The answer is that it is an optical illusion of your brain, the moon doesn't change.
To test this, go outside at noon (when the sun is highest and 'smallest'), take a penny, close one eye, and hold the penny out towards the sun so that it perfectly blocks the sun. Note how far away the penny is from your eye.
Now go out at sunset when the sun is low on the horizon and seems huge. Again take the penny and hold it out to where it perfectly blocks the sun. You will notice that you are holding the penny at the exact same distance from your eye, meaning the size of the sun is exactly the same, you just perceive it to be huge.
This is a natural adaptation of our brain, allowing us to deal with distance. For example, if you see a huge tower far away in the distance, you know that it is huge, even though in actuality it is very small from being so far away, much smaller than something close to you (like a tree or a house).
What happens with a sun/moon that is low on the horizon is this unintended side effect of our brains dealing with distance. Because it is low on the horizon and is so far away, you perceive it as being huge, when in fact it is the exact same size as when it is overhead.
Slashdot Mirror
You're supposed to completely block the sun with the coin, so you're never really looking directly into the sun. That's the point.
The answer is that it is an optical illusion of your brain, the moon doesn't change.
To test this, go outside at noon (when the sun is highest and 'smallest'), take a penny, close one eye, and hold the penny out towards the sun so that it perfectly blocks the sun. Note how far away the penny is from your eye.
Now go out at sunset when the sun is low on the horizon and seems huge. Again take the penny and hold it out to where it perfectly blocks the sun. You will notice that you are holding the penny at the exact same distance from your eye, meaning the size of the sun is exactly the same, you just perceive it to be huge.
This is a natural adaptation of our brain, allowing us to deal with distance. For example, if you see a huge tower far away in the distance, you know that it is huge, even though in actuality it is very small from being so far away, much smaller than something close to you (like a tree or a house).
What happens with a sun/moon that is low on the horizon is this unintended side effect of our brains dealing with distance. Because it is low on the horizon and is so far away, you perceive it as being huge, when in fact it is the exact same size as when it is overhead.