Eve Phillips, age 12, for her question:
WHICH PLANET IS THE SMALLEST?
Mercury is the smallest planet in our solar system. There are nine planets orbiting the sun and Mercury is also the one that is closest to the sun.
Mercury is about two fifths the earth's diameter. Its diameter is 3,100 miles and it is about 36 million miles from the sun, compared to 67.25 million miles for Venus, the second closest planet to the sun.
An elliptical or oval shaped orbit is used by Mercury as it goes around the sun. It is about 29 million miles from the sun at its closest point and more than 43 million miles at its farthest point. It is about 53 million miles from the earth at its closest approach.
Mercury's orbit around the sun is faster than any other planet because it is the closest to the sun.
The ancient Romans named the planet Mercury in honor of the swift messenger of their gods. Mercury travels about 30 miles per second and goes around the sun once every 88 earth days. The earth goes around the sun once every 365 days, or one year.
Rotation of Mercury is slower than that of any other planet except Venus. A day on Mercury lasts about 180 earth days because Mercury rotates so slowly.
Until 1965, astronomers believed that Mercury rotated once every 88 earth days, the same time the planet takes to go around the sun. If Mercury did this, the sun would seem to stand still in Mercury's sky. One side of the planet would always face the sun, and the other side would always be dark.
In 1965, astronomers bounced radar beams off Mercury. The signals returning from one side of the planet differed from those from the other side. Using these beams, the astronomers measured the movement of the opposite sides and found that Mercury rotates once in 59 days.
Mercury's surface appears to be much like that of the moon. It reflects about six percent of the sunlight it receives, about the same as the moon's surface reflects.
Like the moon, Mercury is covered by a thin layer of minerals called silicates in the form of tiny particles. It also has broad, flat plains, steep cliffs and deep craters similar to those on the moon. Many astronomers believe the craters were formed by meteors crashing into the planet.
Mercury does not have enough atmosphere to slow down approaching meteors and burn them up by friction. For this reason, the planet's surface has many craters.
Mercury is dry, extremely hot and almost airless. The sun's rays are about seven times as strong on Mercury as they are on earth. The sun also appears about two and a half times as large in Mercury's sky as it appears on the earth's.
The temperature on the planet is about 625 degrees Fahrenheit during the day and about 80 degrees Fahrenheit at night.