Mercury is the closest planet to the sun. It is difficult to see from Earth. Mercury is about one-third the size of Earth. It completes a trip around the sun every 88 (Earth) days. This makes it the fastest orbiting planet, moving at about 50 kilometers per second. Mercury spins slowly on its axis, however. In two of its years (orbits) only three days pass! The length of one Mercury day is equal to about 58 Earth days.

The daytime temperature on Mercury can reach over 425 degrees Celsius. There is very little atmosphere to absorb this heat, however. The nighttime temperature goes down to about -170 degrees. This is the largest temperature difference of all the planets in our solar system.

The surface of Mercury looks almost like our Moon's surface. There are many craters caused by collisions with meteoroids and comets. There are some areas of smooth surface. In some places there are tall, sharp cliffs. Mercury's iron core has a radius of about 1800 kilometers. It is surrounded by an outer shell that is only about 500 kilometers thick.