Monday, May 07, 2012

Supermoon

All over the news and weather this past weekend. Big story! A Supermoon! Big! Big! Pictures of an orange disk dwarfing the Lincoln Memorial. Dwarfing the Empire State Building. From all the hype, you'd have thought it was the Second Coming.

Now to be fair, the Moon was closer to Earth than usual. Although the Moon's orbit is pretty much circular, it is not centered on the Earth. At its closest -- perigee -- the Moon is about 56 Earth-radii away. At its farthest -- apogee -- about 64 Earth-radii. At midnight Saturday night the Moon was 56 Earth-radii distant, about the same distance it will be on, say, November 13 and December 13. Nothing unusual about that.

What made this somewhat unusual is that perigee coincided with a full Moon; that is, the Moon was opposite the Sun as it reached its nearest point to Earth. Which makes it appear as big and as bright as it gets.

Big deal. The full Moon was almost as close on April 6. Ditto for June 4. I guarantee that without making a careful quantitative measurement you'd never notice the difference.

And those photos of the Moon dwarfing the Lincoln Memorial? You could take a picture like that any month. It's all a matter of perspective.

Well, never mind. If all the hoopla got folks out watching a full Moon rise -- any full Moon rise -- that's no small thing. A full Moon on the horizon, shining through the Earth's atmosphere, always appears golden, even orange or red. And the "Moon illusion" -- seeing the Moon in visual proximity to familiar objects -- tricks the mind into thinking it enormous.

So it was a fun moonrise to watch. It'll be just as much fun -- and almost as "super" -- next month. But you won't hear a thing about it.