If the second Full Moon in one month has a special name, what about the second New Moon?
Most people don't notice New Moons. It's easy to see the Moon when it's full, but the only way to tell when a New Moon is happening is during an eclipse, or by referring to an almanac or using Obliquity's Moon Phase calculator.
To Wiccans, the second New Moon is called the Black Moon, and any magic worked during that period is deemed to be especially powerful.
Of course, the chances of two New Moons falling within one calendar month are just the same as two Full Moons, but because New Moons are generally invisible, most people tend not to notice the occasions when a month has two of them.
That's not to say that New Moons aren't important to non-astronomers. To the world's Muslims, the date of New Moon is of great interest, since the Islamic calendar is governed by the phases of the Moon: the start of each month is marked by the first sighting of the new crescent Moon.
According to an article in the May 1999 issue of Sky and Telescope, the traditional definition of a Blue Moon is the third Full Moon in a season which has four Full Moons. Compilers of almanacs such as the Maine Farmer's Almanac would use a coloured symbol to denote this third Full Moon, hence the name.
Similarly, a Black Moon can be the third New Moon in a season which has four New Moons.
Here is a list, by that definition, of the Black Moons between 1900 and 2099. Dates and times are given in Greenwich Mean Time.
We would like to thank Dr Horst Meyerdierks of the Royal Observatory Edinburgh for pointing out errors in a previous version of this web page.