For yourselves know perfectly that the day of the Lord so cometh
as a thief in the night.
Mercury has been known since at least the time of the Sumerians (3rd millennium BC). It was given two names
by the Greeks: Apollo for its apparition as a morning star and Hermes as an evening star.
Mercury has been visited by only one spacecraft, Mariner 10. It flew by three times in 1974 and 1975.
Only 45% of the surface was mapped (and, unfortunately, it is too close to the Sun to be safely imaged by HST).
A new discovery-class mission to Mercury, MESSENGER was launched by NASA in 2004 and will orbit Mercury starting
in 2011 after several flybys.
Until 1962 it was thought that Mercury's "day" was the same length as its "year" so as to keep that same face
to the Sun much as the Moon does to the Earth. But this was shown to be false in 1965 by doppler radar observations.
It is now known that Mercury rotates three times in two of its years. Mercury is the only body in the solar system
known to have an orbital/rotational resonance with a ratio other than 1:1 (though many have no resonances at all).
This fact and the high eccentricity of Mercury's orbit would produce very strange effects for an observer on
Mercury's surface. At some longitudes the observer would see the Sun rise and then gradually increase in apparent
size as it slowly moved toward the zenith. At that point the Sun would stop, briefly reverse course, and stop
again before resuming its path toward the horizon and decreasing in apparent size. All the while the stars would be
moving three times faster across the sky. Observers at other points on Mercury's surface would see different but
equally bizarre motions.
http://www.nineplanets.org/mercury.html
Kay
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