A sextant is a device used to measure the vertical angular distance of the sun or other astronomical object from the horizon. It can also be used to measure angular distance between any two objects.
The sextant consists of a small tube and a couple of mirrors, one sighting the horizon, the other a celestial object, such as the sun. As one peers through the tube, one half of the view is of the horizon, the other is adjusted to bring the image of the object to match the horizon. Reading the scale determines the angle between the object and the horizon. Referring to a nautical ephemeris, one can determine the declination, or angle above the horizon of the object at the equator on a given date, can mathematically determine the latitude position of the ship.
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