In some languages of Nordic people there are more than a dozen definitions for ‘snow” that shows a role the snow plays in their life. Something similar happens with the Moon, though we have one word “Moon” we love diversifying it as Blue Moon, Cold Moon, Super Moon, Red Moon, Black Moon… In recent years the term ‘Black Moon’ has been made rather popular by astrologers and followers of some religions, but at the same time Black Moon is not well known in the astronomy world where there is even no single accepted definition of Black Moon.
In Astronomy this term has been commonly used to refer to any of the following phenomena associated with the New Moon:
Second New Moon in a calendar month: These Black Moons occur relatively often, once every 2.5 years;
Third New Moon in a season of four New Moons. Astronomers divide a year into four seasons – spring, summer, fall (autumn) and winter. Usually, each season has 3 months and 3 New Moons. When a season has 4 New Moons, the 3rd New Moon is called Black Moon;
Calendar month without a New Moon. This can only happen in the month of February. When this occurs, January and March have two New Moons, instead of the usual one New Moon – this case was mentioned in The Luminaries by Eleanor Catton as a mystic period of time when the most unexpected things can happen;
Calendar month without a Full Moon. About once every 19 years, the month of February does not have a Full Moon. Instead, January and March have two Full Moons each – also looks quite mysterious.
Interestingly that all these cases refer to a beyond the scope Moon position and we will have a chance to see (or not to see) some of them in the future:
30 Oct 2016 Second New Moon in a single calendar month
21 Aug 2017 Third New Moon in a season with four New Moons
Feb 2018 Calendar month without a Full Moon
30 Aug 2019 Second New Moon in a single calendar month