The Marine Sextant — Principles, Use, and Buying Guide

If you're looking for sextants for sale, you may be familiar with this vital piece of marine equipment or be looking to learn more. This post discusses how sextants work, how to use them and potential problems with them. We also provide buying tips and discuss what to look for in a professional sextant. But first, we cover the basics.

1. What Is a Sextant?

A sextant is a device used to measure the angle between two objects. Usually, these two objects are the horizon and an astronomical body, such as the moon, sun, star or planetary body. Navigators then use these measurements at sea to calculate a vessel's latitude. This type of use is known as Vertical Sextant Angle, or VSA. It can also be used to measure the angle between two objects on the shore of known position, which, using trigonometry, can be used to calculate a vessel's position on a map. Using the sextant in this way is known as Horizontal Sextant Angle, or HSA.

The name of this instrument derives from the Latin "Sextus," meaning one-sixth of a circle as the angle of the instrument measures 60 degrees, or one-sixth of a circle.

Originally, an octant was used for this purpose, which covered only 45 degrees. However, the sextant was then invented in 1859 by John Bird, who intended for the larger angle to give accurate measurements of the moon's angle of inclination. After this invention, the octant largely disappeared from use.

2. How Does a Sextant Work?

The sextant is designed around two mirrors, one of which is half-silvered, allowing some light to pass through. When using the device, you look through this mirror at the horizon. The other mirror is attached to a movable arm and is aimed at a bright object, such as the moon or sun. When the arm is moved to the correct position, you will see the moon or sun superimposed on the horizon. You can then read the angle off the scale, which is accurate to within ten seconds of arc. Using this angle, information from almanacs and an accurate clock, your position on the map can then be calculated.

3. The Anatomy of a Sextant

The sextant is a remarkably simple piece of equipment and quite robust, which explains why it is still in use over two and a half centuries since its invention. Despite the advent of GPS systems, navigators still like to have one as electronics are known to fail. It is made up of a few basic parts:

  • The frame — this is the "sextant" part of the device, forming one-sixth of a circle
  • The horizontal mirror — the semi-silvered mirror you use to look at the horizon, which is attached to the frame
  • Telescope — you look through this to observe the horizon and the reflected image
  • The index mirror — the mirror on the movable arm that reflects the bright object you wish to measure
  • Horizon and index shades — these prevent unwanted light and glare from appearing in your view or damaging your eyes
  • Index arm — moves the index mirror
  • Graduated arc — shows the angle of the index arm in degrees
  • Vernier scale — shows minutes of arc
  • Micrometer drum — shows seconds of arc
  • Thimble — allows fine movement of the index arm on the micrometer drum
  • Clamp — enables you to lock the index arm in place

4. How To Use a Sextant

We covered the basics of this in an earlier section, where viewing the sun or moon superimposed on the horizon allows you to measure their angle of inclination. To achieve this, the sextant is held in your right hand while you adjust the index arm with your left after disengaging the clamp by rotating the micrometer drum. Moving the arm away from you increases the angle and moving it toward you decreases it. Turning the drum clockwise increases the angle while turning it anticlockwise decreases it. Once the object to be measured is superimposed perfectly on the horizon, you replace the clamp and read off the angle. Degrees are read on the graduated arc, minutes on the vernier scale and seconds on the micrometer drum.

Reading a Sextant On and Off the Arc

To the left of zero, your sextant will read from zero to 130 degrees, referred to as "on the arc." However, the measurements also extend a few degrees to the right of zero, referred to as "off the arc." When reading on the arc, the angle can be taken as read, but when reading off the arc, you need to deduct the angle from 60 degrees. So, for example, 58 degrees off the arc is read as 60 - 58 = 2 degrees.

5. Sextant Errors

Even with the simplest of devices, there are things that can go wrong, and the sextant is no exception. Fortunately, you can correct for errors on a sextant, which fall into two broad categories; adjustable errors that you can correct on the device and non-adjustable errors caused by manufacturing defects or mishandling. Non-adjustable errors will be picked up during the calibration of the instrument and will be shown on a calibration card supplied with it, enabling you to add or subtract figures from readings you take.

Adjustable Errors

  • Perpendicularity error. If the index glass is not perpendicular to the plane of the sextant, your readings will be inaccurate. You can check for this by clamping the index bar at the middle of its arc and holding the sextant horizontally. Then, with the arc away from you, look obliquely into the index mirror so you can see the graduated arc and its reflection. If the arc and its reflection appear continuous, there is no error of perpendicularity. If there is a step, there is an error, and you need to adjust the screw on the index mirror until the arc and its reflection appear continuous.
  • Side error. This occurs when the horizon mirror isn't perpendicular to the plane of the instrument. To detect this, you need to aim the instrument at the horizon and tilt your head to the right by 45 degrees. If side error is present, the horizon won't appear continuous in its reflection — there will be a step. To correct this, adjust the side error screw until the horizon and its reflection appear in line. Next, return your head to an upright position. If the side errors are eliminated, the horizon will remain in line with its reflection. If not, you may need to correct for index error. You may need to run through adjusting both several times before all errors are eliminated.
  • Index error. If the horizon mirror is not parallel to the index mirror, this results in index error. To check for this, set the index arm at 0 degrees and aim the instrument at the horizon. When you look through, the reflection and the image of the horizon should be a straight line. If they aren't, adjust the micrometer drum until they are and note the measurement it gives. This will give your index error. If the error is on the arc, subtract this number from your readings, and if it's off the arc, you should add it.

