10 things to know about the special 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 September 16, but its waxing and waning put on a good show as well.

Each year as summer in the Northern Hemisphere slowly slips into autumn, we have the September full moon to shine her luminous light on the change of seasons. When the September full moon occurs closer to the autumnal equinox than the October full moon does, it’s called the Harvest Moon. (Alternatively, if the October full moon is closer to the date, which falls on September 22 this year, she takes the title.) While all full moons are special, the Harvest Moon boasts some unique features that make this month’s moon a marvelous must-see.

1. 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 September full moon has been known as the Full Corn Moon or the Full Barley Moon, as it marks the time of harvesting for the two crops.

2. There were, however, variations among some tribes; and the poetry is gorgeous. The Lakota Sioux referred to the September moon as the “Moon When the Plums Are Scarlet.” The Omaha called it the “Moon When the Deer Paw the Earth” and the Sioux knew it as the “Moon When the Calves Grow Hair.”

3. There are just a bit over 12 complete moon cycles every year, on average (there being about 29.53 days in a synodic month, notes the Old Farmer's Almanac), and, as mentioned above, the Harvest Moon is the full moon nearest the beginning of the autumnal equinox. Most often this means that the September full moon wears the crown, but it can happen in early October as well; either way, it happens within two weeks before to two weeks after the equinox. Thus, the Harvest Moon and the change of seasons always go hand in hand.

4. Regardless of the September or October full moon names, the autumnal equinox full moon is also called the Harvest Moon because its unique features traditionally helped farmers; call it a boon moon! The moon after the Harvest Moon is known as the Hunter's Moon, both terms date back to at least the 18th century, according to the OED.

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

5. 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 is a special gift courtesy of the fact that 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.

6. While all full moons rise at sunset, the fact that the Harvest Moon has such a short rising gap from day to day means that we get what appears to be a full moon rising for a number of days; this gave farmers a “sunset extension” of sorts, which went to good use during the very busy time of harvest.

7. The sunset-moonrise concurrence also simply makes for a spectacular spectacle, which we are treated to, again, 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 autumn.

8. 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.

9. This year, the Harvest Moon is even more special for it will also be a penumbral lunar eclipse as it passes through the outer edge of the Earth’s shadow. It will be faint, but visible to varying degrees across Europe, Africa, Asia, Australia and the Western Pacific. The point of maximum eclipse will take place at 2:54 a.m. EDT (18:54:20 UTC).

10. While the 2016 Strawberry Moon (the full moon of June) occurred exactly on the day of the summer solstice, the Harvest Moon last perfectly coincided with the autumnal equinox in 2010, and won’t again until 2029. Until then, we can enjoy this full-figured tangerine-tinged beauty as a harbinger of the season to come, which will be hot on her heels come September 22.

And in parting, this:

Sources: NASA, The Old Farmer's Almanac, SPACE.com.

Tags: Nature | Space


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