prize7-7, Exercise 1: Time and longitude

Model answer (check the details):

The Earth turns through 360 degrees in 24 hours, so it turns 15 degrees every hour (360 / 24 = 15), or 1 degree every 4 minutes.

Method: a navigator finds local solar time by noting when the sun is highest (local noon). A clock set to the time at the reference place, such as Greenwich, shows what time it is there at that moment.

Example: if local noon happens when the Greenwich clock reads 15:00, the ship is 3 hours behind Greenwich in time, because noon here comes 3 hours after noon there. Three hours is 45 degrees, so the ship is 45 degrees west of Greenwich.

Why this needs a good clock: an error of 4 minutes in the clock is an error of 1 degree in longitude, which is about 60 nautical miles at the equator. A clock that loses a few minutes over a long voyage gives a position that is wrong by many miles.