Many hundreds of years ago, the moon was a mystery to people on the earth. What was that glowing ball came out at night? Why did it keep getting smaller, then larger? Why didn't it fall from the sky? Through the ages, people have tried to come up with answers.

    To the native people of Bolivia, in South American, the moon was a great chief who once lived on the earth. The Abaluyian people of Kenya, in Africa, believe the moon and the sun were brothers who fought in the sky. In other parts of the world, people believed they saw different pictures in the moon. The Haida Indians of Canada saw a woman carrying a bucket. The Chinese saw a road.

    People in the past also used the moon to mark time as a kind of calendar. In fact, in some languages, the word for moon meant “month”. A Native American nation called the Sioux made a calendar of 13 moons, or months. Each moon had a name, such as Moon of Black Cherries, Moon of Green Corn, and Moon of Severe Cold.

    Though people of the past found the moon was helpful, mysterious and interesting, they didn't know much about it. They only knew what they could see with their own eyes.

    Then almost 400 years age, an Italian scientist named Galileo made a new telescope(望远镜) that was more powerful than any telescope made before. Galileo decided to take his new implement to look at the moon. He discovered that the moon was not the smooth, shiny ball. The brighter places were mostly hills and mountains. The flat areas were lower, which is why they appeared darker.

    As more people saw the moon through telescopes, they discovered more about it. Yet, people still had much to learn about the moon.

    Have you ever heard someone use the phrase"once in a blue moon"？People use this expression to describe something that they do not do very often．For example，someone might say that he tries to avoid eating sweets because they are unhealthy，but will eat chocolate"once in a blue moon"．Or someone who does not usually like to go to the beach might say"I visit the shore once in a blue moon．"While many people use this phrase，not everyone knows the meaning behind it．

    The first thing to know is that the moon itself is never really blue．This is just an expression．In fact，the phrase"blue moon"has to do with the shape of the moon，not the color．

    As the moon travels around the earth，it appears to change shape．We connect names with certain shapes of the moon．For example，when we can see a small part of the moon，it is called a crescent moon．A crescent is a shape that looks like the tip of a fingernail（指甲尖）．When we cannot see the moon at all，it is called a new moon．When we can see the whole moon，it is called a full moon．Usually，there is only one full moon every month．Sometimes，however，there are two full moons in one month．When this happens，the second full moon is called"a blue moon"．

    Over the next 20years，there will only be 15blue moons．So you can seldom see a blue moon． This fact has led people to use the expression"once in a blue moon"to describe things they don't often do in their lives．

    Some primary schoolchildren have been raised in homes with more green space around. They are likely to come with larger volumes of white and grey matter in certain areas of the brain. These differences are associated(关联) with beneficial effects on cognitive function (认知功能). This is the main conclusion of a study led by the Barcelona Institute for Global Health.

    The study was performed among 253 schoolchildren in Spain .Lifelong exposure(接触) to green space in the living places was recorded—using the information on the children's addresses from birth up through to the time of the study. Brain structure was studied using 3D magnetic resonance imaging (MRI). Working memory and inattentiveness(注意力不集中)were graded with computers.

    “This is the first study that shows the association between long-term exposure to green space and brain structure.” Says Dr. Payam Dadvand, the leading researcher of the study, “Our findings suggest that exposure to green space early in life could result in beneficial structural changes in the brain.”

    The findings show that long-term exposure to greenness is positively associated with white and grey matter volumes in several parts of the brain. Some of them are related to higher scores on cognitive tests. Moreover, larger volumes of white and grey matter in those parts might lead to better working memory and less inattentiveness.

    Exposure to nature has been thought to be necessary for brain development in children. Another study of 2,593 children shows that children in school with more green space have a greater increase in working memory and a greater decrease in inattentiveness.

    Humans are believed to be tied to nature.  Playing in greener areas offers children opportunities to search and learn. Accordingly, green space is thought to prompt important exercises in discovery, creativity and risk taking. These exercises in turn positively influence brain development.

    Dr. Dadvand's study suggests how such structural changes could bring about the beneficial effects of green spaces on cognitive development, it also adds to the proof that suggests the lasting effects of early life exposure to greenness on our health and the benefits of increasing greenness in cities.

    Further studies are needed to prove the findings in other populations, settings and climates. And researchers need to examine differences according to the nature and quality of green space.

    No Car Day first started by 34 cities in France on September 22, 1998. People started it to protect the environment. By now, more than 1,000 cities around the world have a No Car Day.

    The first No Car Day in China was in Chengdu in 2001. Other cities, including Taipei, Shanghai and Wuhan, also support(支持) the day.

    In Beijing, more and more people are joining in the program. It asks drivers to leave their cars at home for one day each month and walk or ride a bike to work. It also calls on Beijingers not to use cars on June 5(World Environment Day). The slogan(口号) for the day is, "If we drive for one less day, we can have one more nice day."

    So far, more than 200,000 drivers Show their support. "We can't decide the weather, but we can choose not to drive," said Wu Zhonghua, a car dub manager. Beijing is trying to have 238 blue sky days this year. In the first quarter of 2017, Beijing only had 52 blue sky days. This was 11 days less than the number for the same period the year before.

    We must do more for No Car Day.

    Sheridan is a town in Indiana, USA. There are lots of trees near the public schools in the town. Their leaves turn sunlight into energy（能量）.

    In Sheridan, the sun not just provides energy for plants. It keeps schools running, too. The schools use solar panels（太阳能电池板） to turn sunlight into energy.

    The science behind solar energy is not new. But Sheridan Community Schools is the first school district(学区) in Indiana to be completely solar-powered. The district finished putting in solar panels for all three of its schools in 2016. Some of the panels can turn to follow the sun across the sky.

    In 2008, fewer than 1. 000 schools used solar power. By 2014, there were 3, 727 schools with solar panels in the USA. These numbers come from a report by the Solar Foundation. Roxie Brown, a program director there said that the number of schools with solar panels has continued to go up since 2014.

    Solar energy has some advantages over other energy sources(来源). Sunlight is a renewable source, which means it won't run out. Solar panels don't harm the environment. Also, sunlight is free. So by using solar power, schools can save money on energy costs.

    Brown thinks more schools should run on solar energy. "The sun is Earth s energy source," she says. "We're trying to make it the energy source for our houses cars and schools, too."

    But using solar energy isn't always easy. Solar panels can be expensive to put in. The panels also take up space. But for many schools, going solar is worth it.

    When we talk about the universe, we mean not only the earth, the sun and the moon, but also the stars and space between them. There are millions of stars in our sky. The sun is one of them. Around it there are eight big planets moving, including the earth. The moon is our nearest neighbor in space. If you look at the sky on a clear night, it is possible to see about 3, 000 stars. They look small, but they are really very big hot balls of burning（燃烧着的）gas. Some of them arc huge（巨大的）, but others are much smaller like our planet Earth. The biggest stars are very bright, but they can only live for a short time. Every day new stars are born and old stars die.

    All the stars are very far away. The light from the nearest star（except the sun）takes more than four years to reach the earth.

    Hundreds of years ago, people used stars, like the North Star, to know which direction（方向）to travel in.

Today you can still see that star.