I'am white,I'am round but not always around~

问题描述:

I'am white,I'am round but not always around~
Question:I'am white,I'am round but not always around.Sometimes I'am half sometimes I'am whole sometimes a slice of me
is all you'll know.
Sometimes I'am light,sometimes I'am dark,sometimes I'am both?Everybody wants to walk on me But only a happy few ever have.---.what am I

I think the answer is moon.