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Longitude and Time

 Longitude and Time: Simple Notes

  • Earth's Rotation:

    • Earth spins around (rotates) 360° in 24 hours (one day).

    • This means Earth rotates 15° of longitude every hour (360° / 24 hours = 15°/hour).

    • Or, Earth rotates 1° of longitude every 4 minutes (60 minutes / 15° = 4 minutes/°).

  • Local Time Changes with Longitude:

    • Eastward Travel: If you go east, you move ahead in time. For every 15° of longitude you travel east, local time becomes 1 hour later.

    • Westward Travel: If you go west, you move backwards in time. For every 15° of longitude you travel west, local time becomes 1 hour earlier.

    • Why? Because the Earth rotates from west to east. Places in the east see the sun earlier in the day.

  • Greenwich Mean Time (GMT): World Time

    • Greenwich, London (0° Longitude) is the starting point for world time.

    • GMT is the time at Greenwich. It's like a standard time that everyone can use as a reference.

    • Sometimes called World Time.

  • Calculating Local Time from GMT:

    • If you know GMT and the longitude of a place, you can find its local time.

    • East of Greenwich: Add time to GMT.

    • West of Greenwich: Subtract time from GMT.

    • How much time? Figure out the difference in longitude from Greenwich (0°). For every 15° difference, it's a 1-hour time difference.

  • Memory Aids:

    • E.G.A.: East - Gain - Add (When you go East, you Gain time, so you Add to GMT).

    • W.L.S.: West - Lose - Subtract (When you go West, you Lose time, so you Subtract from GMT).

  • Examples:

    • London (0°5'W - roughly 0°): Let's say it's noon (12:00 PM) in London (GMT).

    • Madras (Chennai) (80°E):

      • Madras is East of Greenwich, so time is ahead.

      • Longitude difference ≈ 80° East.

      • Time difference ≈ 80° / 15° per hour ≈ 5 hours and a bit. (More accurately: 80° / 15° = 5.33 hours. 0.33 hours * 60 minutes/hour = 20 minutes).

      • So, Madras time is about 5 hours and 20 minutes ahead of London.

      • Local time in Madras ≈ 12:00 PM + 5 hours 20 minutes = 5:20 PM.

    • New York (74°W):

      • New York is West of Greenwich, so time is behind.

      • Longitude difference ≈ 74° West.

      • Time difference ≈ 74° / 15° per hour ≈ 4 hours and a bit. (More accurately: 74° / 15° = 4.93 hours. 0.93 hours * 60 minutes/hour = 56 minutes).

      • So, New York time is about 4 hours and 56 minutes behind London.

      • Local time in New York ≈ 12:00 PM - 4 hours 56 minutes = 7:04 AM.

  • Time Zones in Daily Life:

    • This is why when it's lunchtime in London, it's dinner time in India and breakfast time in New York! Different parts of the world are experiencing different times of day at the same moment because of Earth's rotation and longitude.

  • Finding Longitude using Time:

    • You can find your longitude if you know your local time and GMT.

    • Example: Ship captain.

      • GMT is 8:00 AM.

      • Local time on the ship is noon (12:00 PM).

      • Ship's time is 4 hours ahead of GMT (12:00 PM - 8:00 AM = 4 hours).

      • Since they are ahead of GMT, they are East of Greenwich (E.G.A.).

      • Longitude ≈ 4 hours * 15° per hour = 60° East.

In short: Longitude and time are linked because of Earth's spin. Traveling east adds time, west subtracts. GMT is the world time standard at 0° longitude (Greenwich), and you can calculate local time anywhere if you know its longitude and GMT.

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