What’s Out Tonight?

A general ASTRONOMY site to get you started exploring the night sky


Facts about Mercury 

• First planet from the Sun 
• It’s a Terrestrial Planet just a little bigger than our Moon. Terrestrial means Earth-like, so Mercury has a hard surface that you can stand on.

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 privilege because most people never get to glimpse this planet. At its brightest, which happens a few times a year, it is fairly conspicuous but because Mercury hugs very close to the Sun, you have to work fast for the short time it is visible after sunset or before it gets lost in the glare of the rising Sun. Binoculars can be helpful to locate this illusive planet, enhancing what often escapes the attention of our eyes alone.

More about Mercury

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. 


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.