Early Tuesday skygazers in Europe and Asia had a chance to see something that happens once in a blue moon: a blue moon.

The moon was not blue, but it was the second time in May that the moon was full. The first full moon this month was May 1.

For 12 out of 19 years, there are 12 full moons -- one for each month. The other seven years have 13 full moons. So at least one of the months must have two full moons, one on the first or second day of the month and the second 29.5 days later on the 30th or 31st.

A blue moon can occur in any month of the year except February, since this month always has fewer days than the 29.5-day lunar cycle.