The atmospheric image above isn't a Black Moon. Read on to find out why we can't see a new Moon.

Followers of social media may know what a “Black Moon” is. It's been linked to dramatic predictions of doom and gloom. However it's not an astronomical term, and astronomers who come across it tend to be puzzled, if not outright dismissive. So what is a black moon and would we survive it?

Phases of the Moon
Let's start with a reminder of the phases of the Moon.

As the Moon orbits Earth, it takes about 29.5 days to go from new Moon to new Moon. The apparent changes in the shape of the Moon occur because we see different amounts of the Moon's illuminated side.

The diagram shows that when the illuminated side of the Moon is facing us, we get a full moon. The opposite of the full Moon is the new Moon when the Moon's illuminated side faces the Sun. We can only see a new Moon during a solar eclipse when the silhouetted Moon crosses in front of the Sun's disk. At night, of course, we're facing away from the Sun.

Defining the “Black Moon”
The second new Moon in a calendar month
The most common definition of a Black Moon is that it's the second new Moon in a month. This seems to have been created to correspond to the idea of a Blue Moon as a second full Moon in a calendar month.

It's worth noting that Blue Moon also doesn't mean anything to astronomers. It began as a misunderstanding of the Farmers' Almanac usage of Blue Moon. Their usage actually referred to the third full Moon in a season that contained four, and was important for naming the full moons of the farming calendar. [The link to “Once in a Blue Moon” at the end of this article has more information.]

More uses of the term “Black Moon”
Wikipedia lists three other ways in which Black Moon is used:

1. The third new moon in a season with four new moons – equivalent to the Farmers' Almanac definition of a Blue Moon;
2. A February with no full Moon;
3. A February with no new Moon.

Beware the Black Moon – or be energized by it?
In September 2016 the English newspaper Express reported:

[A] rare Black Moon will occur, which many are linking to the apocalypse. The spectacular Black Moon occurs when the illuminated side of the moon is caught in the shadow of the Earth, making it virtually impossible to see. The phenomenon happens roughly every 32 months.

The Moon is not caught in the Earth's shadow. Sunlight is falling on the side of the Moon that faces away from us. And I'll leave it to readers to consider how “spectacular” the Moon might be when you can't see it.

Some extreme religious groups linked the two new moons in September to the apocalypse, but a number of astrology sites said:

Astrologically speaking, new moons are times for new beginnings and starting over. Black Moons differ from regular new moons in that they magnify and intensify all of the energies, emotions, changes, and feelings that are swirling about in your life.

Some pagans apparently view the Black Moon as increasing the effectiveness of spells and rituals. Others claim the opposite view and counsel against conducting rituals on such a night. A third reaction is puzzlement from those who had never heard of a Black Moon.

So when was the Black Moon?
The Express article said:

The Black Moon will mainly be visible in the Western Hemisphere, over North and South America, with the most western portions of Europe seeing it too. It will reach its peak at 20:11 Eastern Time . . . or 01:11 on Saturday morning UK time.

Since the new moon isn't visible, no one would actually be seeing it. But more interestingly, western Europe did not even have a “Black Moon” on Friday, September 30. The Saturday new Moon was on October 1. And although there was a second new moon over western Europe on October 30, media interest in the Black Moon had waned by then. A few websites were entertained by the notion of such a moon on Halloween, which did happen in eastern Asia and New Zealand on October 31.

So this Moon that caused the media fuss happened in part of the world at the end of September, and in most other places on either October 30 or October 31. Greenland has several time zones, and most of the country had a second new moon in September, but the eastern fringes had one in October instead. Hawaii didn't experience one in either month. Countries using lunar or lunisolar calendars never experience one. [Click on the link to “The Sun the Moon the Calendar” at the end of this article to learn more about calendars.]

An artifact of the calendar
A term isn't very helpful when it could mean four different things. But more importantly, the term doesn't tell us anything useful about the Moon. Whether a new moon is the second in one month or the first in the following month makes no difference to any of its characteristics. The Moon's orbit is not constrained by the oddities of our calendar. In the Gregorian calendar – and the Julian calendar that preceded it – all the months except February are long enough to contain two new moons. In fact, that happens every few years, and has been doing so for centuries without ill effects.

In summary, a “Black Moon” is an invisible non-event that may or may not occur where you live. For those using lunar calendars, it never occurs. Black moons and blue moons are not facts of nature, they're simply artifacts of our Gregorian calendar.

References:
(1) https://www.express.co.uk/news/weird/714924/Black-Moon-END-OF-THE-WORLD-JESUS-apocalypse
(2) https://en.wikipedia.org/wiki/Black_moon