The Challenge: Measure Latitude and Longitude
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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 (360° 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.
How to measure longitude