This series of eight articles will look at each planet individually. Article One will examine Mercury, the innermost planet closest to the sun, and also the fastest moving planet in our solar system.
The International Astronomical Union (IAU) has changed the definition of planet so that Pluto is no longer considered a planet. There are now officially only eight planets in our solar system. Mercury is the smallest of the planets now that Pluto is not included. It is about 3,031 miles in diameter compared to Earth, which is about 7,926 miles in diameter. Mercury is only slightly larger than the Earth’s moon.
The gravity on Mercury is 38% of the gravity on Earth. Its temperatures range from 800 degrees F. to -270 degrees F. There are no seasons on Mercury. Seasons are caused by the tilt of the axis relative to the planet’s orbit. Since Mercury’s axis is directly perpendicular to its motion (not tilted), it has no seasons.
The length of a year on Mercury is 88 days. In other words, it takes 88 Earth days for Mercury to complete one orbit around the Sun. One day on Mercury, which is the time it takes a planet to spin on its axis, is 58 Earth days.
Mercury has no moons. The Earth is three times farther from the sun than Mercury is. Mercury was visited by NASA’s Mariner 10 in 1973 and 1974. About half of Mercury’s surface was mapped out by this spacecraft.
Mercury was named after the mythical Roman winged messenger. It was named after the speedy Mercury because it is the fastest moving planet.
Mercury’s elliptical orbit takes the planet as close as 29 million miles and as far as 43 million miles from the sun. This is in comparison to the Earth which is 93 million miles from the sun.
From a distance, Mercury’s appearance is similar to the Moon. Its surface is very old and heavily cratered, and it seems to have no plate tectonics.
Because Mercury is so close to the sun, it is difficult to observe from Earth. It does make an appearance 13 times in a century. Earth observers can watch Mercury pass across the face of the sun, an event that is called a transit. These transits can be observed within several days of May 8 and November 10.
Scientists used to think that the same side of Mercury always faced the sun. However, astronomers found in 1965 that the planet rotates three times during every two orbits, so that it does sometimes faces away from the sun.