We have always been interested in the moon. 2, 000 years ago, people already knew it moves around the earth and where it would be in the sky at different times of the year. At that time, everything about the moon was learned by watching it carefully in the sky.

When scientists could use telescopes to study the moon more closely, their ideas began to change. They could see the moon was made of rocks. Most scientists thought moon rocks would be different from those on the earth. This was because they believed the moon had once been a planet that had been caught in the earth’s gravity (重力) millions of years earlier.

In 1969, moon rocks were finally brought to the earth and studied. Much to their surprise, scientists found that, except for water, the moon and the earth were made of the same things. Once again new ideas were needed for this new information.

After years of study, most scientists now think that the moon was once part of the earth. They believe very early in its history, maybe 4 billion years ago, something about the size of Mars hit the earth. This sent billions of rocks into space around our planet. These rocks slowly joined together and after many years became the moon.

In the future, though our ideas about the moon may change again, we will still be interested in it.

Look up at the sky on a clear night, you will probably see a lot of stars. Do you ever think about the light (光）from these stars? 【小题1】

First of all, when you see a star, in fact you are looking at the past(过去). Do you know why? 【小题2】 Their light can take many years to get to the Earth. For example, the star Proxima Centauri is around four light years away from our solar system. That is to say, we are seeing light from the star that is four years old. Many other stars are thousands of light years away. So the light we see from those stars is thousands of years old.

Secondly, 【小题3】 At first, they all seem to be the same. However, if you look a little more closely, you will find that the stars are different colors. 【小题4】 And some are even red or blue. There are a few reasons why stars are different colors. 【小题5】 Some are much hotter or cooler than others. Blue stars burn at very high temperatures while red stars burn at very low temperatures. It is interesting, isn’t it?