Monday Puzzle

The Presurfer, in cooperation with pzzlr.com, brings you a puzzle every Monday. Just to tickle your brain.
image credit: Bill Lile cc
Far off in the galaxy there is a star system with two planets. These two planets are unusual in that they have exactly the same orbit. They also both have the same rate of rotation. How can it be that one planet has a year that is 359 days long while the other has a year that is is 361 days long?
You can find the answer here.