What is a sextant instrument?

A sextant is a special navigation instrument that measures the angular distance between the horizon and a celestial body (e.g., the sun, moon, etc.). Measuring the altitude of a celestial body is called “sighting”.

The term “sextant” is derived from the Latin word “sextant”, “sextant” – “sixth part”, from “sextus” – “sixth part”. The instrument’s arc is ⅙ or 60 degrees of a circle, hence the name.

Nowadays, people widely use electronic navigation systems. However, many ships are equipped with sextants. One of the undeniable advantages of this tool is that it does not require electricity or other types of energy. It is worth noting that celestial navigation is used by private sailors even today.

Of course, GPS technology cannot be replaced in most cases. However, a sextant can be an alternative navigation tool, and it is recommended to have a reliable backup.

The hexagram has been used for navigation for more than 200 years.

One of the first hexagrams created by John Baird in the 18th century, this tool is not light; in fact, it requires a stand.

Later, more improved models were made. Sextants are more accurate, better designed, and easier to use today.

Marine Sextant and How It Works?

The brass sextant is a beautiful instrument that comes with knobs, a slider, a vernier, a mirror, a magnifying glass, filters, lenses, and an eyepiece. It is a real historical tool that is still in use today.

Marine Sextant

Even today, all large ships must carry a working sextant, and navigators follow a routine procedure to become familiar with its operation. It is ultimately a gadget that does not require electricity or any type of power source to function, despite the presence of global positioning satellites, gyro compasses, and radio communications. So when everything else fails, the sextant still works. 

Another tool you need when using a sextant is a watch that can keep Greenwich Mean Time with very high accuracy. In addition, you also need tables of the heavens and a good understanding of how to use the tables or make calculations based on the principles. Oh, and yes, you also need to see the object, the sun, the moon, the stars, or a part of the earth. Not that you need to see all of these simultaneously, but seeing just one of them will help.

Using a clinometer at noon was the traditional way for ancient navigators to determine their position.

The different uses of angle measuring tools on boats, explained briefly.

If you know your local noon time, and you know the GMT time at noon, you can work out how far away you are from GMT.

For example, Classic Sailing’s office is located near 5° West longitude. It is noon, 20 minutes from your office in London.

Simply doing the math, there are 60 x 24 minutes daily, 1440 minutes, and 360 degrees of rotation around the Earth. So each degree is 1440 ÷ 360 = 4 minutes. Therefore, 5° West longitude is 5 x 4 = 20 minutes. This might explain why Cornwall is always a little late!

So how do you know when is your local noon time? This is when a sextant is used.

A sextant basically measures the angle between two objects. In this case, it is the angle between the horizon and the sun.

To ensure the sun filter is in place, you look through the sextant and adjust the mirror to superimpose the image of the sun and the horizon. (The filter prevents the sun from blinding you.)

If you watch closely, you will see the sunrise in the sky, and as you adjust the mirror’s angle, you will notice that it stops rising and starts to set again. Recording the time of the highest point gives you the time of noon in your local time. Also, considering the date and many other considerations, the sun angle will tell you how far south or north of the equator you are.

Moon and Stars

So this is the most common use of the sextant, but what if it’s cloudy and you can’t see the sun? Well, in this case, you may need to wait until the night sky clears up. You can see the stars and even the moon if you’re smart. Use a nautical calendar to find out where you are.

The sextant is a development of an old instrument called the Falstaff, which measured the sun’s angle based on the length of its shadow, but this was always a bit difficult at sea. In fact, the sextant wasn’t created to keep precise time, but that’s another story.

Rand Ahoy

So far, we have mentioned measuring the angle between the horizon and the sun, moon, and stars, but it can also be used on the side of the horizon. For example, you are near the earth and see two famous mountain peaks. If you measure the angle between them and use a compass to point to the peaks, you can use the compass direction and the sextant angle to plot your position on a chart.

Distance

With a vertical sextant, you can measure the angle formed between the horizon and the height of a known object, such as a lighthouse, a hill, or the top of a mountain. You also need to know the height of the tide, and then you can tell the distance to the observed object.

Chase me if you can

In the old days, when a ship was chasing or being chased, measuring the angle of the mast to the horizon could quickly tell you if the other ship was approaching or moving away, and by measuring the difference in a short time, you could tell how slow or how fast it was coming from you. It was especially useful when the other ship was far away and difficult to judge by the naked eye alone.

You’ve almost seen it if you’ve seen my favorite movie, Master and Commander – The Far Side of the World starring Russell Crowe, based on parts of Patrick O’Brien’s wonderful 21-book series.

The sextant has some advantages over GPS.

The Americans couldn’t stop him. (Europe’s Galileo GPS system wasn’t fully operational yet, but the British system was just a pipe dream!)

If you lose power but still have an accurate GMT clock, you can use the sextant to calculate your position and correct the paper chart to get your approximate position and the correct date.

Using a Sextant on a Yacht

Yachts are known to lose electronics and there’s no room for two completely independent generators on commercial or marine vessels.

Taking measurements on a yacht rocking in the waves requires a lot of skill.

Now you can try out the many uses of the ocean sextant!

Many classic sailing fleets are equipped with sextants. If you’re interested in putting theory into practice, the best journey is probably across the ocean, where there’s plenty of time to absorb the theory and put it into practice.