On December 31st, we will see the second full moon of the month, or the 13th full moon of the year. These rare occasions are called a blue moon, as in “once in a blue moon”. But that’s not the only thing special about New Year’s Eve this year. There will also be a partial lunar eclipse on the 31st (visible in Europe, Africa, Asia, and Australia. A Blue Moon happens every 2.7 years because of a disparity between our calendar and the lunar cycle. The lunar cycle is the time it takes for the Moon to revolve around Earth: 29 days, 12 hours, and 44 minutes.