| MadSci Network: Astronomy |
No, Mercury does not always keep one side towards the sun. Mercury does have a strange orbit that appears to have changed over time, but its current day (the time from one sunrise to the next) is about 58.65 Earth days. The planet rotates one and a half times ever Mercury year.
It is thought that Mercury once had days that were only 8 hours long, but that solar tides (because it is so close to the Sun) actually "de-spun" the planet over time. Tides are also responsible for why the Moon keeps only one face toward the Earth and why the Moon is slowly moving away from the Earth. (In the Earth-Moon case, the tides are generated from the Earth and Moon themselves as opposed to from the Sun. For more detail, consult the sci.astro FAQ.)
Most of our information on Mercury was collected by the Mariner 10 spacecraft in 1974--75. Before that, we had little information on it as it is pretty hard to observe a planet so close to the Sun. We now know that a Mercurian year only lasts about 88 Earth days, that the surface temperature ranges from -173 to +427 degrees Celsius and that what little atmosphere it has is made up mostly of sodium and helium.
For more information on Mercury, and some good pictures, check out the solarviews.com website or the NASA/JPL site.
[There are also some proposals to re-visit Mercury with a spacecraft. We will have to wait and see if they are approved by the US and European governments.]
Try the links in the MadSci Library for more information on Astronomy.