Look low on the eastern horizon close to dusk this Thursday, October 1, 2020, and you’ll see—clear skies allowing—a massive “Harvest Moon” rising.
The first of a few famous full Moons this coming season, the Harvest Moon is so-called because it helps farmers get the crop in late into the night. Of course, that’s not how modern farmers work, but as with all full Moons it will rise in the east, opposite the sunset, shine all night long, and set in the west, opposite a sunrise.
As its rises and sets the Harvest Moon will also look huge. All full Moons do.
However, this Harvest Moon is different.
Although it’s in its “full” phase this week, the Harvest Moon will be relatively small because come Saturday the Moon will be at apogee, the furthest point along its slightly elliptical 27.5 day orbit of Earth.
It will be 406,000 km from us and be smaller and less bright than usual.
So why will this small full Moon look so big but be so small?
Why will the ‘Harvest Moon’ look so big?
“The Moon will appear massive in the sky for the simple reason that you’ve got a little bit of magnification with the atmosphere,” said Martin Griffiths, a Wales-based science communicator, professional astronomer at Dark Sky Wales and author of Dark Land, Dark Skies: The Mabinogion in the Night Sky. “You’ve also got features in the landscape, which of course are knowable as far as a height is concerned.”
Whether you watch a full Moon rise behind a tree, between buildings or above the roofs of houses, our brains make comparisons between them and the size of the Moon.
It’s an optical illusion that lasts only a few minutes.
As soon as the full Moon has risen above the horizon, that context and those comparisons are lost. The full Moon appears to shrink.
However, not only is it an illusion, but in fact, the opposite is true—the Moon is actually at its smallest as it rises.
Why will the ‘Harvest Moon’ be so small?
“Though it doesn’t appear that way, when you’re looking at the Moon when it’s rising it’s actually at its smallest,” said Griffiths. “That’s because you’ve got the entire radius of the Earth between you and the rising Moon.”
Conversely, when the Moon is high above your head at midnight—when it appears to be at its smallest because there’s nothing for our brains to compare it to—it’s technically at its biggest. “You haven’t got that radius anymore, you’ve got just the distance between you and the Moon, so it’s actually closer then,” said Griffiths. “The problem is, you’ve got nothing to compare it to, so it looks wrong.”
How big is the ‘Harvest Moon?’
All full Moons are tiny.
The apparent diameter of the Moon is only 0.5º. The celestial sphere around our planet is 360º, and from any one place on Earth you can see 180º—the visible night sky (the rest is below the horizon). So 0.5º isn’t much. It’s actually just one 72,000th, the size of the sky.
“It only looks big it’s either dazzlingly bright, or it’s seen against features of the landscape, but you can always cover the Moon with the nail on your little finger held out at arm’s length,” said Griffiths.
Give it a try. It doesn’t matter how long your arm is, or how big or small you are, because all humans are built to scale.
Where to look for the ‘Harvest Moon’ rising and setting
I’ve just posted an exhaustive guide here for watching the Harvest Moon, but the basics are that a full Moon close to the equinox—which this one is very close to—will rise almost due east and set almost due west.
The Harvest Moon is in the constellation of Pisces, and if you look to its left you’ll see rosy red Mars.
All you need is a nice, clear eastern horizon and one of nature’s most beautiful illusions is all yours. Then prepare for a “Halloween Hunter’s Blue Moon” on October 30.
Wishing you clear skies and wide eyes.