Having explained the basic concept of gravitational barycentres, I can now get to the meat of what I wanted to talk about: Lagrange (or Lagrangian) points.
Josh, 7, from London, writes:
I went to the beach yesterday. The weather was nice. I had ice cream. When I was splashing around in the sea with my rubber ring wedged firmly around my waist to stop me sinking, I thought about the planets. Planets have rings, but they do not have to be prevented from sinking. Why do planet rings always go around the middle?
Saying that I have a Ph.D elicits a fairly predictable reaction from most people. They will, in an attempt to appear interested, ask “What subject?” and then when informed that I did Astrophysics – one of the simpler branches of physics if you don’t tangle with cosmology or relativity but which appears to have a fearsome reputation in the eyes of the layman – their eyes glaze over and they either stop talking to me altogether, or else they desperately try to change the subject before I can get a chance to pounce on them, knock them to the ground and inject pure Science into their brains via their ear canal1. There’s a second type of person out there, however; the freakish sort who are genuinely interested in science, and this second type will, after some circumspect small talk, eventually get around to asking me what my thesis was about. And this is a question to which I have gradually evolved a tried-and-tested one-sentence reply:
“I am trying to find out how much energy you need to blow up Pluto.”
Kicking off the science portion of this blog, I’m going to start with an easy question I used to get asked a lot when I did Outreach for the university: why are the various planets, moons etc. round? It’s a fairly simple answer with some wide-reaching ramifications.