What is the moon?

　　The moon is an airless spherical* rock about one-fourth the size of the earth. It is the earth’s only known natural satellite, shining by the sun’s reflected light, revolving* around the earth from east to west in about 29.5 days. It has a diameter* of 3,475 km and a mean* distance from the earth of about 384,000 km.

　　The moon has had a turbulent* history. Most of the craters* on its surface were caused by meteoroids* crashing* into it about three or four billion years ago.

　　Where did the moon come from?

　　The moon and the earth are about the same age. The common theory is: The moon and the earth formed together from the dust and gas cloud.

　　Gravity on the moon

　　The moon is much smaller than the earth and so it has a weaker gravitational* pull*.

　　When the U.S.A. sent astronauts to the moon, they had to be weighed down to help them walk. It is gravity that causes ocean tides* on the earth to rise and fall. The gravity of the sun and moon pulls the earth’s oceans into bulges*. The earth rotates* beneath these bulges causing two high tides per day.

　　Phases of the moon

　　The moon keeps the same side facing the earth at all times. As the moon travels around the earth, we see a varying* degree of this side each day. These stages are called phases* of the moon.

　　The moon’s surface is covered with craters, mountains and valleys. The biggest moon crater that can be seen from the earth is called Bailly. It’s about 290 km across.

　　Little-known facts

　　<25D8> The moon is slowly moving away from the earth at the rate of 3 cm per year.

　　<25D8> The far side of the moon was not seen until 1959 when the Soviet spacecraft, Luna 3, flew by and took pictures.

　　<25D8> The earth’s gravity has pulled on the bright side of the moon, causing bulges several kilometers high.

　　(SD-Agencies)(Difficulty: senior)

　　中秋节就要到了，每到这一天都会有许多人外出赏月。可是你对月亮到底了解多少呢？如果你对我们这个地球的朋友所知寥寥的话，不要着急，读完下面这篇文章，也许你就可以成为一个月亮小专家了。