Black moon is an unofficial term for a new moon that can be defined two ways: when a new moon appears twice in one calendar month, or when a season contains four new moons and the third is labeled the black moon. During a new moon the moon lies nearly between Earth and the Sun so the illuminated side faces the Sun and the Earth-facing side is dark and effectively invisible at night. The moon is tidally locked so the same face points toward Earth. A black moon produces darker skies, aiding stargazing such as viewing the Dumbbell Nebula (M27). This weekend's black moon is the third new moon of summer; the next new moon occurs Sept. 21.
When the moon is nearly between Earth and the sun, one side of the moon faces the sun, and that side is illuminated, but the side the Earth sees is dark. This is called the new moon phase of the lunar cycle when the moon isn't visible at night. During this phase, the illuminated side of the moon is not only facing away from Earth, but it is also up in the sky during the daytime
The moon is gravitationally locked with Earth, meaning we always see the same side of the moon from Earth. It's unclear when it was coined, but "black moon" is an unofficial astronomical term. A new moon is considered by some to be a black moon one of two ways. It can happen when a new moon appears twice in one month (new moons usually happen once per month) or when there are four new moons in one season.
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