What's blue about a blue moon?

You've all heard the phrase, "once in a blue moon," but did you know that this is a phenomenon we will experience in the early months of 1999? Popular tradition holds that when two full moons occur in a month, the second one is the blue moon. And in January and March 1999, we will be treated to two of them.

Sky & Telescope columnist Fred Schaaf says this is an unusual but not extremely rare occurrence. "It is possible because a calendar month can have 30 or 31 days," he explains, "while the astronomical lunar month is 29.5 days."

After being full on Jan. 1 at 9:49 p.m. EST, the moon will be full again on Jan. 31 at 11:06 a.m. EST.