A black moon refers to a second new moon that occurs within a single calendar month. A black moon can only occur when the first new moon in a month takes place early enough so that the second new moon occurs within the same calendar month.
A lunar month is the amount of time it takes for the Moon to complete a cycle through all the phases, from new moon to full moon and then back to new moon again. The lunar month takes 29.53 days, which is very close to the length of a month in the Western calendar, which usually have 30 or 31 days.
Most calendar months typically have one of each of the moon phases — new moon, first quarter, full moon and last quarter moon. But every now and then (2.72 years, to be exact), a month will have two of the same phases. When a month has two new moons, the second one is called a “black moon”.
According to UniverseToday.com, “Another use for the term black moon is when you get a month that does not 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.”
Although the odds of having two new moons within one calendar month is just the same as having two full moons, the new moons often go unnoticed. New moons are almost invisible, so most people don’t notice the first new moon or the black moon when a month has a second new moon.