illusion of the moon size..?

Question

can any one out there explain why the moon sometimes look HUGE and other times looks wee small?

i know its something to do with the angle we view the moon from, i.e. when the moon is low we can compare the size with a building therefore it looks BIG..

can some Boff, put it in plain English plzzzzz!

thanks

I HAVEN"T given anyone a thumbs DOWN......

Answer 1

The 'Moon Illusion' is a common source of mischief. But, it is just that, an illusion.

The Moon usually appears large when close to the horizon and this has led several to state that its large appearance is due to it being close to an object. But, move yourself close to a building, but when the Moon is higher in the sky, and the illusion is not there. (I've tried this by looking at the Moon through a cardboard tube to measure the size of the Full Moon close to the horizon and then later close to a building. The Moon's size did not alter, but my perception was that of the Moon looking bigger near the horizon - very odd!)

It is nothing to do with Moon being close when it is high in the sky. The distance is a few thousand miles at most and cannot account for any noticeable change in size. And on this point another oddity... The Moon is closer to us when overhead but appears smaller than when near the horizon!

Another misconception is a magnifying effect of the atmosphere when the Moon is close to the horizon. The Moon appears flattened but this is due to refraction which appear to squash the Moon but does not make it actually appear larger. (The same effect is noticeable with the Sun.)

Some opinion varies but it is now thought to be an optical illusion due to the way we percieve the shape of the sky which some part of our brain distorts to give the impression that the Moon is different sizes depending on where it is in the sky as well as an issue with our perception of distance.

In all, a puzzling illusion and the source of many 'myths'.

Further reading can be seen at:
www.badastronomy.com

Answer 2

I have often measured the diameter of the Moon using a sextant. It is NOT bigger near the horizon. It should be ever so slightly smaller (because it is slightly further away from the observer) but that difference is too small to be measured by a sextant. I have often measured a Full Moon as it rose, as it was high, and as it set. Never found the size to change. If anything, the "vertical" diameter is ever so slightly smaller near the horizon because of refraction (which raises the bottom limb a bit more than the upper limb).

Answer 3

Quantum's answer is correct. This is the well known Moon Illusion, and five minutes with Google will give you plenty of documentation that it is an optical illusion caused by the Moon's closeness to the horizon. Viewed through a telescope (or even through a toilet roll) the Moon is exactly the same size at the horizon or overhead. Try the experiment yourself!

Answer 4

noo, why do people keep getting confused?
The moon doesn't look big "because we can compare it's size with nearby objects".

For instance, compare a view of the moon right over your neighbor's house, vs a view of the moon on a clear horizon(no trees, no houses, no reference point, just plain land). The horizon moon will look larger, even though there is nothing we can compare it to.
On the other hand, the one right above your neighbor's house we can compare to, well the house, so why doesn't it look bigger?

The effect is called Oculomotor Macropsia. When we look towards a clear horizon, our eyes adjust so we can see far away objects better. That is, by focusing on a very narrow area. The result is that we see the moon bigger than normal.

Now, why doesn't that happen when we look towards space, even though space is 'far away'? Because our brains don't know how far space is - a hundred feet? couple miles maybe? Of course we know by theory how far away a star is, but we can't seem to connect the part that knows that to the part that controls our eyes.

Answer 5

I can't believe 'quantum' got 3 thumbs down!

You already answered your own question - "when the moon is low we can compare the size with a building therefore it looks BIG.." -

When the moon is in the center of the sky we have no reference frame, just a white dot in a black sky.

This is generally referred to as parallax error and a good way to test it's credibility is the next time you're looking at the full moon, take a nickel and hold it up against the moon while it's at the horizon and then hold it up when the moon is high. You'll find the moon is always the same size, it just seems bigger.

Answer 6

Quantum's answer is incorrect. He is stating the illusion happens because we are comparing the Moon to smaller objects on Earth, which is not true.

If what he was saying was true, the moon would look small on a horizon that contained no buildings or trees, but anyone who has been to a desert, or a beach, knows both the Moon, and the Sun look bigger on the horizon of such places, even though there is nothing to compare them to.

Answer 7

The best explanation I've heard goes like this:

Human beings aren't very good at directly identifying such things as distance, size, speed, etc. by sight alone. It's why we invent measuring devices to do it for us. In the absence of such measuring devices, we rely on combinations of other visual cues - combinations which don't necessarily add up to what we think they do.

We tend to subconsciously think of the sky as having the shape of a flattened inverted bowl. Because of this, astronomical objects when directly overhead are (incorrectly) perceived as being closer to the viewer than when on the horizon. Since our experience with terrestrial objects has ingrained the perception that distant objects are actually much larger than they appear, our brains naturally do the same with our perception of the moon.

It's not because we can compare it with other things of known size on the horizon, it's because we think of it as much farther away.

Answer 8

next time you think it looks HUGE, look at where it is - I'll bet it's near the horizon. On the horizon, you know the size of distant objects, but they look very small compared to the moon. When the moon is overhead, there's no objects with which to compare it to, and it looks relatively small. In fact, the size of the moon on the horizon and overhead are the same (or very nearly so), and it's an optical illusion that it looks 'bigger' at times.

Answer 9

without getting technical its down to the orbital paths of earth and moon and the rotation of each body. That means the earth rotates and the moon rotates and sometimes where you stand is closer than at others and hence it looks bigger. If you go near the equator you will see the sun is bigger, because you are nearer to it.

Answer 10

The earth's atmosphere acts as a lens. When the moon is low, you are looking through more atmosphere.