A Moment of Science

Sextant Pie

Captain Cook had one. So did Admiral Nelson. What did both these sailors have? A sextant, and, once upon a time, there was a sextant on every ship. So, what is a sextant, and why did sailors need one?

A sextant is a hand-held gadget that looks like the outline of a large piece of pizza. It is essential for navigation since it helps determine degrees of latitude, telling sailors how far to the north or south of the equator they are located.

The sextant measures the angle between a celestial body, for example the North Star, and the horizon. This angle changes with every degree you move north or south. For example, if you are at the equator, the North Star is at the horizon, or at zero degrees latitude. If you are at the North Pole, the North Star is overhead at ninety degrees latitude.

How does a sextant actually work? Although it’s basically shaped like a piece of pizza, it is a complex piece of equipment, with a scope, two mirrors, a scale of degrees and a moveable arm. The gist of it is that you must look through the scope and adjust the moveable arm until you see the image of the horizon line up with the image of the celestial body. Once you have the arm in this position, you simply read the star’s degree of latitude from the scale.

Why call it a sextant? It is one sixth of a circle, and “sex” is the Latin word for the number six.

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