‘Blue’ moons not nearly as rare as two moon seasons: 287 years until the next one

‘Once in a Blue Moon’ is supposed to mean something exceedingly rare, or something that almost never happens, but an actual ‘Blue Moon,’ the third of four full moons in a season, isn’t all that uncommon, with four-full-moon seasons happening 37 times this century.

A two-moon season – just two full moons in the three-month period between a solstice and an equinox, or an equinox and a solstice – is extremely rare, however, with the last time it happened nearly 65 years ago.

A full moon fell shortly before the December solstice in 1961 and after the March equinox in 1962, leaving only enough room for two full moons during that season.

The next time it will occur? According to EarthSky, the next two full moon season won’t happen for another 287-plus years. In fact, between 1962 and the year 3000, it happens only four more times, in the winters of 2314/5, 2333/4, 2686/7 and 2705/6. In each case, David Harper of obliquity.com told EarthSky, there is a full moon less than five hours before the December solstice, and there are four full moons in both the preceding autumn and following spring.

A two-full-moon season is only possible around the December solstice, which corresponds to the northern winter and southern summer. This is the shortest season of the year, lasting about 89 days.

For a winter season to have only two full moons, the December full moon has to occur just before the December solstice.

Also, EarthSky’s Bruce McClure reported, the full moons from December until March must “closely coincide” with lunar apogee – the moon’s farthest point from Earth in its monthly orbit. “When full moons happen appreciably close to apogee, the time period between successive full moons is longer than average. The shorter season plus longer lunations (lunar months) conspire to give an extremely rare two-full-moon season.”

“Once in a two-moon season” just doesn’t have the same ring to it, but less werewolf howling time might be a good thing, however.