Elusive ‘black moon’ to grace the sky on Friday

Moon phases
CC BY 2.0 Wikimedia Commons

This unusual celestial event happens in the Western Hemisphere on Friday, September 30.

We’ve got blue moons and blood moons, harvest moons and super moons, but the black sheep of the moon family is surely the mysterious black moon. And those in the Western Hemisphere will be treated to one of these gothic beauties on September 30. Though “treated” may be a stretch, as the nature of the black moon is that she remains more or less invisible. Sneaky thing.

While a blue moon refers to a second full moon in a single month, a black moon is a second new moon in a single month. When the moon is full, the side that faces the Earth is fully lit with sunlight; conversely, the new moon occurs when the side facing the Earth is in shadow, as you can see in the illustration above.

The lunar calendar almost lines up with our monthly calendar, but not quite. A lunar cycle – the period from one new moon to the next, known as a synodic month – is on average 29.53 days. Usually this plays out to deliver one full moon and one new moon each calendar month. But given that are months are (mostly) longer than 29.53 days, it means that once in a blue moon … we get a blue moon. Or a black moon, as the case may be.

For those of us in the Western Hemisphere, the black moon officially occurs on Friday, September 30 at 8:11 p.m. EDT. This means that in the Eastern Hemisphere, the clock will have already slipped into October 1, so that part of the world will have to wait until next month for their black moon – right around Halloween. What could be more appropriate than that?

Black moons happen around every 32 months; the next time that two new moons will occur in the same month in the Western Hemisphere won’t be until July of 2019. So enjoy it while you can. Even it you can’t exactly see it … it’s still there, and wonderfully poetic nonetheless.

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