Supermoon? Well, sort of....

I've had several people ask me whether I went out to look at the full moon on March 19. Uhm, no, I didn't. Why not? After all, it made the local (radio) news as "supermoon" since it was going to be closest it has been in nearly 20 years.

First, while there are astronomers who make moon observing their big thing, I'm not one of them. Yes, I do have a lot of images of the moon I've taken and I have gone out for every lunar eclipse that's been visible (meaning it wasn't clouded out). I enjoy views of the moon, especially when it is not full. All the interesting stuff is near the terminator, that line between day and night on the moon where long shadows throw the craters and mountains into sharp relief. Full moon, no terminator, no shadows, no interest (for me anyway).

Second, just how "super" was this moon? Yes it was closer than normal. Many of you realize the moon's orbit is not a perfect circle but slightly elliptical. This means it varies from just over 363 thousand kilometers away just under 406 thousand kilometers distant. Subtract a few thousand for the Earth's radius and adjust for your particular latitude. That's a pretty big different, about 10%. But just how big is this? Well, a full moon at its most distant point has an angular size of about 0.49°. At its closest, this swells to an whopping 0.54°! Are you underwhelmed at this point?

Now, if you take a picture of the full moon with a long telephoto lens, say over 500mm, every month, you will be able to tell which ones are when the moon was closer. It is noticeable with the right equipment. But to the naked eye it really looks pretty much like every other full moon.

If you do decide to take pictures of the moon to compare them, you'll notice other things, but they're tricky to pick up on. Let's say to follow this procedure: take a picture of the moon every clear day when it is due south of your location. Why this time? Because then you can set up you camera with the same orientation relative to the sky. What will you notice? One interesting thing is that 10% size change may be noticeable in spite of the changing phase. Another interesting thing? The moon rocks back and forth!

You can find some simulations of this on YouTube, but the video I like best is the animated GIF that appeared as Astronomy Picture of the Day on Feb 8, 2001 simply because it is a series of pictures of the real thing.

The term for this is "libration" and there are three separate physical effects that cause this, although only two count if you took the pictures as it passed throught the due south point.

First, the moon's axis is tilted just like Earth's, only a much smaller amount, about 6-1/2 degrees. The means as it goes through its orbit, we can see a bit over the the north pole and then a bit over (under?) the south pole two weeks later. The second cause is because of the elliptical orbit: when the moon is closer to us, it travels a little faster and when it is further, it travels a little slower. This causes the rotation that keeps the same face toward us to alternately lag and lead the orbit so we can see around the east and west edges.