The Confusing Case of The Black Moon

Hey, sky fans! A day or so ago, I started noticing things on the Internet shouting about something called a black Moon, which will be in your skies on Friday night, September 30. This is sort of the opposite of a blue Moon, which is most commonly meant as the second full Moon in a calendar month.

Meanwhile, a black Moon is the second new Moon in a month. There are other similar definitions, too. The third of four in a season, stuff like that. So, September 1 had a new Moon, and we’ll have another on Friday, September 30.

I admit, until the other day, I’d never heard the term before, though, for many years, I wondered if anyone ever bothered to give it a name. I certainly didn’t bother.

Having two full or new Moons in a month is rare, but not really that rare. Since a lunar month is about 29 days, and there are 365.25 days in a year, there are, on average slightly more than 12 full lunar cycles a year. This means about every two and half years, really, two years eight months or so, there’ll be two full or two new Moons in a given month.

So, sometimes there are two full Moons. Sometimes there are two new Moons. Let’s take a bigger view, though. Since a 29.5-day lunar cycle is shorter than every month except 28-day February, every month except February is likely to have a repeated phase, every year, though once in a while it falls out of sync.

Put differently, every 29.5 days, the Moon finishes a lap and returns to the same spot in its orbit around the Earth. This means it’s at the same phase. Since 29.5 days is less 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. Blammo! A repeated phase every month.

For the next year:

  • August 2016: thin, waning crescents on the 1st and 31st.
  • September: new Moon on the 1st and 30th
  • October: thin, waxing crescents on the 2nd and 31st.
  • November: thin, waxing crescent on the 1st & 30th.
  • December: waxing crescent on the 1st & 31st.
  • January 2017: waxing crescent on the 2nd & 31st.
  • February: not enough days for a full cycle. No repeating phases.
  • March: waxing crescents on the 1st & 2nd and 30th & 31st.
  • April: waxing crescents on the 1st & 30th.
  • May: near first quarter on the 1st & 2nd and the 31st.
  • June: first quarter on the 1st, not quite first quarter on the 30th.
  • July: first quarter on the 1st and 30th.
  • August: waxing gibbous on the 1st & 31st.

Another exciting definition of a black moon is a month that has no full moon at all. This can only happen in February. The next one of these is in February 2018.

screenshot-from-2016-09-28-11-11-26
The lack of the Moon, September 30, 2016 (screenshot from Stellarium)

The trouble with being really excited about a black Moon is we can’t see it.

Earlier this year, we talked about the “Dark Side of the Moon Day,” as another way to think of the New Moon phase. The moon is between the Sun and Earth at new Moon. This means all of the Sun’s light is hitting the far side of the Moon. From Earth, there’s nothing to see because all of the Sun’s light that hits the Moon is being reflected away from us. So, there’s nothing to see!

Also, lots of these stories say the Moon will be invisible to us, which is true, but they say the Moon will be up at night, “Black Moon In Friday Night’s Sky!” Nope. New Moons rise and set with the Sun; they’re up in the day, not at night. Look at night if you like, but there’ll be even less to see, since the Moon won’t be there.

Since the Moon will up during the day, when clear skies are blue, the Moon wouldn’t be black at all; it’d look blue! You’ve seen this before, when you’ve seen a terrific waning gibbous Moon in the daytime, as it sets in the west with a gorgeous tint to its lit side, and the shadowed side lost in the blue sky. So, maybe, ironically enough, a better name for a black Moon is a blue Moon!

These things always make me smile and chuckle. I hope, whether or not you see the Moon tomorrow, or any other day, your skies are clear. See you soon, everyone!

 

 

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