Why Do We Sometimes See The Moon During The Day?

Jan 4, 2019

Why does the moon change shape? How much does the moon weigh? What color is the moon? Why does the Earth only have one moon? Why does the moon have holes? Where does the moon go when we can't see it? Why do we sometimes see the moon in the daytime? Why does the moon look like it's following you when you're in the car? Answers to your moon questions with John O'Meara, chief scientist at the W.M. Keck Observatory.

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"Why is the moon out in the daytime?" - Elliot, 3, Illinois

We can see the moon during the day for the same reason we see the moon at night. The surface of the moon is reflecting the sun's light into our eyes. But why don't we see the stars during the day?

"The stars are nowhere near as bright as the blue sky during the day, but the moon is approximately as bright in reflected sunlight as the sky during the day, and that's why we can see the moon during the day, but not the stars during the day," O'Meara said.

But we don't see the moon all the time during the day, and that's because of where the moon might be in the sky.

"Sometimes to see the moon you'd have to look through the Earth and we can't do that," O'Meara said. "When we see the moon during the day it's because the moon is in the right spot in the sky and it's reflecting enough light to be as bright, or brighter, than the sky."

"Why does it look like the moon is following you when you drive?" -Anisa, 6, Georgia
John O'Meara is chief scientist at the W. M. Keck Observatory in Hawaii.
Credit John O'Meara

It's an optical illusion. The moon is very far away, compared to anything else you see when you're driving — like the telephone poles that you see that appear to fly past your car as you're going down a highway.

"The moon is so far away that its size and shape on the sky doesn't change much at all because it's really far away. You driving one mile is insignificant in terms of how it makes the moon look on the sky," O'Meara said. "And because the Earth is rotating around once every 24 hours, that means you don't see the moon moving too much in the sky, expect when it's really close to the horizon and you can see the moonset and the sunset happen."

So the moon is just so far away, its shape on the sky isn't changing as you drive along, and so it can feel like the moon is following you.

Click listen to hear the full episode.

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