By Scott Levine
It’s a funny thing about the moon’s phases. They don’t care about our calendar.
Even though the word “month” comes from an old Germanic word for the time it takes for the moon to orbit earth, our months don’t line up with the moon’s cycle. If they did, we’d start every month with a new moon, and it’d be full around the 15th.
The Muslim and Jewish calendars, among others, do and highlight the elegance of the moon’s phases.
Instead, a lunation – a full lunar month from one phase back to that same phase again (usually, from new moon back to new), about 29 earth days – is constantly out of sync with our mostly longer calendar months. With a little quick math, we see that we spend our lives trying to squeeze a bit less than 13 lunar months into our 12-month calendar year.
This doesn’t really affect our lives very much, but occasionally we wind up with a blue moon, a term people used for centuries to mean a rare occurrence. Its popular use as a month’s second full moon is only about 30 years old. We have one every two-and-a-half years or so.
This Saturday, Halloween night will be particularly spooky as a blue moon rises above the witches, zombies and swooping bats.
I don’t care for the hype that tends to come with blue moons, but there’s something fascinating if we take a bigger view. The 29-day lunar cycle is shorter than every month except February; therefore, every month except February is likely to have a repeated phase, full moon or not.
Put differently, every 29 days, the moon finishes a lap and returns to the same spot in its orbit, bringing it back to the same phase. Since 29 days is shorter than the length of all non-February calendar months, the moon is likely to be back at the spot it was in at the beginning of a month when the end of the month comes around.
It might not be one of the fancy primary ones, but just like that, we have a repeated “blue” phase every month.
October’s repeated phase is a full moon. Then, we’ll finish 2020 with waning gibbous moons at the start and end of November and December.
As time goes on, the phases move earlier by a day or two each month. If you keep an eye on this as the months pass, you’ll gradually see interesting patterns develop. After full, the next primary phase is last quarter (on Nov. 8). That floating monthly day or two will add up until we have a blue last quarter next July. Then, we’re off into the exotic-sounding blue waning crescents.
Eventually your favorite phases will float to the top and happen twice in the same month, too. While this happens, the previous blue phase pops off the top and drops to the end of the month.
Our calendar and the moon’s cycle are constantly out of sync, but that imperfection puts its own human twist on how we see the sky. It’s always fun to watch. I hope you will this month.
Scott Levine (firstname.lastname@example.org) is an astronomy writer and speaker from Croton-on-Hudson. He is also a member of the Westchester Amateur Astronomers, who are dedicated to astronomy outreach in our area. For information about the club, including membership, newsletters, upcoming meetings and lectures at Pace University and star parties at Ward Pound Ridge Reservation, visit westchesterastronomers.org. Events are free and open to the public. Please Note: All in-person club activities are suspended until further notice due to COVID-19 concerns.