Because Mercury is so close to the Sun, it is hard to see from Earth except during twilight. Until 1965, scientists thought that the same side of Mercury always faced the Sun. Then, astronomers discovered that Mercury completes three rotations for every two orbits around the Sun. If you wanted to stay up for a Mercury day, you'd have to stay up for 176 Earth days.

 Mercury is the closest planet to the sun. The orbit, or path, it takes around the sun is not in a circle like Earth. The path it takes is called an ellipse, which is like a circle that has been stretched out. For this reason, the distance from Mercury to the Sun varies from about 27 million miles at its closest, to about 41 million miles at its furthest.  Mercury races around the Sun faster than any other planet. A Mercurian year (the time it takes for the planet to go once around the Sun, is only 88 days, while our year on Earth is 365 days. Mercury does not spin as fast as Earth, though, so a Mercurian day (the time it takes a planet to rotate once) is 59 Earth days! 

 Can I See It? 
      Yes you can, and you won't need a telescope, but Mercury is normally visible for a few days only three or four times a year.  Because of its closeness to the Sun, Mercury is normally hidden by sunlight. When the planet is visible, you either have to get up   before sunrise or be watching closely just after sunset. Mercury never gets very far above the horizon, so you will also need a   place where can see clearly to either the east or west. Check the sky maps here at Astronomy for Kids. If Mercury is going to be
      visible, we will tell you when and how to see it. 

 How Big Is It?
      Mercury is the second smallest planet in the solar system. Pluto is the only planet that is smaller. Earth is about 7,200 miles in diameter, while tiny Mercury is less than half that size at about 3,000 miles in diameter. Put another way, if Earth was the size   of a baseball, Mercury would be a little smaller than a ping-pong ball. Mercury is so small that Ganymede, one of Jupiter's  moons, and Titan, one of Saturn's moons, are both larger. 

 How Did It Get Its Name?
      Mercury is named for the Roman god of commerce and thievery. It may have gotten this name because of the way it quickly  appears and disappears from our view. 

 What Is It Made Of? 
      Mercury is one of the rocky planets of the solar system, along with Earth and Venus. Mercury is made up mostly of iron, with a    thin rocky layer on the planet surface. 

 What's It Like on The Surface?
      On thje surface of Mercury, you will see that there are many craters on its surface. The planet has been hit many, many times through the years by asteroids and other objects. These impacts are responsible for all the craters on the surface. Earth  probably had this many craters at one time, but our weather has worn the down over the years.  The weather on Mercury varies from extremely hot to extremely cold. If you were on the side of the planet facing towards the  Sun, you would quickly melt, as the temperature would be around 700 degrees! The "daytime" temperature on Mercury varies,  depending on how close it is to the Sun. At the other extreme, the temperature on the "nighttime" side of the planet (the side away from the Sun) can be over 300 degrees below zero! Mercury is not a very pleasant place.