Here is a picture of Mercury taken by the Mariner 10 spacecraft. Notice how the surface of the planet has been battered by all the objects that have hit it over the years. The surface of Mercury reminds us of our own Moon. Earth probably had this many craters at one time, but our weather has worn them down over the years. The weather on Mercury varies from extremely hot to extremely cold. The "daytime" temperature on Mercury is around 700 degrees, More or less depending on how close it is to the Sun. At the other extreme, the temperature on the "nighttime" side of the planet can be over 300 degrees below zero! The square "patches" you see in the picture are caused because the spacecraft couldn't take a picture of the whole planet at once. Scientists here on Earth put this picture together from a lot of smaller pictures of the planet. A picture together this way is called a mosaic.
Mercury is the closest planet to the sun. Its orbit 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 making the Mercurian year only 88 days, while our year on Earth is 365 days. Mercury does not rotate as fast as Earth, so a Mercurian day is equal to 59 Earth days!
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.
Mercury is the second smallest planet in the solar system with Pluto being the 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.
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.
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.
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