Never
make the statement "That's obvious!" It rarely ever is.
Winter

http://www.fourmilab.ch/earthview/vplanet.html

It
is easy to confuse understanding with knowing. You may know that during
summer in the Southern hemisphere daylight can last all day but understanding
that concept is a different matter all together.
Do
we know about gravity? . . . .well yes.
Do
we understand how it works? . . well maybe not!