- 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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