#167 | ast09 – Using a sextant

Hi guys,

So yeah don’t forget that this blog is a personal one. We are trying to learn something new everyday, but it also means we can learn and make mistakes.

Today will be a short post on how to use a sextant. I was lucky enough to find a sextant during one of my travels so I have already looked for it a few months ago, but never really practice it or even understand it fully. Today is a good time to talk about it thanks to my last post about latitude and longitude coordinates.

The sextant is a nautical and astronomical tool that is still used today for the manual and stable calculation purpose without a computer. Even in the international space station, the sextant is a must-to-have tool to locate some star and locate your position.

Before the sextant, its first form was the octant, a eighth of a circle (45°) measurement tool that could calculate a elevation angle of an object at about 90° from the horizon. The sextant like the name said it’s a sixth of a circle, and can take an elevation angle of 120°. The interesting point to use the sextant compared to the astrolabe is that you can use the sextant without the need to hang the tool, and you can use it on a boat in motion.

How to use it?

The sextant is used to calculate any objects in the sky, but if you want to know your latitude coordinate for example in midday with no stars and only the sun, the sextant can help you for this purpose: by knowing the noon site, the highest altitude of the sun in the sky and with a Nautica Almanac, you can precisely know where you are in latitude.

First thing you need to know is how to use the sextant to calculate the angle between the sun and the horizon. The sextant is built by two mirrors, and an arc of circle that will show you the degrees angle. A telescope will help you to look to the mirrors.

You need to look through your sextant to the horizon, but first you need to check if your sextant is well adjusted to 0°. When it is done, you will see that the mirror you are looking at with your telescope is in fact a half-mirror. The second part is transparent and let you see the horizon. Some filters are also on the sextant and can be used to protect your eye from long exposure to the sunlight.

Description of a sextant – Wikimédia commons

When you finally see the horizon, the half-mirror part reflect the second full mirror view. This index mirror shows the sky and you can adjust it with the clamp at the arc of circle. The deal is to move the visual you have of the lower base of the sun to the horizon by moving the index bar. When the sun is at the horizon, you can then look now adjust the angle with the micrometer drum. The final step before looking at the degrees is to swing your sextant from left to right, always looking at the sun. This way, you will be able to see if the sun is right above the horizon. The degrees will tell you the altitude position of the sun. But it is not finished yet. This is not your latitude.

Wherever you are, the zenith is the point just above your head at 90° from the horizon. When you take your observed sun altitude and subtract it to the zenith, you can defined the latitude. However, it is working only in equinox at the Equator: sun position will be different depending on hour, day and year. So, you need to adjust your zenith distance calculation with this declination due to the sun position depending on the time. It can be done with the Nautica Almanac and by knowing in which hemisphere you are. You can obtain the tricky declination of the sun depending of the day off the year through this list. Then, you can add it or subtract it depending on your Zenith distance :

1) the sun geographic position is not at the same hemisphere than you, it means you need to subtract the declination to get your latitude (the declination is the excess angle to the Equator: zenith distance = latitude + declination).

2) your position is in the same hemisphere as the sun, and the sun geographic position is lower than yours. It means you need to add the declination to the zenith distance.

3) finally, the sun geographic position is above yours, the declination needs to be substract by the zenith distance.

These three cases lead you to obtain your latitude in any moment of the year without the Polaris star.

Thanks for the reading and see you later!

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