PortCities London

Greenwich and the story of time

Measuring latitude and longitude

Finding your way at sea

Imagine you are in a ship at sea, miles out of sight of land. How do you find your position and navigate to safety? On land we learn to recognize and follow paths using known places as points of reference.

Of course, at sea there are no such landmarks, which makes marine navigation much more difficult. The only reference points for sailors are the Earth’s horizon, the Sun, Moon, planets and stars.

Finding latitude

At sea, latitude can be found by measuring the distance between the Sun or certain known stars and the horizon. This gives an estimate of distance north or south of the equator. However, both latitude and longitude are needed for accurate navigation. Finding longitude, or how far you are east or west of a known point, is a far more complicated process.

Finding longitude

Time and longitude
View full size imageTime equals longitude. © NMM

Scientists knew that part of the secret to finding longitude lay in the constant ro