Two friends, one sitting in Porbandar (Gujarat) and the other in Tinsukia (Assam), are speaking on the phone late afternoon.

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. And, as a class activity, calculate the difference in local time between those two cities.

(Hint: for now, consider the difference in longitude between Porbandar and Tinsukia to be 30°; later, you can find out the precise value.)

The Earth rotates on its axis from west to east. That is why the Sun rises in the east and sets in the west. Tinsukia (Assam) is in the east, while Porbandar is in the west. When there is darkness in Tinsukia, there must be daylight in Porbandar. This is due to the difference in longitude between the two.

A full turn of the Earth is 360°.

A full day has 24 hours.

1 hour = 360° ÷ 24 = 15°

There is a difference of longitudes between the two places of 30°.

15° = 1 hour

30° = 2 hours

The difference in local time between the two cities is 2 hours.