Once in a Blue Moon main content.

Once in a Blue Moon

by Neil deGrasse Tyson on


Full Moon
The Full Moon.
Credit: Lick Observatory

On New Year's Eve the Full Moon will rise shortly after sunset—doing just what full moons are supposed to do. In this case, however, it'll be the second full moon in the calendar month of December. Whenever this occurs, recent tradition identifies it as a Blue Moon. The average time between full moons is about 29.5 days. So any month but February can, in principle, harbor a Blue Moon. If you do the math, you will see that somebody gets a Blue Moon every 2.7 years, or so. Not particularly rare—no one thinks of Presidential elections as rare, yet they take place less often than Blue Moons. The last Blue Moon on New Year's Eve was 1990.

The Moon can actually look blue during rare (polluting) atmospheric conditions that involve volcanic eruptions and forest fires.

Each Full Moon of the year has a name, mostly traceable to an era of agricultural living. Most famous among them is the "Harvest Moon" (nearest to the fall equinox), and the "Honey Moon" (June), but others include the "Sap Moon" (March), and the "Grain Moon" (August).

Times have changed, of course, which led me several years ago to update these Moon names with references more relevant to our modern culture. How about:

January Super-Bowl Moon
February Dirty-Snow Moon
March Spring-Break Moon
April Tax-Return Moon (if before the 15th)
Late-Fee Moon (if after the 15th)
May Memorial Moon
June Summer-Vacation Moon
July Independence Moon
August Muggy-Night Moon
September Back-to-School Moon
October World-Series Moon
November Thanksgiving Moon
December White-Christmas Moon (if you live in the North)
I'm-Dreaming-of-a-White-Christmas Moon (if you live in the South)

*Adapted from "Merlin's Tour of the Universe"

Happy New Year to all—'twas just another 584 million miles around the Sun.

-Neil deGrasse Tyson