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How to Use a Sextant

Posted on February 10 2021

How to Use a Sextant

Photo by Matthias Jordan via Unsplash

Story and Graphics by Ali Harford


Imagine you’re an explorer in the 18th century: your ocean voyages are long and arduous, you’re fighting scurvy, singing sea shanties, and navigating using the stars with a brand-new tool called the sextant, perfected in 1759 by a man named John Bird.


The sextant is a useful tool because it can measure the altitude of a celestial object above the horizon, indicating the angle between that object and the horizon, in degrees. This angle can then be used to determine latitude.


If a navigator finds that the North Star or the Southern Cross always appears at the same angular altitude above the horizon night after night, they can be sure that they are sailing along a line of constant latitude. This is because the angle at which those particular objects sit above the horizon is to the measurer’s latitude. Sighting on other celestial bodies can also work, but the calculations to determine latitude from them are more complex.


So the captain of your ship passes you the sextant and tells you to find latitude. But how, exactly?


1. Point the sextant toward the horizon


2. Bring the object into the viewfinder by bringing the index bar forward

3. Line up the object with the horizon and note the degrees and minutes on the sextant

Success! Your ship is on the correct line of latitude and headed in the right direction. You’ll have to take new measurements frequently to make sure you stay on the correct line, but for now, you’re safe.

Read more about celestial navigation on the Geo-Joint blog.

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