Social media and tabloid journalism are full of special names for the Moon and their supposed associated disasters or delights. Have you wondered what a supermoon is? Or a Blue Moon, Black Moon, Blood Moon? Are they really rare astronomical events and portents of doom?

Supermoon, micromoon
Since the Moon's orbit isn't circular, its distance from the Earth varies. Astronomers call the nearest point to Earth perigee and the most distant point apogee.

Supermoon is a term invented by an astrologer, and it's become popular. Even a number of astronomers have taken to using it. If you see it in the media, it will probably refer to a perigee full Moon. A supermoon could also be a perigee new Moon, but you can't see a new Moon, so it gets ignored.

Photographs can show the difference between the extremes of the apparent size of a perigee full Moon and an apogee full Moon. [photo: Anthony Ayiomamitis] However, if you are looking up at the Moon, you can't compare it in size and brightness to some past full Moon.

The Moon does look bigger when it's low in the sky – that's the horizon effect. It's a perceptual effect unrelated to the Moon's distance from Earth. In addition, a photo can show the Moon looking enormous. The photographer gets that effect by being a long way away from the foreground object. [Image: Supermoon over Paris: VegaStar Carpentier]

When the Sun, Earth and Moon are lined up at the time of the full Moon or new Moon, gravitational effects cause higher tides – they're called spring tides. If the Moon is also at perigee, the tides are slightly higher, but not dramatically so.

By the way, it seems the opposite of a supermoon is a micromoon, a full or new Moon at apogee. This term is rarely used. Not only isn't it an astronomical term, but it's too downbeat for a juicy story.

Blue Moon
A Blue Moon isn't blue. It also isn't an astronomical term. That may explain why it was a writer in Sky & Telescope magazine in the 1940s who misunderstood its meaning and inadvertently created a new meaning. Although we use the Gregorian calendar whose year goes from January 1st to December 31st, there are other ways of defining a year. The Maine Farmers’ Almanac used a tropical year going from one December solstice to the next. It also included names for full moons.

Many cultures throughout the world – and probably back into prehistory – have named the full moons of a year according to local conditions. The Maine Farmers’ Almanac, following the Christian ecclesiastical calendar, included names related to the date of Easter. The Lenten Moon was the last full Moon of winter, and had to occur in Lent. The first full Moon of spring was the Egg Moon in the week before Easter.

With each season having three full moons, the 12 full moons marking the months worked well. But sometimes there are 13 moons in the year, so one season will have four full moons. You can't just give the spare moon the next name on the list without upsetting the whole system. So the third full moon was designated as a Blue Moon. (Blue Moon is a very old usage, and there's no agreement about its origin.)

Misunderstanding this complexity led an amateur astronomer to conclude that a Blue Moon was the second full Moon in a month. This was incorrect, yet over a few decades it grew into a new definition, even though it's one that serves no purpose. Blue Moons have no astronomical significance, and our calendar names months, not Moon cycles. Lunar calendars never have Blue Moons.

Black Moon
Black Moon has several definitions, none of them of any obvious use to most of us.

Two definitions echo the common Blue Moon definitions: (1) the third new Moon in a season of four, and more commonly, (2) the second new Moon in a month. The other two definitions are obscure and contradictory. One says a Black Moon is a February with no full moon, and the other that it's a February with no new moon.

As with the Blue Moon, none of the definitions are astronomical. They seem to be connected to some pagan beliefs, but not in a consistent way. The Black Moon may cause concern as a dangerous time. Nonetheless others see it as an enhancing time. Occasionally, a fringe Christian group announces that the Black Moon – however they define it – is a sign of impending doom.

Blood Moon
A blood moon is simply a totally eclipsed Moon. It's not a term I use, but I can see the attraction. It's short, it's dramatic, and it tends to be descriptive. As far back as the eighth century, the Anglo-Saxon Chronicle described an eclipsed Moon as "sprinkled with blood". However, the color of the eclipsed Moon varies considerably, depending on atmospheric conditions on Earth.

When the Moon is wholly in the Earth's shadow, although no sunlight reaches it directly, there's an indirect route. Our atmosphere refracts sunlight, acting like a kind of filter. The farther the light travels, the more the blue light in sunlight is scattered out, leaving only the redder light. (This same process makes for blue skies and red sunsets.) The red light travels through the atmosphere to the Moon and is reflected back to Earth, giving the Moon its unsettling look. [Image credit: Eggishorn]

References:
(1) Donald W. Olson et al, "What Is a Blue Moon?", https://www.skyandtelescope.com/observing/celestial-objects-to-watch/what-is-a-blue-moon/
(2) When Is the Next Black Moon? https://www.timeanddate.com/astronomy/moon/black-moon.html