General Question

andrew's avatar

Why does the moon sometimes look gigantic?

Asked by andrew (16358points) December 12th, 2007

I’ve noticed this usually when she’s low in the sky, and sometimes looks quite a bit larger than normal. Is she closer to us at certain points of her orbit? Is it an optical illusion?

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11 Answers

gailcalled's avatar

Me again. This is a question w. no answer…for an interminable attempt; see
moon size illusion

Sample quote:


” The literature on the moon illusion is vast, indicating an obsession with the problem that borders on lunacy.”

omfgTALIjustIMDu's avatar

Yes, it is an optical illusion called the Moon Illusion. Gail’s site does a pretty good job of explaining it, but there is still some controversy as to the reasons for the illusion.

Fallstand's avatar

I justttt watched a show on this.. People think the moon looks larger near the horizon than when its higher in the sky simply because of the way our brain works. If we see the moon near something large like a house we think the moon looks so big because it dwarfing the size of the house. But when its higher in the sky by itself it looks so small because of the open space around it in the sky.

I’m not sure that makes any sense but i tried

ironhiway's avatar

Gailcalled’s link is very interesting and relates to several areas where because of other items in view cause two items of the same size to be viewed as larger and smaller.

If you take a quarter and put it up to the moon when it’s on the horizon then again later when it’s high in the sky you’ll see it’s the same size in reference to the quarter.

xgunther's avatar

I always thought that it was due to the way that our atmosphere bends light at different angles. I was so wrong.

syz's avatar

I thought so, too, xgunther.

gailcalled's avatar

Light going thru different angles and thicknesses and varying kinds of pollutants in the air will change the color of celestial objects.noticeably the moon.

The moon illusion is really fascinating, as are all optical illusions.

credogen's avatar

i think it is when the moon is at the perigee or the nearest point in its orbit to the earth, the moon looks larger on earth

gailcalled's avatar

@credogen has a good point albeit a slightly different reference. Here’s a terrific site about the moon’s apparent size at perigee and apogee – which is real and not an illusion. There are visuals AND math.

lunar size

rlr718's avatar

It’s an optical illusion I think its the same as the sun setting or rising looks larger. It’s the refraction of the atmosphere and light.

caridgway's avatar

The Moon is actually larger at certain times in its orbit than at others. When it is closest to Earth in its elliptical orbit it is larger than when it is farther away. But unless you photograph it or otherwise measure it you probably won’t be able to tell the difference since those times would be months apart and you wouldn’t remember how big the first Moon was by the time the second rolls around. To illustrate this photograph a first quarter Moon at aphelion and a last quarter Moon at perihelion (or vice verse) with a fixed focal length lens (so you are sure that the focal length is the same for both pictures) and have each frame printed the same size and compare them. (And just to make it even more variable the perihelion and aphelion distances are different on each occurrence so we get the almanac notations for largest full Moon and smallest full Moon of the calendar year.)

And on any given day the Moon is bigger when it is higher up in the sky than when it is on the horizon because it is closer. You can prove this by drawing two concentric circles, the inner representing the surface of the Earth and the outer the orbit of the Moon, and placing a dot on the top of the inner one [you standing on Earth] then draw a line to the outer circle such that it just touches Earth off to the side [the horizon] and measure that distance, the rising or setting moon. Now compare it to the distance measured straight up overhead [a line from the center of the two circles through your location and extending to the outer circle where the zenith would be but only measure from Earth to the Moon’s orbit] and you will see that the distance from your location to zenith is less than to where the Moon would be when viewed when it is rising or setting so it is actually biggest when it is highest in the sky. But the Moon Illusion effect mentioned above will convince you that the opposite is true.

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