Why does the sun appear so much larger when it is on the horizon then when it is high in the sky?
That the Sun appears larger when it is on the horizon is just an optical illusion. The brain thinks that objects on the horizon should be farther away than objects overhead; since the Sun is the same apparent size in both places, the brain concludes that the Sun is physically bigger when it's on the horizon, and thus tricks you into thinking that the angular size is bigger than when it's overhead. This phenomenon is known as the Ponzo Illusion, and occurs for the Moon as well.
To convince yourself that this is, in fact, an optical illusion, put your head between your legs and look at the Sun upside down when it's on the horizon: it should look the same as it does when overhead.
For more information about the "larger Sun" and other astronomical myths, check out Phil Plait's article (now on the Moon and not the Sun but it's the same idea!).
This page was last updated on February 10, 2016.