Photograph by Tim Laman, Nat Geo Image Collection
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A tiny sliver of the moon during the new moon phase in Northern California.
Photograph by Tim Laman, Nat Geo Image Collection
Viral Explainer

The September 30 Black Moon Explained

Find out what this lunar event really means, and why it’s actually impossible to see a black moon rising.

Humans have long had a fascination with lunar events—and with the rise of the Internet, all kinds of moon moments have become media sensations. Consider super moons, harvest moons, and the rare super blood moon total eclipse.

On September 30, sky-watchers in the Western Hemisphere may be tempted to add another lunar moniker to the list: a “black moon,” popularly defined as the second new moon in a month. However, it’s not exactly going to make for good sky-watching.

New moons occur when the moon’s orbit takes it between Earth and the sun, leaving the lunar orb’s unilluminated side facing Earth. At night, this phase of the moon is impossible to see: Since new moons are in the same part of the sky as the sun, they rise and set with the sun and are overwhelmed by its glare.

New moons can be readily seen only when they pass directly in front of the sun, causing solar eclipses. Otherwise, sky-watchers must look to the days before or after a new moon, when just a sliver of the moon’s sunlit side is visible from Earth.

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Usually, new moons occur only once a month, but because there’s a slight disjunct between the moon’s phases—a 29.5-day cycle, on average—and the Gregorian calendar, some months can have two new moons: one at the beginning and one at the end. Joe Rao of notes that this double-dipping occurs once every 32 months or so.

In this sense, a black moon is like the evil twin of a blue moon, conventionally understood as the second full moon in a month. But let’s be clear: This new moon—like any astronomical event—doesn’t bring ill tidings or herald the end of days, despite the astrological fear-mongering that has been seeping through the web.

If anything, this black moon is a harbinger of new beginnings and festivities: On the evening of October 2, the barely visible waxing crescent moon will shine on Rosh Hashanah, the Jewish New Year. And on October 3, the growing crescent will mark the beginning of Muharram, the first month in the Islamic lunar calendar.

The September 30 new moon won’t even be a true black moon for the majority of humankind. In the Eastern Hemisphere, the upcoming new moon starts after midnight on October 1, putting it in the wrong month to fit the popular definition. (In London, for instance, the moon reaches its new phase at 1:11 a.m. local time, meaning that the United Kingdom won’t experience a black moon at all in the coming days, much less the apocalypse.)

As Rao points out, the Eastern Hemisphere will instead get a black moon at the end of the month, with a second new moon arriving on October 30. And for people living in East Asia, Japan, New Zealand, and Australia, the black moon will come on the perfect spooky date: October 31, or Halloween.