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 sun sets earlier in Assam than in Gujarat because Assam is located to the east of Gujarat. The Earth rotates from west to east, so eastern places experience sunrise and sunset earlier than western places.
With a 30° longitude difference, the time difference is 30° ÷ 15° per hour = 2 hours.
Therefore, when it is dark in Assam, it is still daylight in Porbandar because Porbandar's local time is about 2 hours behind Assam's local time.
Explanation
The chapter explains that longitude determines local time. The Earth makes a full rotation of 360° in 24 hours, meaning 15° of longitude equals 1 hour of time difference. Places to the east experience sunrise and sunset earlier than places to the west. Tinsukia in Assam is approximately 30° east of Porbandar in Gujarat, creating a 2-hour time difference. When the sun sets in Assam, Gujarat still has about 2 more hours of daylight.
Solution Steps
Step 1: Identify the longitude difference: 30°
Step 2: Recall that 15° longitude = 1 hour time difference
Step 3: Calculate: 30° ÷ 15° per hour = 2 hours
Step 4: Eastern locations have earlier sunrise and sunset
Step 5: Assam (east) experiences sunset about 2 hours before Gujarat (west)