Facts about Mercury

• First planet from the Sun
• It’s a Terrestial Planet just a little bigger than our Moon

Diameter: 3,032 miles
Mass: 0.055 Earth’s mass
Density: 5.43 where water = 1
Gravity: 0.38 times that of Earth
Albedo (% of Sunlight reflected): 11%
Rotation on Axis: 58.7 days
Inclination of Axis to Orbit: 0.0°

Distance from Sun: 35,980,000 miles
Revolution about Sun: 87.97 days
Inclination of Orbit to Earth’s Orbit: 7°

Atmosphere: Mercury has no atmosphere.
Surface Temperature: 800° F/day side. –300° F/night side.

Moons: Mercury has no moons.

The Challenge to See Mercury: Just to find and see Mercury with your eyes is almost a privledge because most people never get to glimpse this planet. Mercury hugs very close to the Sun, so you have to work fast just to see it after sunset or before sunrise because you don’t have much time before it is “lost.” At its brightest, it is fairly conspicious but time is limted before it sets or is lost in the glare of the Sun. Binoculars can be helpful to locate this illusive planet, enhancing what often escapes the attention of our eyes alone.



The planets were named after ancient Roman and Greek mythological gods. Mercury, the closest planet to the Sun, was identified with the Roman god who had wings attached to his feet and a helmet on his head. He swiftly delivered messages to the other gods. As the name so well implies, the planet Mercury revolves rapidly around the Sun, more swiftly than any of the others.

Mercury resembles our Moon. Like our Moon, it is small in size, pitted with craters, and has no atmosphere. Its craters were formed from a heavy bombardment of asteroids and comets during the first billion years of the solar system’s existence. The interior of Mercury, once molten, has cooled and is now solid. It is composed mostly of iron ore.

Since Mercury orbits inside Earth’s orbit, it cycles through phases like our Moon. When we see phases, we are seeing nothing more than the day and night sides of the planet at the same time.

Mercury is difficult to study with a telescope because it is so close to the Sun. All of the close-up pictures of it were obtained by the two spacecraft, Mariner 10, that visited in 1975 and the more recent, Messenger that flew by in 2008 and went into orbit in 2011. Messenger has provided the greatest wealth of information and a complete mapping of the surface.

Mercury in the sky
Mercury is visible as a fairly bright star several times a year; however, most people never see it, because it can only be seen for a short time after sunset or before sunrise. Consult the What’s Out Tonight? monthly star chart for the best times to view Mercury.


A false-color picture of Mercury to enhance a big impact basin (the yellowish part). Astronomers often colorize pictures to bring out detail.


When looking at the pictures above and below, one could mistake them for the Moon instead of Mercury because the two bodies are covered in craters, have plain-type areas and are appear crystal sharp because neither has an atmosphere.


What’s Out Tonight? is sponsored by Ken Press, publisher of astronomy books and charts.
Phone: (520) 743-3200 • Fax: (520) 743-3210 • Email: