The Moon has been worshipped by primitive peoples and has inspired humans to create everything from lunar calendars to love sonnets, but what do we really know about it? The most accepted theory about the origin of the Moon is that it was formed of the debris from a massive collision with the young Earth about 4.6 billion years ago. A huge body, perhaps the size of Mars, struck the Earth, throwing out an immense amount of debris that coalesced and cooled; in orbit around the Earth.

The development of the Earth is inextricably linked to the Moon; the Moon’s gravitational influence upon the Earth is the primary cause of ocean tides. In fact, the Moon has more than twice the effect upon the tides than the Sun does. The Moon makes one rotation and completes a revolution around the Earth every 27 days 7 hours and 43 minutes. This synchronous rotation is caused by an uneven distribution of mass in the Moon (essentially, it is heavier on one side than the other) and has allowed the Earth’s gravity to keep one side of the Moon permanently facing the Earth. It is an average distance from the Earth of 384,403 km.

The Moon has no atmosphere; without an atmosphere, the Moon has nothing to protect it from meteorite impacts, and thus the surface of the Moon is covered with impact craters, both large and small. The Moon also has no active tectonic or volcanic activity, so the erosive effects of atmospheric weathering, tectonic shifts, and volcanic upheavals that tend to erase and reform the Earth’s surface features are not at work on the Moon. In fact, even tiny surface features such as the footprint left by an astronaut in the lunar soil are likely to last for millions of years, unless obliterated by a chance of meteorite' strike. The surface gravity of the Moon is about one-sixth of that of the Earth’s. Therefore, a man weighing 82 kilograms on Earth would only weigh 14 kilograms on the Moon.

The geographical features of the Earth most like those of the Moon are, in fact, places such as the Hawaiian volcanic craters and the huge meteor crater in Arizona. The climate of the Moon is very unlike either Hawaii or Arizona; however, in fact the temperature on the Moon ranges between 123 degrees centigrade to -233 degree centigrade.

The underlined word “they” in the passage refers to __________