Mercury

 
	Distance from sun ...........36x10^6 miles/0.387 AU
	Diameter ....................4,878 km/3,030 miles
	Length of year ..............87.97 Earth days
	Length of day ...............58.65 Earth days
	Mass ........................3.30 x 10^23 kg
	Average density .............5420 kg/m^3
	Surface temperature:
	Average .....................452 K
	Maximum .....................700 K
	Minimum .....................100 K 

Mercury is the closest planet to the Sun. It is in a highly eccentric orbit around the Sun. At its closest (perihelion), it is only 46 million km from the Sun. At its most distant (aphelion), it is 70 million km. Its orbit perihelion moves backward (precesses) around the Sun at a very slow rate.

Mercury always appears near the Sun when viewed from Earth, but is rarely seen because of the Sun's glare. Many astronomers have never seen it because it's only visible a few days out of the year, and does not rise very far above the horizon. Mercury can only be seen from Earth just before sunrise or just after sunset, but not in the middle of the night.

Mercury is very difficult to study from the Earth because it is always so close to the Sun. Even at elongation, it is never more than 28 degrees from the Sun in our sky.

Mercury
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The Planets

Our Earth is one of 8 planets orbiting the Sun. The planets are vast balls of rock, metals, and gases that orbit a star. Other stars may have planets too. A planet is a celestial object which revolves around a star and reflects that star's light.

Eight planets have been discovered in our solar system. Mercury, Venus, Earth, and Mars are the planets closest to the Sun. They are called the inner planets. The inner planets are made up mostly of rock.

The outer planets are Jupiter, Saturn, Uranus, and Neptune. Jupiter, Saturn, Uranus, and Neptune are large balls of gases with rings around them.