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.

计算：$\sqrt{18}-\sqrt{2}$=____ .

下列各句中，有错别字的一项是(　　)

发电机是把机械能转化为电能的装置。下图中能作为发电机设计原理的装置是(      )