English

Time Zones and Local Time

Middle schoolGeographyLocal time = UTC + zone offset; nominal zones are 15° of longitude wide; solar time shifts 4 minutes per degree

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About this experiment

When it is noon in London it is evening in Beijing and early morning in New York. The Earth turns 360° in 24 hours, so the Sun appears to move 15° of longitude every hour, and the world is divided into time zones roughly 15° wide. Run the clock and watch day and night sweep across the map, tap any place to find its zone time and solar time, and plan flights that seem to land before they take off.

Learning goals

  • Explain why the Earth needs time zones, using its rotation of 15° per hour
  • Calculate local time from UTC and a time zone offset
  • Calculate time differences between places from their longitudes
  • Explain the International Date Line and why the date changes when you cross it
  • Solve flight arrival time problems across time zones

How to use it

  1. 1Run the clock. In which direction does the night side move across the map? Why?
  2. 2Stop at 12:00 UTC. Which cities are in daylight and which are in darkness?
  3. 3Tap a point on 90°E. What is its nominal zone, and how many hours ahead of London is it?
  4. 4Tap near Beijing and then near far western China. Compare the zone time with the solar time. What do you notice?
  5. 5Find two places on either side of the date line at the same moment. What are their dates?
  6. 6Plan a flight from Tokyo to Los Angeles leaving at 17:00 and lasting 10 hours. When does it land, in local time?
  7. 7Plan the return flight. Why does it take a whole extra day on the calendar?
  8. 8Change the month to June and to December. How does the shape of the night side change?

The model

How fast the Sun appears to move across the longitudes
Zone time from Coordinated Universal Time
Nominal zone of a longitude, rounded to a whole hour
Arrival in local time at the destination

Assumptions

  • Cities use their standard offset; daylight saving time is left out because each country changes clocks on different dates.
  • The coloured bands are nominal zones, exactly 15° wide. Real zone borders follow countries, so China uses one zone and India uses UTC+5:30.
  • Solar time is mean solar time from longitude only; the equation of time, which shifts it by up to about 16 minutes, is ignored.
  • The night side uses the Sun declination for the middle of the chosen month and ignores refraction and twilight. Land outlines are from Natural Earth.

Try this in class

  1. Q1A place is at 75°W. What is its nominal time zone, and what time is it there at 15:00 UTC?
  2. Q2It is 09:00 on Monday in London. What time and day is it in Sydney (UTC+10) and in Honolulu (UTC−10)?
  3. Q3A plane leaves New York (UTC−5) at 22:00 and flies 7 hours to London. When does it land in local time?
  4. Q4Why does China use only one time zone, and what problem does that cause in the far west?
  5. Q5A traveller crosses the date line going west. Do they skip a day or repeat a day?

Frequently asked questions

What is the difference between GMT and UTC?

GMT was originally mean solar time at Greenwich. UTC is the modern standard kept by atomic clocks, adjusted with leap seconds to stay close to Earth rotation. For everyday use they differ by less than a second.

Why are some time zones offset by half an hour or 45 minutes?

Countries choose their zone to suit their geography and daily life. India picked UTC+5:30 to sit in the middle of the country, and Nepal uses UTC+5:45 to match solar noon in Kathmandu.

Why is the date line not straight?

It bends around island groups so that each country keeps a single date. Kiribati moved it in 1995 so that all its islands share the same day, which put some of its islands at UTC+14.