Why this week’s full moon will look big but be small

Look down on the eastern horizon at nightfall this Thursday, October 1, 2020, and you will see – the clear sky permits – a huge “harvest moon” rising.

The first of some famous full moons this coming season, the Harvest Moon is so called because it helps farmers harvest late at night. Of course, this is not how modern farmers operate, but as with all full moons, she will rise in the east, facing the sunset, shine all night, and set in the west, facing at the Sunrise.

As she gets up and down, the Harvest Moon will also look huge. All full moons do.

However, this Harvest Moon is different.

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Although it is in its “full” phase this week, the Harvest Moon will be relatively small because come on Saturday. the moon will be at its peak, the farthest point in its 27.5-day slightly elliptical Earth orbit.

It will be 406,000 km from us and will be smaller and less luminous than usual.

So why will this little 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 have a little magnification with the atmosphere,” said Martin Griffiths, a Wales-based science communicator, professional astronomer at Dark skies of Wales and author of Dark Land, Dark Skies: The Mabinogion in the Night Sky. “You also have features in the landscape, which are of course knowable with regard to the height. ”

Whether you watch a full moon rise behind a tree, between buildings, or over the roofs of houses, our brains make comparisons between them and the size of the moon.

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It’s an optical illusion that only lasts a few minutes.

As soon as the full moon has risen above the horizon, that context and those comparisons are lost. The full moon seems to be shrinking.

However, not only is this an illusion, but in fact the opposite is true: the Moon is actually at its smallest size when it rises.

Why will the “Harvest Moon” be so small?

“While it doesn’t appear that way, when you look at the Moon as it rises, it’s actually at its smallest,” Griffiths said. “It is because you have 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 is nothing our brains can compare it to – it is technically at its largest. . “You don’t have that ray anymore, you just have the distance between you and the Moon, so it’s actually closer then,” Griffiths said. “The problem is, you don’t have anything to compare to, so that sounds wrong. ”

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How big is the “Harvest Moon?” “

All full moons are tiny.

The apparent diameter of the Moon is only 0.5 °. The celestial sphere around our planet is 360º, and from anywhere on Earth you can see 180º — the night sky visible (the rest is below the horizon). So 0.5 ° is not a lot. It’s actually just a 72,000th, the size of the sky.

“It only looks big, it’s either dazzlingly luminous or seen against the features of the landscape, but you can still cover the moon with your pinky fingernail stretched out at arm’s length,” said Griffiths.

Try. It doesn’t matter how long your arm is, how tall or how tall, because all humans are built to scale.

Where to look for the sunrise and sunset of “Harvest Moon”

i just posted a comprehensive guide here to watch the Harvest Moon, but the basics are that a full moon near the equinox—which this one is very close– will rise almost due east and set almost due west.

The Harvest Moon is in the constellation Pisces, and if you look to its left you will see Mars red pink.

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All you need is a beautiful, clear eastern horizon and one of nature’s most beautiful illusions is yours. So get ready for a “Halloween hunter’s blue moon“October 30.

I wish you clear skies and big eyes.

Troy M. Hoffman