The Challenge: Measure Latitude and Longitude Page: 1 2 3 4 5 6 7 8 9 10

How to Measure Longitude

1. Find a radio station that broadcasts the 'GMT (Greenwich Mean Time) pips' on the hour, giving the time at the Prime Meridian.

2. Construct a device to work out local noon, like a north-south line.

3. Time the difference between local noon, measured from the sun, and the 12 o'clock noon pips on the radio.

4. If you reach local noon before the radio signal, you're east of the Prime Meridian (0° longitude). If the radio signals noon while the sun is still climbing, you're on a western longitude.

5. For every four minutes that you time with your pendulum between the GMT radio signal noon and local noon, your longitude will increase by one degree, because that's how much the Earth has rotated in four minutes. For example, local noon on the Mall in Washington D.C. would occur about five hours and eight minutes before the noon GMT 'pips', making the longitude about 77° west.

6. If it's not practical to use GMT, then the local time zone can be used instead, as long as 15° (the amount the Earth turns in one hour) east or west are added for every 60 minutes that the time zone is ahead of, or behind, GMT, respectively.