Hope for a clear sky next Thursday, August 31: There will be a full moon! But this is not your typical full moon — this will be a Full Moon, Supermoon, and Blue Moon all at once.
Full Moons
As most everyone knows, a full moon occurs about once a month. Actually, it is every 29.5 days. Many cultures have names for the full moons — names for the July full moon include Buck Moon, Thunder Moon, or Hay Moon, while August’s full moon names include Corn Moon or Grain Moon.
Blue Moons
Occasionally we may have two full moons in a month. If so, the second is referred to as a Blue Moon.
I have never heard a definitive explanation as to how they came to be called Blue Moons. One explanation, however, is that it is a mistranslation from the Old English term blewe, which had two definitions: One was the color blue, the other was “to betray,” implying two full moons in a month betraying the typical one.
Supermoons
This full moon is also a Supermoon. It’s the third one this year.
The term Supermoon was coined a few years ago to refer to a full moon that occurs when the moon is near its closest approach to the earth. The moon looks bigger when it is close to the earth’s surface, as opposed to high in the sky. That is a well-known illusion called the "moon illusion." The moon, of course, is still the same size.
But the moon does not orbit the earth in a circular orbit. The orbit is elliptical. The average distance between the earth and moon is around 239,000 miles, however, when the moon the farthest away from the earth, it’s out there around 252,000 miles. At its closest point it is about 225,000 miles away. So, there is a difference of about 27,000 miles, which is enough of a distance to make the moon look a different size.
Some say a Supermoon is about 14% larger and 30% brighter compared to when the moon is farthest from the earth. Those are the values I have seen comparing the closest point (perigee) and farthest point (apogee).
I suppose you could call a full moon furthest away a minimoon or a micromoon, but that doesn’t have much pizzazz.
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More Information:
- Phases, Eclipses & Supermoons (NASA Science)
