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阅读理解。
Do you know that you can use your watch as a compass(指南针)on a sunny day?Take a watch and point the hour-hand in the direction(方向)of the sun．Imagine a line which runs from the center of the watch through a point half-way between the hour-hand and number one．This line will surely point south．If you know astronomy a little, you will understand why this works．In the northern half of the earth, the sun is in the south at noon．In our country the real noon is one o'clock．If at that time we point the hour-hand at the sun, both the hour-hand and number one will point south．That is why you must imagine a line which will run from the center of the watch through a point half-way between the hour-hand and number one．If you are in the southern part of our earth, you must point number 12 at the sun．The line which you can imagine between number twelve and the hour-hand will then show north．
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