21（）

A．in
B．of
C．from
D．on

Everyone uses the expressions AM and PM. But do you exactly know what they mean and how they came into being

As you know, the turning of the earth makes the sun and the stars seem to move across the sky. Daylight,of course, (21) . when the sun rises in the east and ends in the west,when the sun is high (22) the sky, between these two positions, half of the daylight hours have been spent.

Therefore, by noticing (23) the sun stood in the sky, early man know they could tell the time of the day. At night the motion of the stars (24) the same purpose. The important thing in keeping time is to know the exact moment of (25) . For each of us,wherever we are, noon is when the sun is directly over head. Think of an imaginary line (26) the sky, stretching from the north point of your horizon just to the south point.

When the sun cross your meridian (子午线), it is noon for you. When the sun still (27) of this line, or the meridian, it is morning. After the sun has crossed this line, it is (28) .

The Latin word for" midday" is" deridian" , (29) which comes our word meridian. So AM is an abbreviation(缩写词) for "ante meridian " ,of before midday, and PM for after midday. Each of the world’s time zones(时区)is about fifteer degrees wide in longitude(经度) ,which is about the distance the sun moves through the sky in an hour. (30) lives in the same zone observes noon at the same moment. In this way, the time is different by one hour. as you move through each time zone.