Mercury - Astronomy Matters

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Mercury . . .

Mercury was the messenger of the gods and the god of roads and travel in Roman mythology. The ancient Romans also worshiped Mercury as the god of commerce, property, and wealth. The words commerce, merchandise, and merchant are related to his name. The Romans considered Mercury crafty and deceptive. They even saw him as a trickster or thief. Criminals regarded him as their protector. Because Mercury resembled the messenger god Hermes in Greek mythology, he took on many of Hermes' myths.

Mercury delivered his messages with miraculous speed because he wore winged sandals called talaria. He also wore a broad-brimmed winged hat called a petasus and carried a winged staff. The Greeks called the staff a kerykeion, from the Greek word for messenger. In Latin, the language of the Romans, the word was changed to caduceus. Mercury's caduceus had two snakes curled around it. In ancient times, most messengers and travelers wore a hat similar to Mercury's petasus to protect them from the sun.

Mercury is the planet nearest the sun. It has a diameter of 3,032 miles (4,879 kilometers), about two-fifths of Earth's diameter. Mercury's average distance from the sun is about 35,980,000 miles (57,910,000 kilometers), compared with 67,240,000 miles (108,210,000 kilometers) for Venus, the second closest planet.

Because of Mercury's size and nearness to the brightly shining sun, the planet is often hard to see from the Earth without a telescope. At certain times of the year, Mercury can be seen low in the western sky just after sunset. At other times, it can be seen low in the eastern sky just before sunrise.

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 about 59 days. The 59-day rotation period is two-thirds of the 88-day period that makes up a year on Mercury.

Mercury's surface appears to be much like that of the moon. It reflects approximately 6 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 many deep craters similar to those on the moon. Many astronomers believe the craters were formed by meteorites or small comets crashing into the planet as it does not have enough atmosphere to slow down meteoroids and burn them up by friction.