With a structure and shape both marvellous and perplexing, the humble Sextant is a sight quickly spotted by anyone wishing to pursue the high seas. First introduced in its iconic, 60° form in 1759 by John Bird, it's gone from its support-necessitated size to a handheld model you can hold in a single hand. They've gone from having wooden and brass frames to high quality plastic, and even with the advent of GPS, they're still relied upon as electricity-free beacons of navigation. But why are they that essential? In this blog we'll be finding out what these unique instruments are for, and giving a basic outline on just how to use a Sextant.
What are Sextants?
Sextant's use a combination of simple math, observation and a particular science known as 'celestial navigation' (or astronavigation) for a variety of navigational purposes whilst at sea. A Sextant is a its most basic a precise measuring tool that features a telescope augmented with a variety of parts imperative for measurement; a micrometer drum, horizon & index mirrors and an index arm.
One can use a Sextant to measure the angle between two visible objects, for example, finding the angle between the Sun and the horizon. Very much like school trigonometry, this form of measurement - known as sighting, shooting the object or taking a sight - lets you use the acquired angle to calculate distance. With various other pieces of information, you can begin to narrow down your position to an exact point.
How Do You Use It?
- Finding the Altitude of the Sun: Using the index shades (adjustable, tinted glass squares on the Sextant that prevent damage from looking directly at the Sun), the index arm should be adjusted until the sun is visible both in the two mirrors and index shades. By physically moving the Sextant, the Sun's image should just touch the horizon line. The Sun's altitude can then be taken with the scales on the sextant. By using a table of Increments and Corrections in a Nautical Almanac, you can find the Degrees, Minutes and Tenths to determine the position of the Sun at your exact minute and second.
- Latitude: By measuring the angle between the horizon and the Sun when the Sun is at its highest point (around noon), the declination tables in your almanac will tell you which line of latitude the Sun should be above on that exact calendar date.
- Longitude: Once you have your latitude, working out the longitude is fairly straight forward. You need both an exact time (best done with a chronometer set to an accurate GMT before journeying) and the equation of time. To get the latter, at each day at sea, use the sextant to figure out exactly when the Sun is at its local apex ("local apparent noon"). Every hour the Earth spins 15 degrees, so for example, if the Sun is directly above at noon, the longitude is 0°. By calculating the time difference between GMT noon and your local noon, you can calculate your exact longitude with an accuracy of 10'.
- Calculating Location: We've already mentioned this a few times, but once one is familiar with ideas like Great and Small circles, the Nautical mile and of course latitude & longitude, one can work out their position with remarkable accuracy. When trying to work out your location using the Sun and observations of celestial bodies made at any other time than noon, than a knowledge of the navigation triangle, circles of equal altitude, assumed position is required, as are navigation tables like Nautical almanacs, and Sight Reduction Tables. Nearly all navigators around the world use these prevent errors and omissions when conducting celestial navigation.
Celestial Navigation and the correct use of the Sextant is a highly complex practice that requires a great deal of knowledge and understanding of how to use the associated tables with measuring the Earth and its neighbouring bodies (not just the Sun, but stars and planets too). It's understandable why GPS systems have been so quickly adopted, but maintaining the traditions of sailors like these, that have been practised for centuries, is still a highly rewarding process.
For more in depth, comprehensive guides to using a sextant, we recommend the following online guide:Davis Master Sextants User's Guide
If there's any important hints and tips with using a Sextant, especially ones that are often overlooked, let us know in the comments, or by sending them to us via our Facebook page, on Twitter or Google+.
