by Joe Rao,

Friday’s sky on September 30th, 2016 is host to a somewhat unusual lunar event in the Western Hemisphere: a second new moon in a single month, which some people call a “Black Moon.”

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).

The lunar calendar almost lines up with Earth’s calendar year, so 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.” A Black Moon is supposedly the flip side of a Blue Moon: the second new moon in a single calendar month. The next Black Moon takes place on Sept. 30 (in the Western Hemisphere).

From the Western Hemisphere, the new moon occurring on Friday, Sept. 30, is a Black Moon. Officially, it occurs at 8:11 p.m. Eastern Time (5:11 p.m. Pacific Time).

For the Eastern Hemisphere (Europe, Africa, Asia, Australia), this new moon occurs after midnight on the calendar date of Oct. 1. So for this part of the world, this particular new moon is not the second one in the calendar month, but rather, the first! So it does not qualify as a Black Moon, and that hemisphere will have to wait until the end of the month for theirs. Indeed, for the billions living in the Eastern Hemisphere, the Black Moon will arrive on Oct. 30 or, if you live in eastern Asia, Japan, Australia or New Zealand, not until Oct. 31 (Halloween).

The Black Moon is a somewhat unusual celestial event — they occur about once every 32 months.