A February without a full moon

The average interval between two full moons is 29 1/2 days. With a full moon on Jan. 31, and February only 28 days long, the full moon skips that month entirely and lands on March 1, leaving February fully moonless. Then, in 29 1/2 days, a full moon lands on March 31, making for two Blue Moons in three months!

It feels surprising to have a month without a full moon, but it makes sense based on our calendar. Eleven months have 30 or 31 days, with two moons possible in any one of them from time to time. On average, we get a Blue Moon month once every two to three years. But February is too short to ever have two full moons, even when it bulks up to 29 days during leap years. Twenty-nine days is half a day shy of the time between one full moon and the next. If a full moon were to land on Feb. 1 in a leap year, the next would occur on March 1.

And of course, if the timing is right, as it is this year, February gets skipped altogether. Interestingly, if February has no full moons, then both January and March must have two. That's the unavoidable math of the matter. The last time a fully moonless February occurred was in 1997, and the next will be in 2037, so this calendrical oddity happens infrequently.

What February will have are two barely-full full moons. On Feb. 1, the moon will be one day past full, and on Feb. 28, one day before full. To our eyes, they'll appear so close to full many of us will strain to tell the difference.

The moon takes 27.3 days to go around the Earth once but 29.5 days to go from one full moon to the next. Why the difference? As it goes around the Earth, the Earth also revolves part-way around the sun. For the sun, Earth and moon to line up again so we see a full moon, the moon has to travel an extra 2.2 days along its orbit.

No one knows for sure why February has only 28 days, but it goes back to pre-Christian Rome when the calendar was based strictly on the moon's comings and goings. In 45 B.C., emperor Julius Caesar reformed the calendar to sync it with the sun rather than the moon. He added 10 days to the year (which before then had 355 days) and added an extra day to February every four years. This lengthened the year to 365.25 days, which is very close to the actual average day length of 365.2425. Further refinements to keep the dates of the equinoxes and solstices (start of spring, summer, fall and winter) in lockstep with the position of the sun were made by Pope Gregory XIII in October 1582.

That's the calendar we use to this day.