Well, the Moon orbits the Earth in roughly 30 days (1 month), while the Earth orbits the Sun in 365 days (1 year). The Sun lights the Earth sky for roughly 12 hours a day, while the Moon rarely gets 12 hours of exposure. In fact, at some point throughout the month, the Moon is never visible in the night sky. AND, on top of all that, you can fit hundreds of Moons within the Sun.
Obviously, the Moon has something to be pissed about.
My best guess would be that whoever has that tattoo is pissed about something and can't do anything about it, so he has to compare his struggle to that of the Moon's.
-Warik