## Archive for September, 2011

### Doom from the sky

Saturday, September 17th, 2011

I was on the Channel 8 Richmond TV news last night. You can see the video here.

Since I’m apparently the only astrophysicist in the greater Richmond area, I sometimes get asked to comment on space stories. I think this is my first time on this channel; I’ve been on Channel 6 from time to time.

In this case, they wanted to talk about the UARS satellite, which is going to reenter the atmosphere in the next couple of weeks. Some pieces are predicted to survive reentry and reach the ground.

1. The reporter says that there’s a 1 in 3200 chance of “being hit” by the debris. This is NASA’s estimate of the chance of someone, somewhere in the world being hit. The chance of any given person (such as you) being hit is 7 billion times smaller — i.e., one in 20 trillion. I stated that in the interview, but they chose not to use that part. The way they stated it is extremely misleading.
2. The Santa Claus line is mine.

### Music of the spheres

Thursday, September 1st, 2011

I’m teaching about the Copernican revolution in my first-year seminar these days. Before getting to Copernicus, we’re taking a look at what people thought about the motions of the planets at earlier times. We’re particularly focusing on the ancient Greeks, since that’s largely what Copernicus and pals were responding to.

Most astronomy textbooks talk about the Ptolemaic system, with its epicyles, deferents, and the like. But before there was Ptolemy, people like Eudoxus came up with pretty good, detailed models to explain planetary motion based on the idea that all of the heavenly bodies were attached to nested, concentric spheres. This “homocentric sphere” picture was very important, largely because it’s the one Aristotle championed.

To explain the complicated motions of the planets in this model, you need to use multiple spheres, all rotating about different axes at different rates. You’d think there’d be nice animations out there on the Web somewhere to show how this all worked, but I couldn’t find any, so I made my own:

More images and detailed explanations here.