Everyone on the planet wants the sun to be at its highest
point in the sky (crossing the meridian) at noon. If there
were just one time zone this would be impossible because the earth rotates
15 degrees every hour. The idea behind multiple time zones is
to divide the world into 24 15-degree slices and set the
clocks accordingly in each zone. All of the people in a given zone set their clocks the
same way, and each zone is one hour different from the next.
In the continental United States there are 4 time zones:
Eastern, Central, Mountain and Pacific. When it is noon in the Eastern time zone it is 11
AM in the Central time zone, 10 AM in the Mountain time zone and 9 AM in the Pacific time
zone.
All time zones are measured from a starting point centered at
England's Greenwich Observatory. This point is known as the
Greenwich Meridian or the Prime Meridian. Time at the Greenwich Meridian is known as
Greenwich Mean Time (GMT) or Universal Time. The Eastern time zone in the United States is
designated as GMT minus 5 hours. When it is noon in the Eastern time zone it is 5:00 PM at
the Greenwich Observatory. The International Date Line (IDL) is located on the opposite
side of the planet from the Greenwich Observatory.
meridian
วงกลมที่ผ่านขั้วโลกในท้องฟ้า
rotates หมุนรอบ
multiple หลายอย่าง
slices ชิ้นบางๆ
continental แห่งทวีป
measured ซึ่งมีขนาด
Observatory หอดูดาว