In the universe, there is the sun, the moon, the earth and many stars. Through our geography lessons, we know the earth goes around the sun, and the moon goes round the earth. We have day and night because the earth keeps turning all the time. When our part of the earth turns to the sun, it is day. When our part of the earth turns away from the sun, it is night. The other part of the earth is always contrary.

Just because the moon is closer to the earth than the sun, it looks much bigger than the sun. Big things will look smaller when they are further, and small things also look bigger when they are nearer.

The sun is big enough to give out very strong light. The moon can’t give any light at all, but it looks quite bright, too. Why? In fact, the light from the moon comes from the sun. The moon looks much bigger and brighter than the stars. But in fact, the stars are much bigger and brighter than the moon. They look smaller than the moon also because they are further away from the earth.

Earth is so important because living things and people can only live on the earth. And some countries have sent astronauts to the space to learn more about the universe. At 9:00 a.m. on October 15th, 2003, Chinese astronaut Yang Liwei was sent up to space. He stayed there for 21 hours. It was the first time for Chinese to get into space.

Venus, the evening star, is a brother planet of our earth. 【小题1】 It’s covered with air and clouds. Scientists once thought that there might be some kinds of life under the clouds. Recently, they have found that it’s too hot—400℃. 【小题2】 Why is it so hot then? 【小题3】 CO₂ allows the heat of the sun to go to the ground. At the same time, it prevents the heat from going out. 【小题4】 Thus（因此）most of the heat from the sun is kept in the air forever. The temperature goes up every year, and it’s 400%℃ now.

The United Nations suggested that all leaders in the world should meet to discuss cutbacks（缩减）of CO₂. Other people in Europe have started a battle called “Green Peace Movement”. 【小题5】 No one would like to change our green earth into a red hot planet like Venus.

Some people believe that humans could live on the planet Mars by the year 2100. Our own planet, the Earth, is becoming more and more crowded and polluted. It is hoped that people could start all over again and build a better world on Mars. Here is what life there could be like.

At present, our spacecraft are too slow to carry large numbers of passengers to Mars—it would take months. With the development of technology, by the year 2100, the journey might only take about 20 minutes in spacecraft that travel at the speed of light! However, the spacecraft would travel really fast, so the journey to Mars might be quite uncomfortable. Many people would feel ill.

Humans can’t live without water, air or food. So far, nobody knows whether there would be enough water or air on Mars for people. Moreover, scientists are not sure whether plants could grow on Mars. Food would most probably be in the form of pills (药片) and would not be so tasty.

Compared with life on the earth, life on Mars would be better in some ways. People would have more space. They might live in houses with huge comfortable rooms. Also, robots would do most of their work so that they could have more time to relax. There would probably be no schools on Mars. Every student would study at online schools with “e-teachers”.

Life on Mars would be interesting and challenging (有挑战性的). However, it may take hundreds of years to make it true. So the most important thing we need to do now is to protect our beautiful blue planet—the Earth!