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You might not have heard the term before, but there are few times where some astronomers use the term “black moon”. Here are some of the situations, that might call for a black moon.
When you get the second new moon in a month, some astronomers call that a black moon. This is similar to the situation of when you get a blue moon, for the second full moon in a month. The moon only takes 29 days to go from full moon to new moon and back again. This is shorter than an average month (30 days or so). When you get a new moon in the first couple of days in a month, then you can get another near the end of the month. It’s that second occurrence which would be considered a black moon. Like blue moons, they happen about once every 2.5 years.
Another use for the term black moon is when you get a month that doesn’t have a full moon. This can happen in February, because they have 28 days, on average, and the Moon takes 29 days to go through its phases. These are very rare. That last one happened in February 1999, and the next will occur in February 2018.
You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?