While a full moon refers to the moment when the moon's Earth-facing side is fully illuminated by sunlight, a new moon refers to the moment when the moon's Earth-facing side is fully in shadow. (Unfortunately, that means the Black Moon will be more or less invisible, even if the moon is high in the sky).
Although "Black Moon" is not an official astronomical term, there are two common definitions for it. Because the lunar calendar almost lines up with Earth's calendar year, there is typically one full moon and one new moon each month. A second full moon in a single calendar month is sometimes called a "Blue Moon." By this definition, a Black Moon is the flip side of a Blue Moon: the second new moon in a single calendar month.
By the second definition, the term refers to an extra full moon in a season; because Earth's seasons are approximately three months long, they typically have three new moons. The Northern Hemisphere summer of 2020 (or winter in the Southern Hemisphere) has four new moons, in June, July, August and September. June's new moon arrived one day after the summer solstice, and September's new moon will arrive five days before the autumn equinox.
Related: Black Moon, Blood Moon, Blue Moon: What's with all these weird moon names?
A Black Moon (in some parts of the world)
The Black Moon is a somewhat unusual celestial event — they occur about once every 32 months, on average, and they sometimes only occur in certain time zones. The new moon occurring on Tuesday, Aug. 18 is a Black Moon. Officially, it occurs at 10:41 p.m. EDT (0241 GMT Aug. 19).
The next Black Moon won't arrive until April 30, 2022, and it will be the second new moon in a single calendar month. The next Black Moon by the seasonal definition of the term will occur on May 19, 2023.
Seeing (or not seeing) a Black Moon
At its "new moon" phase, the moon is always black. It happens at that time of the month when the moon passes through the same part of the sky as the sun and as such, the moon's dark or unilluminated side faces Earth. So there really is nothing to see.
Actually, that's not always true, since there are times when the new moon passes directly between Earth and the sun and Earthlings can then see the moon's black silhouette crossing in front of the sun, causing a solar eclipse.
Wait for the crescent
If you have ever wondered where the term "new moon" originated, it simply refers to the start of a new lunar cycle.
The time frame from one new moon to the next is called a synodic month, which, on average, lasts 29.53 days. This is the period of the moon's phases, because the moon's appearance depends on the position of the moon with respect to the sun as seen from the Earth. The word "synodic" is derived from the Greek word sunodikos, which means "meeting," for at new moon, the moon "meets" the sun.
But unlike a "supermoon," which gets countless numbers of people scurrying for vantage points to see a slightly larger and slightly brighter-than-average full moon, with a Black Moon, you simply can't see it.
A couple of evenings later, however, you'll be able to pick out a slender sliver of a waxing crescent moon low in the western twilight sky about 30 or 40 minutes after sunset local time.
Some people mistakenly refer to the appearance of any thin lunar crescent as the "new moon." This fallacy has even spread into popular literature. In his classic work "A Night to Remember," about the sinking of the Titanic, author Walter Lord quotes a fireman in a lifeboat who caught sight of a narrow crescent low in the dawn sky and exclaimed, "A new moon!"
A note on branding
As one who has been involved in the broadcasting field for nearly 40 years, I'd like to point out that we live in a time when the news media is seemingly obsessed with "branding." This marketing strategy involves creating a differentiated name and image — often using a tagline — in order to establish a presence in people's mind. In recent years in the field of astronomy, for example, we've seen annular eclipses — those cases when the moon is too small to completely cover the disk of the sun — become branded as "Ring of Fire" eclipses. A total eclipse of the moon — when the moon's plunge through the Earth's shadow causes the satellite to turn a coppery red color — is now referred to as a "Blood Moon."
When a full moon is also passing through that part of its orbit that brings it closest to Earth — perigee — we now brand that circumstance as a supermoon. That term was actually conjured up by an astrologer back in 1979 but quite suddenly became a very popular media brand after an exceptionally close approach of a full moon to Earth in March 2011. It surprises me that even NASA now endorses the term, although it seems to me the astronomical community in general shies away from designating any perigee full moon as "super."
Then there is Blue Moon. This moniker came about because a writer for Sky & Telescope Magazine misinterpreted an arcane definition given by a now-defunct New England Almanac for when a full moon is branded "blue," and instead incorrectly reasoned that in a month with two full moons, the second is called a Blue Moon. That was a brand that quietly went unnoticed for some 40 years, until a syndicated radio show promoted the term in the 1980s and it then went viral. So now, even though the second full moon in a month is not the original definition for a Blue Moon, in popular culture we now automatically associate the second full moon in a calendar month with a Blue Moon.
So are you ready for yet another lunar brand? The newest one is Black Moon.
Editor's note: This article was originally published for the Black Moon of July 31, 2019 and updated for the Black Moon of Aug. 18, 2020.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmer's Almanac and other publications, and he is also an on-camera meteorologist for News 12 Westchester, N.Y. Follow us on Twitter @Spacedotcom and on Facebook.