In U.S. Time Zones you saw that the United States has six time zones and that it gets earlier as you go west. Now we can see why, using longitude.
Key idea
Earth turns all the way around, 360°, in 24 hours. So it turns 15° every hour (360 ÷ 24 = 15). Noon, when the Sun is highest, sweeps west around the world, and each time zone is about 15° wide.
Why it works
Before time zones, every town set its clocks by its own Sun: noon was when the Sun was highest. A town 15° east of you had noon one hour earlier than you did. That worked until trains made people travel fast; railroad timetables needed shared times. In the 1880s, railroads and then countries adopted zones, each about 15° wide, using the same clock time across the whole zone. The zones are counted from Greenwich, as hours ahead of (+) or behind (−) UTC, the time at the prime meridian.
Worked example: Degrees to hours
Earth turns 15° each hour. How many hours does it take to turn 75°?
- Divide by 15: 75 ÷ 15 = 5.
Answer: 5 hours.
Worked example: Sun time
Town A is 45° of longitude east of Town B. When it is noon by the Sun in Town B, what is the Sun time in Town A?
- 45° ÷ 15° per hour = 3 hours.
- Places to the east are ahead: they have already had their noon.
- So Town A is 3 hours later: 3 p.m.
Answer: 3 p.m.
Common mistake
Thinking time zones follow straight lines every 15°. They roughly do over the oceans, but on land they bend around countries and states, because a country wants its towns on the same clock. China uses one time zone, UTC+8, even though it spans about 60° of longitude, from about 73° E to 135° E; at the same clock time, the Sun is much higher in the east than in the far west. India uses UTC+5:30, halfway between two whole-hour zones.