There is a rare astronomical event happening on Friday, May 19 that you won’t be able to see. It's a new Moon, the lunar phase where the Moon is not visible to us because the Moon is on the day side. Friday’s one is a rarity because it is an extra new Moon this season, making the so-called Black Moon the new Moon equivalent of a Blue Moon.
Celestial rhythms don’t perfectly align. The Moon goes around the Earth 12.37 times every year, So while it is a good approximation to say that there is a full Moon and a new Moon every month, we end up having extras. These extras are, in non-astronomical terms, known as Black and Blue Moons. They occur on average every two or three years. A more precise value is there are seven extra new Moons within the 235 new moons that happen in almost exactly 19 years. This is known as the Metonic Cycle, which is the basis of many lunar calendars.
What is a Blue Moon?
The Moon's phases have traditionally been used to know when to plant and harvest. The term "Blue Moon" does not originate in astronomical circles but from the 1937 edition of the Maine Farmer's Almanac. These days it refers to two scenarios. One is the third full Moon in a season with four full Moons. Seasons usually have three months and so three Moons. In seasons with four Moons, the third one is known as the Blue Moon.
The second meaning refers to the second full Moon in a month, and was created by accident. A misunderstanding of the original Almanac reference by J.H. Pruitt in a 1946 edition of the American magazine Sky and Telescope described it as being the second full Moon in a month, a definition that is still very popular.
What is a Black Moon?
The term "Black Moon" is far more recent – originating in Wiccan and astrological circles in the last decade – and, like a Blue Moon, doesn't have a single definition. Astrology aside, it is often referred to as a mirror version of the Farmer’s Almanac's Blue Moon, so the third new Moon of a season. This is the type of Black Moon that is occurring this Friday and occurs around once every 33 months.
According to TimeandDate.com, it can also refer to the second New Moon in a month, which occurs roughly every 29 months and is slightly more common.
In either case, the Moon does not change color. It is neither blue nor black (or pink for that matter). It does however present good sky viewing opportunities as the night sky is not polluted with the bright light of a full Moon, so if you want to catch any last remaining Eta Aquarids, now is a good time to look up. So, enjoy not seeing the spectacle on Friday regardless of your preferred definition.