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Mercury is the closest planet to the Sun and the smallest
(with the exception of Pluto).
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Orbit:
Diameter:
Mass:
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57,910,000 km (0.38 AU)
from Sun
4,880 km.
3.30e23 kg
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Astronomy
The ancient Samaritans, who occupied the Northern Kingdom of the
ancient Hebrews between the River Jordan and the Mediterranean,
have known Mercury since at least the third millennium BC.
Mercury was given two names by the Greeks: Apollo for its appearance
as a morning star and Hermes for its appearance as an evening star.
Greek astronomers knew, however, that the two names referred to
the same body. The Greek philosopher, Heraclitus, even believed
that Mercury and Venus orbit the Sun, not the Earth, prefiguring
the theory of Copernicus.
Mercury's orbit is highly eccentric; at its closest it is only
46 million km from the Sun but its orbit extends as far out as 70
million km. The 19th Century astronomers could not adequately explain
the differences in the orbit using Newtonian mechanics. The tiny
differences between the observed and predicted values were a minor
but nagging problem for many decades. There was speculation that
another planet (sometimes referred to as Vulcan) might exist in
an orbit near Mercury's to account for the discrepancy. The solution
was far more dramatic. Just as there had been a paradigm shift from
a Ptolmaic view of the universe to a Copernicum view (see Unravelling
the Mystery), now Einstein's General Theory of Relativity replaced
Newtonian mechanics. Einstein's theory correctly predicted the motions
of Mercury, something that was an important factor in the early
acceptance of the new theory.
Mercury's highly eccentric 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 appear to increase in
size as it slowly moved toward its zenith. At that point the Sun
would stop, briefly reverse course, and stop again before resuming
its path toward the horizon, apparently decreasing in 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.
Temperature variations on Mercury are the most extreme in the solar
system. At midday on Mercury temperatures soar to 430 degrees Centigrade
(800 degrees Fahrenheit). In comparison, the temperature on Venus
is slightly hotter but very stable. At night temperatures on Mercury
plunge to minus 180 degrees C. (-290 F).
Mercury is in some ways similar to the Moon. Its surface is ancient
and extensively cratered, and it has no plate tectonics. On the
other hand, Mercury is much denser than the Moon. After the Earth,
Mercury is the second densest major body in the solar system. Interestingly,
Earth's density is due in part to gravitational compression, and
without this, Mercury would be denser than Earth. Thus Mercury's
dense iron core is thought to be relatively larger than that of
the Earth, comprising the majority of the planet, which has only
a relatively thin silicate mantle and crust. Mercury has a small
magnetic field with a strength of about 1% of that of the Earth.
Mercury is often visible with binoculars or even to the naked eye,
but it is always very near the Sun and difficult to see in the twilight
sky because it appears just above the horizon and may be obscured
by twilight or trees and other topographical features.
If you would like to know more about the solar system, Starscine
suggests the following websites:
www.nineplanets.org
www.nasa.gov
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