The Mudcat Café TM
Thread #108225   Message #2254402
Posted By: Charley Noble
05-Feb-08 - 04:55 PM
Thread Name: BS: A Spider on Mercury (NASA photo)
Subject: RE: BS: A Spider on Mercury (NASA photo)
Shimrod-

No, more current observations confirm that Mercury does rotate, though slowly as it orbits the sun:

"Spin–orbit resonance

After one orbit, Mercury has rotated 1.5 times, so after two complete orbits the same hemisphere is again illuminated.

For many years it was thought that Mercury was synchronously tidally locked with the Sun, rotating once for each orbit and keeping the same face directed towards the Sun at all times, in the same way that the same side of the Moon always faces the Earth.
However, radar observations in 1965 proved that the planet has a 3:2 spin–orbit resonance, rotating three times for every two revolutions around the Sun; the eccentricity of Mercury's orbit makes this resonance stable—at perihelion, when the solar tide is strongest, the Sun is nearly still in Mercury's sky. The original reason astronomers thought it was synchronously locked was that whenever Mercury was best placed for observation, it was always at the same point in its 3:2 resonance, hence showing the same face. Due to Mercury's 3:2 spin–orbit resonance, a solar day (the length between two meridian transits of the Sun) lasts about 176 Earth days. A sidereal day (the period of rotation) lasts about 58.7 Earth days."

Cheerily,
Charley Noble