A rare black moon event will happen Aug. 23.The new moon/black moon phase is not visible from Earth.Slightly offset solar/lunar calendars lead to events like black moons and blue moons.
Once every lunar cycle the moon becomes invisible to Earth-bound viewers as the side we can see falls into a full shadow in a phase known as the new moon.
When the antithesis of the full moon occurs twice in a month, or is the third of four seasonal new moons, it earns the non-astronomical moniker of “black moon,” a rare occurrence that will appear once again on Aug. 23.
But “appear” needs to be taken with a grain of salt because this weekend’s black moon, as with all new moons, will pass unseen.
NASA explains that in this phase, the moon is in the same part of the sky as the sun and rises and sets with the sun. Not only is the illuminated side facing away from the Earth, it’s also up during the day. In this phase, the moon doesn’t usually pass directly between Earth and the sun, due to the inclination of the moon’s orbit. Of course, those rare instances when it does are known as solar eclipses.
A quirk of conflicting calendars?
In an interview published earlier this week, Northeastern University astrophysicist Jonathan Blazek noted the slight difference between the Gregorian and lunar calendars is at the heart of “rare” lunar events like the black moon, blue moon and others.
“Because a month and the lunar cycle are similar in length, we typically have one full moon and one new moon each month,” Blazek said. “Similarly, since the seasons are each one quarter of the year (about three months), we typically have three full moons and three new moons in each season. However, because the lunar cycle (29.5 days) is slightly shorter than a month, we can sometimes get an extra full or new moon. There is nothing different about these full moons or new moons — it is simply a quirk of using a non-lunar calendar cycle.”
That quirk occurs about once every 33 months or so and is similar to the occurrence of blue moons — two full moons in a single month — which happen about once every two or three years.
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, according to a report from Live Science.
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.
Weekend stargazers may also catch the tail end of summer’s best meteor show, the Perseids, which wraps up its active phase Aug. 23.