Q&A: Super Moon

Question: I heard that our last Full Moon was a Super Moon. What’s that? — GK, Prescott, AZ

Answer: Indeed it was. The Full Moon of May 5th, 2012, was a Super Moon, so called because it was physically closer to us at that time. The technical name for this phenomenon is perigee sysygy. Perigee means “closest point to the Earth,” and sysygy means “three bodies in a line” (in this case, SunEarthMoon).

By the way … if you’re a Scrabble player, the word “sysygy” will score you 21 points, even before any bonuses. It’s a word well worth remembering. But you’ll need a blank tile (wild card) to spell it, since there’s only two Y’s in the game.

The animation above left shows the Moon’s orbit around Earth. It is not to scale, and the orbit’s ellipticity (or more formally “eccentricity“) is exaggerated for clarity. If the animation was to scale, you would hardly notice that the Moon’s orbit isn’t a perfect circle. But the Moon-Earth distance does vary by about 50,000 km (30,000 miles). Your eyes can see that difference in how large the Moon appears in the sky. The animation above right shows the effect. Watch the Moon as it moves closer to and farther from Earth. The change in apparent size is about 14%. Note that the two animations are not synchronized.

I should also note that the Moon actually rotates as it orbits, keeping the same side facing Earth. This is why we always see the same familiar features. The Moon completes one orbit, and one rotation, every 29.5 days. It’s a gravitational effect called tidal locking. The animation didn’t really need to show this.

Perigee can occur at any phase from New to Full. When it happens at the time of a Full Moon, we have what’s called a Super Moon. On average, we get one pretty much every year, though the exact coincidence of perigee and a Full phase is rare. If it’s better than 90% aligned, we call it a Super Moon. If the sky is clear, you’ll see the Moon about 30% brighter than average.

Whether the Moon is at perigee or apogee (farthest distance from Earth) has a lot to do with eclipses. The one we had yesterday, on May 20th, occurred at apogee. The Moon appeared smaller compared to the Sun, so we had an annular eclipse rather than a total eclipse. If you missed last week’s Sky Lights, check it out. And watch for my eclipse photos in my next blog next Monday.

May 20, 2012 Solar Eclipse
The May 20, 2012 Annular Eclipse