11 things to know about this year’s Harvest Moon

Harvest moon
CC BY 2.0 Roadcrusher

This year’s full moon closest to the autumnal equinox rises in all its giant, gorgeous glory on October 5, but its waxing and waning put on a good show too.

Each year as summer in the Northern Hemisphere slowly slips into autumn, we have the September and October full moons to shine their luminous light on the change of seasons. When the September full moon occurs closer to the autumnal equinox, which fell on September 22 this year, it’s called the Harvest Moon. But if the October full moon is closer to the date, she takes the title. All full moons are special, but the Harvest Moon has some unique features that make this month’s moon a marvelous must-see.

1. This year's Harvest Moon will be officially full at 2:40 p.m. Eastern Daylight Time on October 5 – this is when it is directly opposite of the sun. But will appear full for a day or two before and after.

2. In the 80 years between 1970 and 2050, the October full moon will only claim the Harvest Moon title in 18 of those years altogether. The last time was in 2009, and the next time will be in 2020. According to Space.com, October Harvest Moons come every three years on average, although as many as eight years can elapse between them.

3. Next year, we'll be back to the more usual schedule, with the Harvest Moon occurring on September 24, a mere two days after the 2018 autumnal equinox.

4. Harvest moons can happen as early as September 8 or as late as October 7, making this year’s relatively late.

5. Sometimes the full moon happens on the equinox itself, the Harvest Moon last perfectly coincided with the autumnal equinox in 2010, and will do so again in 2029.

6. According to early Native American tradition, the calendar was marked by cycles of the moon and thus, each one has a seasonal significance and name. The October moon has been variously known as the Full Hunter's Moon, the Travel Moon, and the Dying Moon.

Harvest moonThe Harvest Moon, 1871 (George Hemming Mason)/CC BY 2.0

7. The autumnal equinox full moon is called the Harvest Moon because its unique features traditionally helped farmers. The term dates back to at least the 18th century, according to the OED.

8. During the rest of the year, the moon rises up to a full 73 minutes later each successive day of its cycle; but the Harvest Moon rises in as little as 23 minutes later each day. This happens because during this time of year, the moon’s orbit is more nearly parallel to the horizon and thus its relationship to the eastern horizon does not change much from day to day.

9. While all full moons rise at sunset, the fact that the Harvest Moon has a shorter rising lag on successive days means that we get what appears to be a full moon rising near sunset for more days than usual; this gave farmers a “sunset extension” of sorts, which went to good use during the very busy time of harvest.

10. The sunset-moonrise concurrence also simply makes for a spectacular spectacle, which we are treated to for several days. Low-hanging moons at sunset are reddened by clouds and dust, giving them that surreal “giant floating pumpkin” effect that so perfectly ushers in the fall.

11. And about the giant part; the Harvest Moon is like the moon poster child for the so-called “moon illusion,” in which a horizon-hovering moon appears to be gigantic. The moon actually doesn’t change size, it’s just a trick of the eye – one theory suggests that in relation to terrestrial objects the moon appears much larger than when it's floating in the vast heavens. Though, it's a bit more complicated than that, as you can see in the video below.

Tags: Nasa | Nature | Space

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