The Moon's Size?

[Undomiel]

I hope this isn’t a silly or obvious question ... the way that the moon looks much larger on the horizon than it does when high in the sky? I previously believed that this was an optical illusion, either because when the moon is closer to the horizon the observer’s subconsciously comparing it with whatever’s on the horizon - mountains, trees? Or, an alternative theory I’ve heard of, that it’s an optical illusion caused by seeing the moon through the atmosphere’s thickness ... looks bigger because the atmosphere’s thicker when it’s closer to the horizon than higher in the sky? Are these hypotheses wrong? Because I’ve lately read that there actually IS a measurable difference in the moon, at different times, of up to 10%? Does anyone know if this is true, and whether it’s related to the moon appearing larger when closer to the horizon?

[Janus]

Your first inclination is correct; It is due to the fact that the eye has something to compare it with at the horizon.

The atmospheric effect is very small, and doesn't make the Moon look larger anyway, it tends to flatten the Moon from top to bottom instead.

Yes, the apparent size of the Moon does change, this is due to the fact that the orbit of the moon is not a perfect circle, and thus its distance from the Earth changes. This has no connection with its position with respect to the horizon, but is a day to day comparison.

[Labguy]

About Janus' last paragraph, see the link for some cool size comparisons and lots of Moon info:

http://www.fourmilab.ch/earthview/moon_ap_per.html