In the universe, as we all know, 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 around 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.

Just because the moon is closer to the earth than the sun, it looks much bigger than the sun. Big things 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 actually the stars are much bigger and brighter than the moon. They look smaller than the moon also because they’re further away from the earth than the moon.

We all know that at 9:00 a. m. on October 15th, 2003, Chinese astronaut Yang Liwei went to space. He stayed there for 21 hours. It was the first time for Chinese to get into space. Then in the following years, more and more people get into space. I hope I will go to space one day.

We live on Earth. It’s our home. But how much do you know about it? Let me tell you something about our Earth.

The Earth is not flat as we see but it is like a huge sphere in the space. Like the other seven planets (行星), the Earth is running around the Sun. It’s the third nearest one to the Sun. It takes the Earth about 365 days to run around the Sun.

At the same time, the Earth is going around itself. If you are in space, you can see lots of white clouds over the surface (表面) of the Earth. Through the clouds, you can see the blue colour of the oceans, and the brown colour of the land. About 70% of the Earth is covered with water. Why do we have day and night? When the half of the Earth is facing the Sun, it’s daytime. As the Earth turns, and this half is away from the Sun, night is coming. And it’s daytime for the other half.

Do you ever get the feeling that you’re being watched? Well, you might be right.

According to a study published in Nature on June 23, astronomers(天文学家) have found that 1,715 stars have had a direct view of Earth since humans were here.

In order to do this, scientists used a method that looked for life on other planets. But instead, they changed the method so it could try to decide what places could see us.

The team looked at 331,312 stars within 326 light-years of Earth. Out of all those stars, only 1,715 of them could see Earth within the last 5,000 years, with an extra 319 stars that will be able to see us in the next 5,000 years.

“When I look up at the sky, it looks a little bit friendlier because it’s like, maybe somebody is waving,” said Lisa Kaltenegger, director of the Carl Sagan Institute at Cornell University, US, and the study’s lead writer.

If a planet circling around one of those 1,715 stars is home to advanced life, they could easily see that there is life here because of the oxygen on Earth. If that didn’t give it away, then the radio waves we have sent out into space would also be an indicator. In fact, human-made radio waves have already travelled through 75 of the closest stars on Kaltenegger’s list.

Why haven’t we heard from anyone yet, then?

It takes a long time for messages to travel between star systems. By the time a message could be received, that advanced life would probably not exist anymore.

Alan Boss, a scientist at the Carnegie Institution for Science in the US who wasn’t part of the study, wrote in an email that this long time would limit the chances for different life to exchange “emails and TikTok videos”.

“So we should not expect aliens to show up anytime soon,” Boss said.