon will rise during or near twilight on these nights, making it seem as if there are several full moons for a few nights in a row around the time of the Harvest Moon.

If you can spot the full moon just as it is rising it may appear much larger than when it is over head. I'm afraid this is just an optical illusion, but even so it does make the Harvest Moon seem special.