A rare black moon is set to occur this week, though night sky observers will not be able to see it.

The astral phenomenon is set to leave a moon-sized dark spot in the heavenly panorama on the night of Saturday, Aug. 23.

Friday's black moon is considered a "seasonal" version of the event as it will be the third new moon in a season that has four – according to Space.com . Normally a season has three new moons but since the lunar cycle does not exactly align a fourth one occurs about every 33 months.

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