Nib asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
why does the moon look bigger near the horizon
Here's a puzzle you've seen. A moon sitting on the horizon looks enormous, but the same moon high overhead looks small and far away.
But the moon did not grow. Hold a pencil eraser at arm's length and it covers the moon in both places. Both span about half a degree of sky.
It isn't the air acting like a magnifying glass, either. The horizon moon is actually about one point seven percent smaller, because you're a bit farther from it.
So the trick is in your head. The sky doesn't look like half a ball. Nobody has one settled answer, but a leading idea: it feels like a squashed bowl, so the horizon feels far away.
A leading idea is a brain rule: two things with the same size image, the one that seems farther may be judged bigger. Scientists still argue.
So here's the whole picture. One moon, one half degree. Nobody has one settled explanation, but a far-feeling horizon sky may play a part. Next moonrise, watch it shrink as it climbs.
The moon never changes size; the illusion is in your head, and a pencil eraser can prove it.