Why does the Moon seem larger when it is close to the horizon? I've noticed that when I look into the sky at night, sometimes the moon appears very large near the horizon. Sometimes it also looks like it has a yellow tint. 



Is this the light rays bouncing off the moon differently, or the atmosphere of the earth, or some type of sunspot blocking part of the light to the moon?
Interesting fact. It Isn't bigger!

This is a well known optical illusion that dates back hundreds of years. It's all in your head, it seems bigger because your brain perceives size as something that is relative. 

This is an effect called perspective distortion and is will understood and used in photography. Phil Plait's Blog (A.K.A - The Bad Astronomer) also writes on this matter and would be worth reading.

Consider the below picture:



As the focal length of a lens gets longer the difference in relative size of foreground and background objects becomes more pronounced. Your brain then interprets this and respectively the moon seems huge!

But why does it look yellowish?

It appears yellow sometimes because our atmosphere blocks and stretches certain wavelengths. So when the moon is low in the sky, it has to pass through the most atmosphere to get to you, stretching and distorting the light through the yellows and oranges.