How is this view of Earth's daily cycle similar to a clock?
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Did you notice?
  • The line between darkness and daylight seems to move clockwise, like the hands on a clock.

The Why Behind What You See
The Earth rotates on its axis. It makes one full turn every 24 hours.

Our clocks show these 24 hours, too. Most are divided into 12 a.m. and 12 p.m. hours. When we want to use a standard time that's the same everywhere on Earth, we use Greenwich Mean Time (GMT), also called Universal Time (UT). This is based on a 24-hour clock.

The Earth is divided into 24 time zones. All clocks in the same time zone show the same time. Each time zone is an hour earlier than next zone east and an hour later than next zone west. On the equinox, as shown in the animation above, each hourly movement of the sun roughly shows the movement from one time zone to the next.

Math Challenge
Each day, the earth rotates once on its axis, which equals 360 degrees.

  1. There are 24 hours in a day. How many degrees longitude does the earth turn each hour?
  2. The earth is divided into 24 global time zones. How many degrees of longitude does each one cover? (See answer, below.)


  1. 360 degrees ÷ 24 hours = 15 degrees per hour.
  2. Each time zone covers 15 degrees of longitude.

Greenwich Mean Time

24-hour clock