A ‘black moon’ is not an official astronomical term, and it does not refer to anything unusual from a physical perspective, says Northeastern University astrophysicist Jonathan Blazek.
A purportedly rare “black moon” event is slated for this weekend — August 22 and 23.
What is a black moon, and what does it mean for stargazers?
Not a whole lot, it turns out — at least, in lunar terms.
Northeastern University astrophysicist Jonathan Blazek breaks it all down. His comments have been edited for brevity and clarity.
A ‘black moon’ is not an official astronomical term, and it does not refer to anything unusual happening from a physical perspective. Instead, like the term ‘blue moon,’ it refers to the timing of the lunar cycle with respect to our calendar. These two methods of tracking time are fairly similar — indeed, several cultures continue to use lunar calendars — but there is a slight difference in periods.
Our months are 30 or 31 days (except for February), while the lunar cycle is around 29.5 days, which is the time to go from one full moon to the next, or equivalently one new moon to the next.
A new moon is when the moon is on the same side of the Earth as the sun, so the side that is illuminated is facing away from us, and the side we can see is dark. A full moon is when the moon and sun are on opposite sides of the Earth, and thus the side we can see is fully illuminated.
Because a month and the lunar cycle are similar in length, we typically have one full moon and one new moon each month. 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 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.
A blue moon usually refers to the second full moon in a single month. It can also mean the third full moon in a season that has an extra, fourth full moon. (I have only ever heard it used in the first way.)
A black moon is the third new moon in a season that has an extra, fourth new moon. This summer, we have four new moons (the first was June 25 and the fourth will be on Sept. 21, just before the end of summer). The new moon on August 23 will be the third of the summer, and it is, therefore, a black moon.
Black moons occur roughly every 33 months (about three years), which comes from the difference between the average month and the average lunar cycle.
There is nothing unusual to see with the black moon. Indeed, because it is a new moon, there is nothing to see at all.
This situation is unlike a ‘blood moon,’ for example, which refers to the red color the moon has during a total lunar eclipse. In this case, there is something physically different happening. A lunar eclipse occurs when the Earth is perfectly aligned between the sun and moon, blocking almost all of the sun’s light. Only a small amount is able to pass through the atmosphere, and this light is preferentially red, giving the blood moon its color.