Longitude and Time

LOCAL TIME. Since the earth makes one complete revolution of 360 degrees in one day or 24 hours, it passes through 15 degrees in one hour or 1 degrees in 4 minutes. The earth rotates from west to east, so every 15 degrees we go eastwards, local time is advanced 1 hour. Conversely, if we go westwards, local time is retarded by 1 hour. We may thus conclude that places east of Greenwich see the sun earlier and gain time, whereas places west of Greenwich see the sun later and lose time. If we know G.M.T., we merely have to add or subtract the difference in the number of hours from the given longitude. A simple memory aid for this will be East-Gain-Add (E.G.A.) and West-Lose-Subtract (W.L.S.). You could coin your own rhymes for the abbreaviations. The local time for Lagos (3 degrees east) will be 12 minutes ahead of London or 12.12 p.m. But the local time for New York (74 degrees west) will be 4 hours 56 minutes behind London or 7.04 a.m. We can put it in another way: when Londoners and Nigerians are having lunch, New Yorkers will have breakfast. This is difficult to believe, but it is true. The rotation of the earth round the sun means that at any point in time different places are experiencing a different time of day.
HOW TO CALCULATE LOCAL TIME
1. Work out the longitude difference.
2. Convert this to a time difference.
3. Adjust the time according to the direction of movement (east or west).
Example. What is the time in Calcutta (longitude 96 degrees east) when it is 9.00 a.m. in Munich (longitude 11 degrees east)?
1. Longitude difference = 85 degrees
2. Time difference 85/15 = 5 hours 40 minutes
3. Calcutta is east of Munich, therefore the time is ahead. Thus 9.00 a.m. plus 5 hours and 40 minutes = 2.40 p.m.
There are many ways of determining the longitude of a place. The simplest way is to compare the local time with G.M.T. by listening to B.B.C. radio. For example, the captain of a ship in the midst of the ocean wants to find out in which longitude his ship lies. If G.M.T. is 8.00 a.m. and it is noon in the local region, it means that he is four hours ahead of Greenwish, and must be east of Greenwich. His longitude is 4 x 15 degrees or 60 degrees east.