Cosmic Horizons

There was a movie, in the old days, Journey to the Far-Side of the Sun  (also known as Doppleganger) which (spoiler alert) posits that there is a mirror version of the Earth hidden on the other side of the Sun, sharing the orbit with our Earth. The idea is that this planet would always be hidden behind the Sun, and so we would not know it there there. This idea comes up a lot, over and over again. In fact, it came up again last week on twitter. But there's a problem. It assumes the Earth is on a circular orbit. I won't go into the details here, but one of the greatest insights in astronomy was the discovery of Kepler's laws of planetary motion , telling us that planets move on elliptical orbits. With this, there was the realisation that planets can't move at uniform speeds, but travel quickly when closer to the Sun, while slowing down as their orbits carry them to larger distance.  There has been a lot of work examining orbits in the Solar System, and you can  s

A long time ago, I wrote a post about the  Physics of Rendezvous with Rama , a science fiction story by  Arthur C. Clarke  set on an immense alien spaceship. The spaceship rotates, providing the occupants with  artificial gravity , a staple of science fiction. I mentioned in the article that I am not an immense fan of a lot of science fiction, as much of it relies on simple "magic", but Clarke knew his physics and so he knew that the "gravity" experienced in the rotating ship will differ to that on Earth, and previously I wrote about what happens if you jump off a cliff. In the last week, there was a question on twitter (it's an internet thing) about the movie  Elysium  which has a spectacular rotating spacecraft with the Earth's rich abroad. While it's a shame that the plot was not as spectacular, the question was how can such a station keep it's atmosphere. This is an interesting question, as you might think that it would all simply zip off