Exploring the Planets

Mercury Facts

Mean distance from Sun: 0.3871 AU
(57,910,000 km/35,980,000 mi)
Diameter: 4,878 km (3,031 mi)
Length of year: 88 days
Rotation period: 58.65 days
Mean orbital velocity: 48 km/s (30 mi/s)
Inclination of axis:
Average temperature: 800° F (427° C) day
                                  -300° F (-183° C) night
Number of observed satellites: 0

Comparisons with Earth
Mean distance from Sun: 0.387 x Earth's
Diameter: 0.38 x Earth's
Mass: 0.055 x Earth's
Density: Slightly less than Earth's

  • Mercury is one of the five planets known to the ancients. They called these planets "wandering stars."
  • Mercury may be seen as an evening "star" near where the sun has set, or as a morning "star" near where the sun will rise.
  • The ancient Greeks called the evening star Hermes and the morning star Apollo, believing them to be different objects.
  • The planet is named for Mercury, the Roman messenger of the gods.
  • To the Teutonic peoples, the planet was named for their god Woden. Wednesday (wodnesdaeg in anglo-saxon) is named for him.
  • Mercury's atmosphere is so thin that it is barely detectable.