Q1.
Two friends, one sitting in Porbandar (Gujarat) and the other in Tinsukia (Assam), are speaking on the phone late afternoon. The latter remarks that the sun has set in Assam and it's now dark. The former is surprised and says, “But it's still full daylight here!” Explain why.
Answer
Because the two friends are far apart in longitude, and the Earth turns from west to east.
- Tinsukia is in the extreme east of India (about 95°E); Porbandar is in the extreme west (about 70°E).
- As the Earth spins eastward, the eastern places are carried into the sunlight first and out of it first. So the Sun both rises and sets earlier in Assam than in Gujarat.
- By the time the Sun has set in Tinsukia, Porbandar has still to turn through about 25 more degrees before its own sunset — roughly one and three-quarter hours of daylight left.
Earth turns 360° in 24 hours → 15° in 1 hour
The farther east a place is, the earlier its sunrise and its sunset.
The farther east a place is, the earlier its sunrise and its sunset.
What confused the friends: both their clocks showed the same time, because all of India follows one Indian Standard Time. Their clocks were equal, but their local times — the times told by their own Sun — were not. That is the whole difference between standard time and local time.
Did you know? This is a real and much-discussed problem. In Tinsukia the Sun rises before 4:30 am in June and sets soon after 4:15 pm in December, so a great deal of daylight is wasted while offices still work to IST. Assam has often asked for a separate “Chaibagan (tea-garden) time”, one hour ahead of IST.