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10.5: Local Time (LT) and Time Zones

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  • Longitudes are measured from zero to 180 east and 180 west (or -180), and both 180-degree longitudes share the same line, in the middle of the Pacific Ocean.

    As the Earth rotates around its axis, at any moment one line of longitude — “the noon meridian” — faces the Sun, and at that moment, it will be noon everywhere on it. After 24 hours the Earth has undergone a full rotation with respect to the Sun, and the same meridian again faces noon. Thus each hour the Earth rotates by 360/24 = 15 degrees.

    When at your location the time is 12 noon, 15 to the east the time is 1 p.m., for that is the meridian which faced the Sun an hour ago. On the other hand, 15 to the west the time is 11 a.m., for in an hour's time, that meridian will face the Sun and experience noon.

    In the middle of the 19th century, each community across the US defined in this manner its own local time, by which the Sun, on the average, reached the farthest point from the horizon (for that day) at 12 o'clock. However, travelers crossing the US by train had to re-adjust their watches at every city, and long distance telegraph operators had to coordinate their times. This confusion led railroad companies to adopt time zones, broad strips (about 15 wide) which observed the same local time, differing by 1 hour from neighboring zones, and the system was adopted by the nation as a whole.

    The continental US has 4 main time zones: eastern, central, mountain and western, plus several more for Alaska, the Aleut islands and Hawaii. Canadian provinces east of Maine observe Atlantic time; you may find those zones outlined in your telephone book, on the map giving area codes. Other countries of the world have their own time zones; only Saudi Arabia uses local times, because of religious considerations.

    In addition, the clock is generally shifted one hour forward between April and October. This “daylight saving time” allows people to take advantage of earlier sunrises, without shifting their working hours. By rising earlier and retiring sooner, you make better use of the sunlight of the early morning, and you can enjoy sunlight one hour longer in late afternoon.