The word “day” has two meanings. When we talk about the number of days in a year, we are using “day” to mean 24 hours. But when we talk about day and night, we are using “day” to mean the time between sunrise and sunset.

Since the earth looks like a ball, the sun can shine on only half of it at a time. Always one half of the earth is having day and the other half night. A place is moved from day into night and from

night into day over and over by the spinning（旋转）of the earth. At the equato（r   赤道）day and night

are sometimes the same length. They are each twelve hours long. The sun rises at 6 o’clock in the morning and sets at 6 o’clock in the evening.

For six months, the North Pole is tilted（倾斜）towards the sun. In those months the Northern

Hemisphere（半球）gets more hours of sunlight than the Southern Hemisphere. Days are longer than nights. South of the equator nights are longer than days. For the other six months the North Pole is tilted away from the sun. Then the Southern Hemisphere gets more sunlight. Days are longer than nights. North of the equator nights are longer than days. Winter is the season of long nights. Summer is the season of long days.

Thumbs Down to Texting (屈指发短信)

Do you have a mobile phone? Do you send text messages to your friends and family?

Text messaging or “texting” is becoming very popular. But, scientists have discovered that texting can give us problems with our hands. Be careful! Too much texting can cause swelling in our thumbs. Our thumbs were not made for pushing small buttons, over and over . Scientists call this problem RSI. We spoke to a doctor called Harriet Wilson, who treats many sick people with RSI. She says we need to see how much time we are spending in typing text messages. If we are spending more than 10-15 minutes at once, we could have problems in the future.

As mobile phones develop, they are getting smaller with buttons closer together. Texting with a smaller phone is worse than using a larger phone with bigger buttons. Using a computer keyboard and playing video games can cause the same problem. So more and more people could find themselves with RSI. Among them, most are university students and teenagers, as they do not know the risks of spending so much time using computer, video games and mobile phones. If we don't do something about this, too many young people could grow up in terrible pain.