The moon looks enormous on the horizon and normal overhead, is that the atmosphere, and why does a photo not show it?

Your camera is telling the truth and it settles the first half of the question: the moon is the same angular size in both cases.

In fact it is very slightly smaller at the horizon, because you are further from it by roughly the radius of the Earth. The difference is tiny and it is in the opposite direction from what you see.

So it is not magnification and it is not the atmosphere. The atmosphere does two things near the horizon - it reddens the moon by scattering, and it flattens the disc very slightly by refraction: and neither makes it bigger.

What you are experiencing is entirely a perceptual effect, which is why it is called the moon illusion and why a camera cannot record it. The image on your retina is the same size; the interpretation is not.

That is also the answer to why photographs are always disappointing. People take them expecting the camera to capture what they saw, and the camera has no perceptual system. The only way to get a photograph that looks like the experience is a long lens with foreground objects in the frame: which, as the next answer explains, is not a coincidence.

And it is genuinely old: the illusion was written about thousands of years ago and it is still not fully settled.

On what the perceptual explanation actually is - there are several and the honest position is that none is universally accepted.

The leading family of explanations involves context and distance judgement.

Near the horizon the moon is seen alongside trees, buildings and hills - things whose distance you understand. Your visual system judges it to be far away, beyond all of them. Overhead there is nothing to compare it with and no distance cue at all.

When something occupies the same angular size but is judged to be further away, the system concludes it must be physically larger, and you perceive it that way. This is the same mechanism behind several classic size illusions, and it is why the effect is stronger over a landscape with depth than over a flat sea.

There are competing accounts - some involving how the eyes converge, some involving how the brain handles the sky as a flattened dome rather than a hemisphere: and there is evidence that more than one thing contributes.

What everybody agrees on: it is in perception, not in optics, and the size on your retina does not change. The disagreement is about which perceptual mechanism dominates.

Three demonstrations you can do the next time it happens, which is much more convincing than any explanation.

Hold something up to compare. An aspirin at arm's length, or the nail of your little finger, roughly covers the moon. Do it at the horizon and again overhead. Same coverage both times, and it is startling how much you disbelieve your own hand.

Remove the context. Look at the horizon moon through a rolled-up tube of paper, so nothing else is in view. The moon shrinks immediately. This is the strongest one and it is essentially a direct demonstration of the leading explanation: take away the landscape and the illusion goes with it.

Look at it upside down. Bend over and view it between your legs, or lie down. The effect weakens or disappears for most people, which is hard to explain by anything in the light and easy to explain by perception.

On photographing it: use the longest lens you have and include something recognisable - a building, a ridge: at a distance. That gives the viewer of the photograph the context your eyes had, and it is why the dramatic moon photographs you see are always shot from a long way back with a long lens rather than up close with a wide one.