Every morning the Sun seems to rise in the east, climb across the sky and set in the west. It looks as if the Sun is moving. In fact Earth is spinning, like a merry-go-round under a fixed lamp. Earth turns from west to east once about every 24 hours, and you ride along.
Key idea
Earth rotates once a day, from west to east. The half facing the Sun has day; the half facing away has night. Because we turn toward the east, the Sun appears to rise in the east and set in the west.
Why it works
Worked example: How far in 6 hours?
Through how many degrees does Earth turn in 6 hours?
- Earth turns 15° each hour.
- $6 \times 15 = 90$ degrees, a quarter turn.
Answer: 90°
Worked example: Noon somewhere else
The Sun is at its highest point (solar noon) over a town at 75° W. What is the solar time in a town at 45° W?
- The second town is $75 - 45 = 30$ degrees farther east.
- Each 15° is one hour: $30 \div 15 = 2$ hours.
- Eastern places are ahead, since they turned into the sunlight first: 2:00 p.m.
Answer: 2:00 p.m. solar time
Common mistake
Saying the Sun travels around Earth each day, or that night comes when clouds or the Moon block the Sun. Night is simply our side of Earth facing away from the Sun. A check: at every moment it is day on one half of Earth and night on the other, which a cloud or the Moon could never do.
Clock time zones follow country and state borders, so they bend around the 15° bands. Solar time is what the Sun itself shows.