The moon is     1     away from the earth. We can only see a small moon with our own eyes on the earth. When we see a small part of the moon, we call it a crescent moon (新月). When we see the whole moon, we call     2     a full moon. That’s all people knew about the moon in ancient times. They even didn’t know the moon goes     3     the earth. But they thought that there might be life on the moon. However, in July 1969, scientists sent Neil Armstrong to the moon and found that there weren’t any     4     on the moon.

Have you ever heard of the satellite (卫星) called Early Bird, which was built by a group of scientists     5     14 countries? So far, a few satellites have been put into space and they helped people to     6     photos of the earth and other stars.

    7     we know a lot about the moon and the earth now, people always ask the questions: Is there anyone out there in space? Are we alone? Our scientists haven’t got a clear answer     8    . It is still one of the greatest mysteries (谜底) and some people think it will be a secret for     9     long time. But we believe we can     10     the problem some day.

We see the sun every day. If it is     1    （sun）, we can see the moon at night, too. The moon is near us. So far astronauts     2    （visit） the moon several times. The moon is almost     3     nearest satellite（卫星） to the earth in space.

Light     4    （travel）at 300,000 kilometers a second. Light reaches us from the moon in less than a second and a half. The moon is about 382,000 kilometers away.

Light from the sun arrives in the earth in about 8.5     5    （minute）. The sun is about 149,640,000 kilometers away     6     the earth. Other stars are much     7    （far） away than the sun. Light from the nearest star reaches     8    （we）in about four years. When you look at a star, it is very small.     9     in fact many of them are much bigger than the sun. They look small only because they are     10    （real） far away.