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Longitude & Time

  • Earth: Divided into twenty-four time zones of one hour each
  • Each zone: Covers 15° of longitude
  • Best means to measure time: By observing the movement of the earth, the moon and the planets

􏰁  Regularly rises and sets every day

􏰁  Local time can be reckoned by the shadow cast by the sun, which is the shortest at noon and longest at sunrise and sunset.

􏰁  When the Prime Meridian of Greenwich has the sun at the highest point in the sky, all the places along this meridian will have mid-day or noon.

􏰁  As the earth rotates from west to east, those places east of Greenwich will be ahead of Greenwich Time and those to the west will be behind it

􏰁  How to calculate the rate of difference:
􏰂 The earth rotates 360° in about 24 hours, which means 15° an hour or 1° in four minutes. Thus, when it is 12 noon at Greenwich, the time at 15° east of Greenwich  will be 15 × 4 = 60 minutes, i.e., 1 hour ahead of Greenwich Time, which means 1 p.m. But at 15° west of Greenwich, the time will be behind Greenwich time by one hour, i.e., it will be 11.00 a.m.

􏰂  Similarly, at 180°, it will be midnight when it is 12 noon at Greenwich.

􏰂  At any place a watch can be adjusted to read 12 o’clock when the sun is at the highest point in the sky, i.e., when it is mid-day. The time shown by such a watch will give the local time for that place.

  • Tonga Islands (in the Pacific Ocean) and Mauritius Islands (in the Indian Ocean) are situated on the same latitude (i.e., 20° S).
  • As the earth rotates from west to east, those places east of Greenwich will be ahead of Greenwich time and those to the west will be behind it
  • There will be a difference of about 1 hour and 45 minutes in the local times of Dwarka in Gujarat and Dibrugarh in Assam.
  • Russia: Eleven Time Zones

see image carefully- focus on where is +/-

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