Good evening, Kibitzers! Did you all see the “blue moon” on Saturday night, the one responsible for Sunday being Easter? [Warning: pedantry ahead!]
It’s sort of a misnomer; a “blue moon” was originally the name for the third full moon in an astronomical season that has four full moons. If that sounds a little random (it did to me!) there’s an explanation: People used to care about the names of the full moons in each month, although now I think most of us would be hard-pressed to come up with any names beyond “Harvest Moon”.
The time between full moons is less than 30 days, so a 13th full moon can wedge itself into a year. When that happens, whatever three-month season has four full moons won’t have a name for one of them. Calling the third one the “Blue Moon” kind of splits the difference and keeps the other moon names close to the months they should be in. This original usage is now called a “seasonal blue moon”.
The more straightforward definition, that a blue moon is the second full moon in a month, arose as an error in a magazine article in 1946, but it stuck around, probably because it was easier to understand, and is now what most people mean by the term. Any kind of blue moon is somewhat rare: the next seasonal blue moon will be May 18, 2019, but the next blue moon that is the second in a month will not be until Halloween of 2020. This year had two months with a blue moon, January and March; that won’t happen again until 2037. (This page is an excellent source for moon phase information. You can choose your location at the top.)
So anyway, Easter. Easter is a “moveable feast” because the events surrounding Jesus’s crucifixion and resurrection are said to have taken place in connection with Passover, and Passover occurs in a lunar month that moves around relative to the Julian and Gregorian calendars. In the year 325 CE, the First Council of Nicaea attempted to address controversy over properly finding the date of Easter (among other controversies!) [marge has contributed a wonderful poem about Easter, Passover, and the moon. Thanks, marge!]
While they were able to determine to their satisfaction that the Jews were wrong about how they identified the time of Passover (!!!), it took a mind-numbing number of additional centuries to establish the actual specific rule that is used today: roughly, Easter is the first Sunday after the first full moon on or after the vernal equinox. So, Saturday, March 31, was the full moon in question this year. Had the full moon been on Sunday, Easter would have been this coming Sunday instead. (I am sure it will not surprise you to learn that it is not even as simple as this, because there is an “ecclesiastical full moon” and “ecclesiastical equinox” that may or may not coincide with the astronomical ones. The US Naval Observatory has a good writeup of the details.)
It did snow on Monday, several inches here, but it could be worse — by this rule, Easter can occur as early as March 22, if the full moon is on the equinox and the equinox is on Saturday. We had a March 23 Easter in 2008, but that will not happen again until 2160, and it will next fall on March 22 in 2285 (Stardate 8191.2). I am afraid snow will not be a problem by then. The latest possible Easter date by this method is April 25.
That’s way too much about Easter, and not enough about the blue moon. There’s an obvious cure for that. If you don’t like these, just ask YouTube, because nearly everyone has recorded this Rogers & Hart song. Do you have a favorite cover?