spaceSpace and Physics

Tonight Is Going To Be A Black Moon. This Is What That Means


Tonight, stargazers based in the Western Hemisphere can enjoy a black moon – an astronomical phenomenon that creates the perfect conditions for watching celestial activity.

Confusingly, a black moon can mean several different things. It can mean a month with exactly zero new moons (a scenario that can only take place in February, a month that is shorter than the 29.53-day lunar cycle) or the third new moon in a season of four moons. In this case, it refers to the second of two new moons in one calendar month.


The length of the lunar cycle almost (but not quite) aligns with the months in the Gregorian calendar, so there tends to be one full moon and one new moon each month. But because the alignment is not perfect, there can, on occasion, be a second full moon or new moon in a month (provided that the month is not February). That is what is happening here.

It is the opposite of a supermoon, which refers to a full moon when it is at its closest point to the Earth. The result is that it looks just a bit larger than usual and while the change might be slight, it can make for some pretty impressive photos.

A black moon like this takes place roughly once every 32 months – the last one was in September 2016. It will be good news for astronomers because a black moon means a dark sky, which is perfect for stargazing and this time it just so happens to coincide with the Southern Delta Aquarids, a meteor shower that takes place every year from mid-July to mid-August and peaks in late-July.

July's black moon will only be visible to people in the Western Hemisphere because the next new moon in the Eastern Hemisphere falls on August 1. Astronomers in the Eastern Hemisphere will get to enjoy a black moon soon enough – there will be a black moon on August 30. 


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