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How to use a Sextant ? And what are the differences between a Sextant and an Astrolabe ?

The sextant is the symbol of navigation instruments. Invented in the 1730s, it can be used to determine its position on Earth (latitude and longitude) using the sun or the stars.

It allows great sea voyages, explorations and adventures, like that of Captain Nemo, and will be used until the end of the 20th century.

How to use a Sextant ?

The Sextant is used to measure the height of a star (sun, or star), ie its angle to the horizon. The sextant is held at eye level.

Captain Nemo’s Sextant

1-Aim the horizon with the telescope through the fixed semi-transparent mirror (Index mirror).

2-Turn the Index Bar which rotates the main mirror (Horizon mirror), and aim at the sun (or the star) to bring its reflection back to the horizon, by double reflection.

3-Measure the angle on the arc.


The height measured with the sextant thus makes it possible to determine the latitude of the place where we are on Earth. Indeed, for example, the height of the sun in the sky depends on:

-The time of observation (different heights between morning and noon)

-The date (at the winter solstice, the sun is at its lowest. While at the summer solstice, it is at its highest)

Latitude (if we are at the equator, at the equinox, at 12 o’clock (local time), the Sun is at the zenith, height = 90°. But if we are at the North Pole at the same time, the height of the sun is 0°, the sun is on the horizon)


If we know the date and time, and measure the height of the sun (using the sextant), we can then find the latitude. Astronomical tables show the positions of the sun, and stars according to dates and times, and help us determine latitude.

The differences between a Sextant and an Astrolabe

About the astrolabe, see blog article, Senarius: How to use an Astrolabe ?


Captain NEMO’s Sextant

An illustration from 20 000 Leagues Under the Sea shows Captain Nemo using a sextant aboard the Nautilus. Senarius imagined Captain Nemo’s sextant, inspired by Jules Verne.




Université Lémans, Physique et simulations numériques

Planétarium Ventoux – Provence