How can we know the position at sea? Quite simple.

All we need is the sun, the Northern Star, time in Greenwich, a sextant and some mathematical tables.

LATITUDE: The extended axis of the earth shows to the North Star. That means, standing at the north pole, the Northern Star is exactly above the head, 90 degrees from the horizon, that means 90 degrees northern latitude. Moving from the north pole southward, the North Star comes lower. At the equator the North Star is in the horizon, 0 degree, 0 degree latitude. The latitude of Glasgow is appr. 56 degrees north latitude. The Northern Star is in 56 degrees over the horizon.

The sun, being over the equator, can be used as well to find latitude without needing exact time. At the north pole the sun is in the horizon, at the equator in 90 degrees, in zenith. For Glasgow it means the sun reaches midday highest position in sky, that is 34 degrees, 90 minus 34 equals 56, 56 degrees north latitude.

LONGITUDE: At 12.00 hrs Greenwich time the sun goes through the Greenwich meridian, is exactly in the south and reaches highest position in the sky, 0 degree longitude. The same thing happens one hour later on 15 degrees west longitude. All we need is to always know what the time is in Greenwich. When at 11.00 A.M., Greenwich time, the sun reaches highest point in sky, one is on 15 degrees east longitude. It takes the sun one hour to move 15 degrees from east to west, 360 degrees in 24 hours.

To find longitude, exact Greenwich time is needed. Before John Harris developed the first usable chronometer, longitude was just guesswork.

LATITUDE: Measuring height in degrees over the horizon, no exact time necessary. Longitude: Exact Greenwich time essential.