More Moon Madness

Black Moon Rising

Before we get started, I’d like to wish a happy first birthday to StupidBadMemes. One year ago I set out to rid the world of idiot memes (or at least to ineffectually gripe about them). I’m not sure whether to be happy or not that I’m still in business.

Okay, this meme comes from a Wiccan Facebook group, so I’m not going to spend much time talking about “omens of change” and “hidden truths”, nor will I take any more time to talk about how the term Black Moon is really only used by Wiccans. These are their beliefs, and since they’re not hurting anybody or trying to legislate their convictions (as far as I know), I’ll leave them be.

There is a bit of science misrepresented, though, and I would like to discuss that. Let’s start with the comparative rarity of black moons.

Black moons are apparently the astronomical cousins of blue moons, and should occur with similar frequency. A blue moon was originally meant to be the third full moon in a season with four full moons, but the term has evolved in popular usage to mean the second full moon in a calendar month. Since the average time between full moons is about 29.5 days, most calendar months have only one full moon (and one new moon) but the small difference in day length between the lunar cycle and the average month means that the cycle of lunar phases slowly shifts with respect to the framing of the calendar months. Every two to three years (seven times in a 19-year cycle, to be exact), there will be two full moons in a month. Since new moons and full moons occur with equal frequency, we should expect to see two new moons in the same month as often as you see two full moons, which means seven times in a 19-year period.

Actually, we shouldn’t expect to see new moons…because new moons are largely invisible. Unlike a full moon, which dominates the nighttime sky, a new moon happens when the Moon is nearly between the Sun and Earth. The sunlit hemisphere of the Moon faces away from Earth, which means we can only see the half that is in darkness. Now from the perspective of the Moon, Earth is nearly “full”, and the bright blue Earth casts a lot of light back on the surface of the Moon. The dark face of the Moon can often be seen, particularly during its crescent phases, faintly lit by reflected Earthshine. But during a new moon, the Moon appears very close to the Sun in Earth’s sky, so the pale glow of Earthshine is utterly washed out by the intense glare of the nearby Sun. In other words, the new moon is for all intents and purposes invisible to the unaided human eye.

Now, about this Supermoon nonsense. I’ve discussed this before, but apparently not everybody got the message. The term Super Moon or Supermoon was created by astrologers to refer to a full moon that happens roughly when the Moon is at perigee, or the closest point in its orbit to Earth. Apparently the term can now apply to any phase of the moon that coincides with perigee, which means that we should have a Super Moon every 27.6 days*.

The tides are caused largely by the Moon’s gravity (and to a smaller extent, the Sun’s gravity). When the Moon, Sun, and Earth are roughly aligned – as they are during a new or full moon – the tidal effects of the Moon and Sun combine to produce spring tides. Spring tides are characterized by higher-than-average high tides, but also lower-than-average low tides. This happens about every two weeks, regardless of how close the Moon is to Earth.

The relative proximity of the Moon at perigee can contribute to more pronounced spring tides than normal, but the effect is minimal…inches at best. You wouldn’t be likely to notice the difference unless you took detailed measurements. Of course a perigean spring tide would be a slightly worse time for a hurricane to come ashore, but there doesn’t seem to be any threat of that, at least in the United States.

Even if you’re not Wiccan, there is one benefit of a new moon…the night skies will be especially dark and well-suited for stargazing. So bundle up, grab a cup of hot cocoa, and go watch the stars.


*The time between two perigees, called the anomalistic month, is shorter than the time between two full moons (the synodic month) because Earth moves in its orbit while the Moon orbits around it. The Moon takes a couple of extra days to “catch up” and return to being full.

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