The universe (宇宙) is everything out in space. Nobody ______ how large the universe is. When we talk about the universe, we mean the Earth, the Sun, the Moon and all the ______ , and the space between them.

Now let me tell you something about the ______ . It is like a large ball. It ______ around the Sun. And ______ and land cover it. If you are in a spaceship and ______ the Earth from space, you will find that there are some blue parts. They are the seas.

We live on the outside part of the Earth. Around us is the air. It takes the Earth 24 ______ to turn in a full circle (整圈). When you see the Sun shining ______ , it’s the day for you. But when the Earth turns you away from the Sun, everything around you ______ dark(黑暗的).

Can you tell me something else about the universe? I think it is ______ and I want to learn more about it.

What do you know about Mars? What does it look like?

Mars is the fourth planet from the sun in our solar system and is ___________ after Mars, the Roman god of war.___________ diameter(直径) is 53% as ___________ as that of the earth. Mars goes around the sun at a distance of about 228 million kilometers. You can tell ___________ planet is Mars because it is bright red in the night sky. It ___________ like a red and orange ball. It’s very beautiful. But during spring and summer, the surface of Mars is covered ___________ strong storms. The gravity on the surface of Mars is about ___________ as strong as it is on earth. So a person who weighs 90 kilos on earth weighs only 36 kilos on Mars. The temperature on Mars is ___________ -138℃ and 28℃. The air has only 0.13% oxygen.

It takes a spaceship about eight months to ___________ Mars from the earth when the two planets are ______________ to each other. Scientists are still searching for more information about Mars.

We have always been interested in the moon. Two thousand years ago, people already knew that the moon _________ 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 _________ it carefully in the sky.

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

In 1969, moon rocks were finally _________ to the earth and studied. To their surprise, scientists found that, except 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 _________ billions of rocks into space around our planet. These rocks _________ joined together and after many years became the moon.