why does the moon seem to be bigger when it is close to the horizon?

Answer 1

This one again...

The most common explanation is that the atmosphere acts like a lense and magnifies the moon. This is only half true. The atmosphere *does* have a lensing effect, but it actually makes the moon's apparant size *smaller*, not bigger. So obviously this is not what causes this phenomenon. Also, the moon is actually slightly farther from you (not the Earth, but you) when it's near the horizon (the reasons for this become obvious when illustrated visually, but it's hard to actually explain with words), which will actually make it appear slightly smaller too.

The idea that it seems to be bigger due to the presence of objects on the ground to comapre it to is not necessarily correct. That can be a contributing factor but it doesn't seem to fully explain it becaue the phenomenon is reported to occur even in areas where the horizon is pretty much flat and barren, giving nothing of note to compare size with. Not everyone experiences this effect and those that do don't seem to always notice it every time.

It boils down to it really being a trick of the mind and a side effect of how our mind perceives our environment. It's not a terribly satisfactory answer, but it's what it is.

Answer 2

The major reason for this illusion is that at horizon, your eyes perceive the Moon as being further away than all other objects (trees, houses, cars). Therefore, the eyes adjust for this distance and will cause the visual angle to be a little larger than its true value (oculomotor macropsia). On the other side, when the Moon is at zenith, you have no other objects to compare it with. This will cause your eyes to adjust to a resting focus position, and will make the visual angle slightly smaller than it really is (oculomotor micropsia). Therefore, due to the oculomotor macropsia, the Moon appears to have a larger diameter at horizon.

Another part of the explanation is the atmospheric refraction.

At horizon, the atmospheric refraction is larger, i.e. the rays of light coming from stars near the horizont are more bent. This phenomenon will cause stars (and other objects on the sky, of course) to appear at a higher altitude above the horizon. The atmospheric refraction is measured by the index of atmospheric refraction, which is 0 at zenith and 30' (half an arc degree) at horizon.

Astronomical refraction won't only cause an object on the sky to have a larger apparent altitude, but, in the case of large objects, with a clear disk shape (only Moon and Sun in the case of Earth, because the other planets' angular diameter is too small), will also increase the apparent diameter.

Why this happens? Well, I couldn't find a nice drawing on the internet, but I can tell you what to draw and you'll figure it out by yourself.

First, draw a small circle, somewhere in the middle of the page; this is the Earth. Now draw a concentric circle with a radius two or three times greater than that of the first circle. And now draw another concentric circle, so that the distance between this circle and the last one is about 1.5 cm. The space between the last two circles is the atmosphere of our planet. Ok... Now take a circular object somewhere "above" the these circles. From each part of the disk, draw parallel rays of light toward Earth, until you reach the atmosphere. When you get to the atmosphere, bent this rays, both at approximately the same angle (the lower one a little more, because it's closer to the horizon). You will notice that the image will be projected at an angle which increases as you take the object lower on the horizon (not so "high above" the circles you draw).

What you see from this drawing is what also happens in the case of Moon. As it gets closer to the horizon, its projected disk on the atmosphere covers a larger angle. But a larger angle also has a larger length, so the diameter of the Moon will appear larger on the horizon.

Answer 3

It is an optical illusion. It is not an atmospheric lensing effect; it is not caused by refraction, scattering, diffusion or any other process. If any of these were the answer, the difference in size would be measurable - it isn't.

It was thought that objects on the horizon made the moon seem further away and therefore larger but the large moon appears at sea and on plains with no objects on the horizon.

What causes it is our perception of the dome of the sky above us. Our brains do not register the sky as being hemispherical but more like an inverted saucer. We think things above us are closer to us than things on the horizon. We know that the sky is not a dome but our brains perceive it to be so. Consequently, they tell us that the horizon is further away than the sky above us so the Moon (or the Sun) looks bigger when it is on the horizon.

Answer 4

Because, as you look at the moon, the lower in the sky it is, the more atmosphere you have to look through to see it.

So when its low, the air acts as a lens and distorts the shape of the moon, making it appear larger.

Is the same reason we get a red sky.
The light of the sun is refracted more by the atmosphere when the sun is low to the horizon. But when the sun is higher, its light has less air to travel through, and the sky is seen as blue.

Answer 5

It works with the constellations as well. If you go to the tropics and see Orion overhead it looks smaller

Answer 6

I think the atmosphere acts as a lens but I have heard they disproved that theory.

Answer 7

Image magnification by a thicker layer of atmosphere.

Answer 8

because you have something to scale it against....IE trees or buildings.
When it it high up there is nothing to give you scale.

Answer 9

It's an illusion.