A rare 'black moon' rises this weekend: What is it, and what can you see?

At precisely 2:06 a.m. EDT Saturday (Aug. 23), the moon will enter its new moon phase — something it does every 29.5 days. However, this new moon will have a special title and significance on the calendar. Meet the "black moon."

It's not something that can be seen in the sky, however. A new moon occurs when the moon passes roughly between Earth and the sun, making the surface of the moon invisible from Earth. So why is this particular new moon called a "black moon"?

A black moon is the opposite of a blue moon — and just as rare. As with blue moons, there are two types of black moon. A new moon can get that name if it's the second new moon in a single calendar month. That can happen when there's a new moon on or around the first day or two of a month. It's then guaranteed that a second new moon will occur later that month. This kind — called a monthly black moon — occurs approximately once every 29 months, according to Time and Date. (The next monthly black moon will occur on Aug. 31, 2027.)

However, astronomers also use the term "black moon" to refer to the third new moon in a season of four new moons. That particular calendar quirk is what's happening this weekend — and it's all down to a new moon occurring soon after a solstice or an equinox.

The current season — summer in the Northern Hemisphere and winter in the Southern Hemisphere — began with the solstice on June 20 or 21 (depending on your time zone) and will end with the equinox on Sept. 22. Within that period, there are new moons on June 25 (just four days after the solstice), July 24, Aug. 23 and Sept. 21 (one day before the equinox). It's a tight squeeze, but there's just enough time for four new moons to occur in a single summer.

The third of these new moons (on Aug. 23) is known as a seasonal black moon. This type of new moon occurs once about every 33 months — making it slightly rarer than a monthly black moon.

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Although you won't be able to see the black moon with the naked eye, its timing offers a special opportunity for stargazers: a moonless night perfect for enjoying the summer stars just as the Milky Way is looking its best from the Northern Hemisphere.

The best way to get a good look at the arc of our galaxy overhead is to find a location away from light pollution, preferably somewhere with no cities on the southern horizon. Find the three bright stars of the vast Summer Triangle in the southeast — Vega, Deneb and Altair. The Milky Way will be streaming through the left side of the Summer Triangle, roughly from Deneb down to Altair and, from there, down to the southern horizon.

Although the Milky Way is visible in any moonless night sky, the night of the black moon is the perfect opportunity to see it at its best.