On Wednesday night, June 18th, step outside at sunset and look around. You'll see a giant form rising in the east. At first glance it looks like the full Moon. It has craters and seas and the face of a man, but this "moon" is strangely inflated. It's huge!
You've just experienced the Moon Illusion.
There's no better time to see it. The full Moon of June 18th is a "solstice moon", coming only two days before the beginning of northern summer. This is significant because the sun and full Moon are like kids on a see-saw; when one is high, the other is low. This week's high solstice sun gives us a low, horizon-hugging Moon and a strong Moon Illusion.
Sky watchers have known for thousands of years that low-hanging moons look unnaturally big. At first, astronomers thought the atmosphere must be magnifying the Moon near the horizon, but cameras showed that is not the case. Moons on film are the same size regardless of elevation: example. Apparently, only human beings see giant moons.
The full moon rising over Manchester, Maryland. Credit: Edmund E. Kasaitis.
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