How to measure Longitude
It’s easy to work out your longitude using a radio, watch and sundial, but as castaways on Capraia we had none of these. Even though we built a radio, we still needed some sort of timepiece, so we made a pendulum of a certain, known length which we knew would swing with a fixed frequency (determined by this length).
The Earth rotates one full turn (360º of longitude) in one day. It therefore turns one degree of longitude in 1/360th of a day, or every four minutes. To calculate your longitude, you therefore simply need to work out the time difference between noon at your location and noon at the Prime Meridian.
What to do
Find a radio station that broadcasts the ‘GMT (Greenwich Meantime) pips’ on the hour, giving the time at the Prime Meridian.
Construct a device to work out local noon, like a north–south line.
Time the difference between local noon, measured from the sun, and the 12 o’clock noon pips on the radio.
If you reach local noon before the radio signal, you’re east of the Prime Meridian (0º longitude). If the radio signals noon while the sun is still climbing, you’re on a western longitude.
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. So, in parts of Cornwall for example, local noon would be 20 minutes after the ‘pips’, so the longitude would be 5º west.
For details of the calculation that we used to determine the longitude of our castaway island location, see ‘Difficulty with maths? No problem!’.
If you’re further away from the Prime Meridian, then the local time zone can be used, as long as 15º (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.