Sextant usage tips

Sextant usage tips: Using the sextant itself is not complicated, but it’s crucial to understand that the challenge lies in the associated calculations.

Sextant usage tips, image principe of the sextant

In this course, I do not explain the sextant in detail.

You can find many descriptions of sextants on the internet.

Perhaps the easiest part in using the sextant is in its practical application

How to use a sextant, image birds on the Ganges
Birds on the Ganges in Bénarès. author image: Ji-Elle

Sextant usage tips: perhaps the easiest part in using the sextant is in its practical application

Hence, for this reason, my website focuses on the theoretical aspect, specifically on calculating and completing the worksheet that comes after the sextant observation.

In my classes with approximately ten students, I needed one hour so that each student could lower the sun on the horizon and measure the height of the sun and the index error using the sextant.

Spending an hour by the sea and watching one or two YouTube videos on the use of the sextant, focusing on how to set the sun on the horizon and how to take the index error. You can also practice on a distant hill, mountain ridge, or the horizon of an open landscape


Thunderstorm. Author image: Tulumnes

Sextant usage tips, text to emphasize the importants of the calculation part

Sextant usage tips: practice with the sextant

First we start with the index error.

Sextant usage tips, image index error
index error

Then the height of the sun above the horizon.

Sextant usage tips, image sextant height (Hs)
sextant height (Hs)

The trickiest part of using a sextant is lining up the sun with the horizon.

Choose the filters before lowering the sun.

First, focus your eye on the sun and adjust the drum to move it up and down a bit.

Finally, lower the sextant with locking system (clamp) until it is perfectly level and you can see the horizon as well as the sun.

After roughly aligning the sun with the horizon using the sextant, a very slight rotation of the handle, without moving your arm, resulted in this movement where the sun touches the horizon.

If the clamp does not allow the sun to stop precisely on the horizon, slightly adjust the drum until the horizon and lower limb of the sun are perfectly aligned.


Ocean City. the spectacle of Hurricane Earl.  Author image: Mrs.Gemstone U.S.A.

Sextant usage tips:

Swinging the lower limb on the horizon

And now swing the lower limb of the sun over the horizon to make the sun tangent to the horizon.At the same time, use your other hand to gently turn the sextant’s drum to fine-tune the alignment of the lower edge of the sun with the horizon to achieve a precise tangent.

Sextant usage tips, image sextant direct and reflected image and horizon
Sextant usage tips, image balancing with a sextant
It is possible to use the upper limb, perhaps in a cloudy sky. I never used it.
Sextant usage tips, image lower limb and upper limb

In practice, we almost always take the lower part of the sun’s limb.


Sextant usage tips:

Once you have recorded the time, you should proceed to read and record the reading from the sextant.

“top” UT

Take the “top” UT. (if someone on board can do it for you, that is fine too).

We go to our chart table and note the:

1 sextant height (Hs )

2 the index error

3 top UT.

Then put the sextant back in its case !!

This is all simulated in the most important exercise of this website, ‘La Rochelle’