Since 1757, long before the advent of satellite navigation in the 90’s – with Global Positioning Systems able to calculate a location to the centimeter – sailors, aided only by a sextant and a chronometer, could, and still can, positioned themselves by latitude and longitude even when land was out of sight. It was a huge leap forward for navigation, allowing sailors to plot their position with relative accuracy, with a margin of error of about only 1 nautical mile.
The sextant is used to measure the angle between a celestial body – the Sun, stars or visible planets – and the horizon at a specific time (universal time), allowing the navigator to understand their position on the terrestrial sphere.
The officer responsible for navigation used to do three measurements throughout the day:
– In the morning at dawn (when the stars and the horizon are visible);
– At noon with the sun (at a point called “the meridian”). At this time the sun is high, and the latitude is given by the sextant and the longitude given by the chronometer;
– In the evening at dusk (when the stars and the horizon are visible).
The sextant has other useful functions. It can determine the distance between two daymarks* or one’s distance vis-à-vis daymarks if one knows his/her height on the water.
This tool has also been used to find navigate across deserts.
*A daymark is a fixed and identifiable point used for maritime navigation.