Sommers-Bausch Observatory - University of Colorado

Blue Moon for June

Original article by Keith Gleason appeared in The Old Lyons Recorder, Lyons, Colorado, on June 27th,1996.

There's nothing more romantic than a full moon in June - the traditional month for lovers and weddings - except, perhaps, two of them! Two and a half hours before end of June, starry-eyed lovers in Lyons will have the benefit of a second full moon in a single calendar month - a phenomenon known as a Blue Moon. Europeans will miss out entirely, since for them July will already have begun.

A Blue Moon is really the result of ancient Roman politics rather than an astronomical event. In the good old days before Julius Caesar, the calendar was directly tied to the cycle of lunar phases, or "moon-ths". Since the Moon returns to its alignment opposite the Sun (where it appears fully illuminated) every 29.5 days on the average, the Romans developed a calendar year consisting of twelve months alternating between 29 and 30 days in length. The calendar added up to only 354 days, or 11.25 days short of a full year. The priests, who were placed in charge of such things, made up the difference by occasionally throwing in an additional month (sandwiched in the middle of February) to benefit their friends in public office.

In 46 B.C., Julius Caesar decreed a new calendar scheme intended to put an end to political favoritism and bureaucratic disregard for the seasons. The year was padded to its correct length by adding an extra day to each of eleven months, with the remaining month receiving a bonus day every four years. The corrupt Roman Senate was underwhelmed with appreciation, assassinated Caesar, and then placated its constituency by renaming one of the months in his honor. Political prudence dictated that they also rename the following month to honor the new incumbent, Caesar Augustus - and just to play it safe, a day was robbed from February (which was always getting messed up anyway) and tacked onto August so that the two months would be of equal importance.

The expression "once in a Blue Moon" is the familiar phrase used to describe events that are rare. But thanks to the seven long months of the Julian calendar, Blue Moons aren't really that scarce - they come along about every two and a half years or so. Two full moons can fit into a 31-day period whenever the first occurs within the initial 36 hours of the month - a 5 percent, or 1 in 20 chance. A Blue Moon in a 30-day month, however, is only one-third as likely, since the "window" for the first full moon is only 12 hours long. (Actually, the odds are slightly better for June: one chance out of 47 - but to explain why, I'd first have to convince you that the Earth is physically closer to the Sun in wintertime than in summer, so you probably wouldn't believe me anyhow!) Of course, poor little February has no chance at all, even in leap years.

History and statistics aside, few celestial events offer up such a catchy rhyme-scheme as the Blue Moon of June. The next such opportunity for the romantic poet won't come along again for 57 more years, until the year 2053. And so, young lovers, make the most of it - the next one is reserved for your great-grandchildren.