24 hours.

Earth is one of nine planets which revolve (绕转) around the sun. This family of planets, with their moons, is known as the solar system. Since we live on Earth, we think it is the most important planet. But the other planets are also interesting. Some are smaller than Earth, some larger;some are hotter, some colder. Each one is different from all the others and has something special about it. No one knows for certain how the solar system began. We do know a lot about the planets, however, from the science of astronomy (天文学 ). Astronomers have done research on the planets and other objects in space for thousands of years. Astronomy is a very complicated science. We look at the other planets to study them, but the planet we are on is moving all the time. Computers have helped us in the past twenty or thirty years. The satellites we sent into space to measure and take pictures also tell a lot about the planets and help us to understand them.
    Mercury (水星) is the closest planet to the sun. It is also the smallest and fastest planet. The speed of the planet gave it its name: Mercury was the rapid messenger of the Roman gods. Mercury completes its orbit around the sun in only 88 days, while Earth’s orbit takes 365.25 days. A day on Mercury, however, is much longer than a day on Earth. Earth rotates on its axis once every 24 hours. Mercury rotates on its axis once every 59 days.
    Venus (金星) is the second planet in distance from the sun. It is about the same size as Earth, and it comes closer to Earth than any other planets. Next to the sun and the moon, Venus is the easiest object in the sky to see. People of ancient times loved its brightness and beauty so much that they named it after the Roman goddess of love and beauty.
    Venus revolves around the sun once every 225 days and rotates on its axis once every 224 days. Information from telescopes and satellites shows us that the planet is very hot. The temperature is usually between 300 and 675 degrees Celsius. We can observe both Venus and Mercury either in the evening or early in the morning just before the sun rises.
    Mars( 火星) is called the red planet because of its definite (明确的) red-orange color. It is the fourth farthest planet from the sun. A day on Mars is only about 40 minutes longer than a day on Earth, but a year lasts 687 days. The planet was named after the Roman god of war.
    For many years, people believed that there was life on Mars, but we now know that there probably isn’t. The Mariner space program sent many satellites to Mars. We have excellent photos and information on this neighbor planet, and nothing suggests that there is any life there.
    The fifth planet from the sun is also the largest of all the planets, Jupiter (木星). Jupiter revolves around the sun once every 12 years, yet its day is amazingly(惊人地) short. Its rotation, or its day, is only about 10 hours long. This is the fastest rotation—the shortest day—of any of the planets.
    The planet’s name is appropriate, since Jupiter was the largest Roman god—the king of all the other gods. If you can imagine Jupiter as a hollow ball, all of the other planets would fit inside and there would still be a lot of room left. Jupiter has fourteen moons, the largest number of any planets.