Longitude and local time

Longitude and time. The earth rotates west to east, so the sun rises in the east and sets in the west. One rotation of 360° takes 24 hours.

• So the sun appears to cross 15° of longitude every hour, or 1° every 4 minutes.
• Places east of Greenwich see the sun earlier, so their local time is ahead; places west of Greenwich are behind.
• Local time of a place = Greenwich time plus 4 minutes per degree east, or minus 4 minutes per degree west.

The book's examples (Greenwich at 12 noon):

• Thimphu (Bhutan), 90° E: 90 × 4 = 360 minutes = 6 hours ahead, so 6 p.m.
• New Orleans (United States), 90° W: 6 hours behind, so 6 a.m.
• One line in the book says time 'decreases' as we move west to east; that has the direction reversed. The worked examples show the correct rule: east is ahead, west is behind.

Example (new): when it is 9 a.m. at Greenwich, what is the local time at 30° W? 30 × 4 = 120 minutes = 2 hours. West is behind, so it is 7 a.m.