Full moon.... why does it sometimes look much larger and closer than at other times?

Question

Tonight it looks like you could hop on an EasyJet flight and pop over there in an hour or so, yet at other times it looks much smaller and further away?

Answer 1

It is an illusion, but NOT an optical one. It is NOT due to "comparing its size to nearby objects" when the Moon is near the horizon.

Frankly, no one knows why the Moon appears larger near the horizon, but we do know some things about it.

First, it is not really bigger on the horizon than high in the sky. You can determine this for yourself by comparing its size to the size of your finger whith your arm outstretched.

Second, it is not an optical illusion; if you take a photograph of the Moon on the horizon, the Moon looks normal sized in the picture. It is not due to comparing the Moon to objects on the Horizon; it also looks large on the horizon when seen from the ocean, where there is nothing to compare it to. Also, if you look at a distant lighted helium balloon the apparent size of the moon near the horizon, it will not appear too big. Finally, if you stand on your head (try it!) while looking at the moon on the horizon, it also looks normal sized.

So it is a *cognitive* illusion. There appears to be an instruction in the cognitive processing centers of the brain that when the sun or moon is on the horizon, it should be processed to appear bigger than it is. Why our brains should be wired this way no one knows.

Answer 2

The funny thing is, it is actually larger by a small amount when high in the sky! That this is so is because when the moon is on the horizon it is one earth radius farther away (neglecting any change due to the eccentricity of the orbit of the moon) than when it is directly overhead.

And, the difference is not small. The moon is 1/60th times closer when it is at the zenith than on the horizon (4,000 miles/240,000 miles), so it would appear nearly 1.7 percent larger (not enough to detect without a photograph).

Actually, both the optical illusion effect and the magnification effect come into play, and they work on the sun, as well. I believe that the magnification effect is more important, because the moon as seen over nearby mountains (not near the horizon if the earth were non-mountainous) does not seem that much larger.

But, here again, when it is near the horizon you are only comparing it to a small part of the sky (the earth takes up a larger part of your view), while when high in the sky you see a lot of sky and a much smaller-appearing moon.

Answer 3

The Harvest Moon is only called that because it's the full moon at harvest time,in September.It is the full Moon that is closest to the Autumnal Equinox which is usually on September 23rd.The next full moon after that,in October, is called the Hunter's Moon,and the one after that,in December is the Snow Moon. The Harvest Moon has been made famous by romantic stories and various songs,and many people expect it to look bigger than other full Moons,but actually it isn't.Some full Moons are bigger than others because the Moon's distance from us varies. When the Moon is closer to the Earth it looks bigger.Nowadays,in the developed countries,farmers don't have to work late at night to get the harvest in because modern machinery ('combine harvesters') can get the work done quickly. Years ago,corn,wheat and other cereal crops were cut by hand using scythes and sickles. It took a lot of people,often working late into the night, to get the job done. The full moon at harvest time gave them enough light to see by, and it became known as the Harvest Moon. All full moons look big because of the glare of the light.Look at it through a pinhole or coloured glass so not so much light enters the eye and you will see that the Moon seems a bit smaller.When the Moon is near the horizon it looks bigger than when it is high in the sky. That is called 'the Moon Effect'.But so do the constellations. Orion rises in the evening at this time of year in the South-East. Notice how big it looks as it's rising and then look at it again a few hours later when it is higher in the sky.It looks a bit smaller.It's an illusion caused by two things. Firstly,by the denser air near the horizon bending the light,which has a small effect (and often makes the Moon actually look a bit smaller),and secondly,because when things have a lot of space round them they look smaller than when they haven't. It's a phychological illusion. When there is a lot of sky around the Moon it looks like a small thing in a big space,phychologically.On the horizon there are trees,hills,or other things to compare it's size to,and it looks big . Very occasionally the Harvest Moon occurs in October instead of September. It's happened five times since 1900. The last time, in 2006, it was on October 7th.

Answer 4

If you Google "moon illusion" you'll find ample evidence that this is an "optical illusion" or as one of the answerers more aptly called it, a "cognitive illusion." The Moon is exactly the same size when on the horizon as when it is overhead. Refraction has no effect on it. Sure its orbit is elliptical, but you need a micrometer to detect the difference in its diameter from perigee to apogee. This is a phenomenon that has been studied in detail by psychologists specializing in perception over the years, and their findings are not speculation, unlike most of the answers here. As I said, look in Google if you don't believe me.

Answer 5

It will appear larger as it rises above or descends to the horizon. This is due to the earths atmosphere refracting the light from the moon. The atmosphere acts as a magnifying glass and distorts the image. This also occurs at sunrises and sunset, if the atmosphere is right the sun or the moon can look two or three times its normal apparent size.

Answer 6

One of the earlier responders hit upon the correct answer: it's an optical illusion. When the moon is near the horizon, it looks larger, since your mind compares it to the size of objects on the horizon.

Photographers have made time lapse photos of the moon passing through the night sky. These photos prove that the move indeed is the same size all along.

While it's true that the moon's orbit around the earth is elliptical, it's close enough to circular so that the naked eye wouldn't really perceive a size difference due to lunar distance. The size difference that we perceive is just an optical illusion.

Answer 7

Supposedly (according to scientists), it's the same size, whether you see it near the horizon, or near it's zenith.

The reason it looks different sizes (they say) is because when it's near the horizon, you can 'gauge' it against something else you can see - for example, a tree. So it looks big.

When it's high in the sky, there's nothing near and so you have nothing to judge it against. So it looks smaller.

Can't believe this myself - sometimes it's a huge Moon and sometimes it's a tiny Moon. That's it.

Answer 8

Because the atmosphere defracts light waves. If there is more atmosphere, the light can get defracted more, but it all depends on the relative angles in the sky. This is why when the moon is really big, it usually low in the sky-->the light is passing through the most amount of atmosphere. It's also why the moon is usually a different color (like orange) when its big. A magnifying lens works the same way by defracting the light to make the image appear bigger.

Answer 9

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Lots of answers, some correct and some misleading.
However, none so far have provided a reference.

Here it is;
http://en.wikipedia.org/wiki/Moon_illusion

Yes, it is an optical illusion and nothing to do with refraction, etc.
(sorry Kennyk, seems that it is not the other answerers that are the idiots.....)


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Answer 10

To me, it always looks really large when it's close to the horizon, so I think the reason is that you have something to compare it to, like houses or trees. When it's high in the sky, there's nothing nearby, so your mind reduces it a bit. At least that's my grand theory of the day ;-)