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ETpro's avatar

Why doesn't the full moon look like a globe?

Asked by ETpro (34552points) July 9th, 2010

When you look at a full moon, it appears to be a grayish-white disk hanging in the nighttime sky. It doesn’t look spherical, with gradually shaded sides. Why?

Look at a ping pong ball, even in full sun, and it is clearly a sphere. Same goes for a basketball or for even big spherical objects like those bulbous water towers. What makes the moon such a unique sphere?

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6 Answers

NaturallyMe's avatar

Huh? It looks spherical to me…

ragingloli's avatar

Because of the low angle and the high light intensity of the sun, coupled with the high contrast between the dark space and the illuminated moon at night?

Austinlad's avatar

Check it out through a telescope or bionculars. You’ll get a very different perspective.

ETpro's avatar

@NaturallyMe Compare this picture of the full moon to the this one of a ping pong ball. Notice the difference in the gradient of shading?

Good guess, but there has to be more going on than just the gradations in color. Check this soccer ball for instance.

@Austinlad The reason that difference exists might provide a clue to the illusion.

downtide's avatar

A lot of the light you see on the moon is not just from the sun, it’s also reflected off the Earth, which means that it’s always getting a blast of light from full face on, which will illuminate the whole visible surface equally. It would be like looking at a ping pong ball on a black background but in full floodlights. The shading at the edges is just washed out by the intensity of the light.

ETpro's avatar

@downtide Nice try, but use a mirror to light up a ping pong ball, and it still looks quite spherical. Light it with a flashlight and mirrir. Still a clearly globular shape. There is something else at work here. It has to do with the assumptions the human brain makes about reflective surfaces in the real world.

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