Non-Adjustable Errors

  • Collimation error. This is an error in the mounting of the telescope, where it is not parallel to the body of the instrument. We have included this as a non-adjustable error, as modern sextants don't have collimation screws that allow you to adjust the telescope's angle. When the telescope isn't parallel, the light's path will bend slightly as it hits the mirrors. This causes errors that increase as the angle of the instrument becomes greater. Due to accurate modern manufacturing methods, collimation error at low angles is almost non-existent.
  • Centering error. The pivot of the index arm may not be exactly at the geometric center of the instrument, causing inaccuracies that are greater at large angles. If, as a result of manufacturing, this will have been accounted for during calibration. However, it can occur through mishandling of the instrument.
  • Graduation error. This is where the arc or the vernier graduations have been placed inaccurately during manufacture. Again, it's an error that should be found during calibration, so you shouldn't see it on your new sextant.
  • Worm and rack errors. If the worm and rack in the micrometer hub become worn, this produces a looseness in the mechanism, leading to inaccuracies. You will become aware of this when there is excess play in the micrometer hub. Proper care and maintenance should prevent this.
  • Shade error. If the shades aren't perpendicular to the line of the instrument and parallel with each other, this results in small errors in where the image appears on the mirrors. Multiple shades result in multiple errors, although this should be kept to a minimum by the manufacturing process and will be found during calibration. However, if you don't care for your instrument properly, the mountings can become worn and introduce shade error.
  • Prismatic error. Ideally, the two surfaces of the mirrors should be perfectly parallel to each other. This is because light is refracted through the glass when it hits a mirror. If the surfaces are parallel, the refraction from passing into the mirror is canceled out when it bounces out again. But, if the surfaces aren't parallel, the angle at which the light comes out won't be the same as the angle at which it came in. Modern manufacturing techniques largely eliminate prismatic error, and if present it will be caught on calibration, where faulty mirrors will be replaced. Therefore, prismatic error can largely be ignored on a modern sextant.
  • Dip. Ideally, measurements off the horizon should be taken at sea level. Of course, this is not feasible due to the height of the deck and the person measuring it, so this introduces an error known as dip. Nautical tables and almanacs provide information to account for this.

6. Tips For Using a Sextant

To get accurate readings from your marine sextant, it's best to check for errors before every use, as described above. Once you've done that, here are a few more tips to get accurate readings:

  • To save time on each use, focus the telescope on the horizon the first time you use it and make a mark on the stem so you can easily refocus next time should it get moved.
  • Holding the sextant steady is vital; the best way to do this is to stand with your feet slightly apart and hold the instrument with both hands.
  • To keep motion to a minimum, position yourself as closely as possible to the centerline of your vessel.
  • Direct sunlight can damage your eyes, so ensure you use the correct shades when observing the sun.
  • Take measurements on stars and planets around twilight
  • Unless absolutely necessary, nighttime measurements are best avoided as moonlight can create a false impression of where the horizon is.

7. Sextant Care and Maintenance

Your sextant is a valuable piece of equipment that needs to be cared for properly to get accurate use and long life from it. Here are a few things you need to do:

  • When grasping your sextant, don't put too much pressure on the index bar.
  • Avoid touching the arc as this may smear it.
  • Ensure that the rack and worm are clean before use and coat in vaseline if your sextant is out of use for a while as this helps avoid corrosion.
  • When cleaning mirrors, lenses and shades, wipe them gently to avoid pushing them out of alignment. Use a clean cloth to gently wipe them after each use.
  • When not using your sextant, ensure it's properly stored in its box.
  • Always take care to avoid bumping your sextant as this could cause movement and raise some of the errors noted earlier in this post.
  • Keep the sextant away from direct sunlight and store it away from damp or extreme temperatures as this can cause parts to shift, putting the instrument out of calibration.

8. Tips on Buying a Sextant

Now that you have a basic working knowledge of sextants, we can outline some tips for buying them. Firstly, when looking for a marine sextant for sale, you might wish to consider whether you are going for a plastic or metal instrument. There are plenty of low-cost metal models on the market that offer excellent accuracy and ease of use. But if you have a low budget, plastic models are adequate for use on near-shore craft such as lifeboats.

Next, you need to ask yourself if you're choosing a used sextant or a new one. Older models often have smaller telescopes and mirrors, making them more difficult to use. Those looking for professional sextants usually opt for new ones as there is less chance of problems such as bent arcs, and new sextants come fully calibrated with a warranty. However, if you decide to go for a used sextant, you should always look for trusted, reputable sellers.

Mirror size is the next thing to look at. The larger the mirrors, the more accurate your sextant will be. Large mirrors also allow more movement of the sextant when you're using it —definitely an advantage on the open sea.

The weight of your sextant is another factor to consider. Sextants are made from different metals such as brass, aluminum or bronze, all suitable materials in a corrosive marine environment. Some users prefer the weight of the brass and bronze models as they don't move much in the wind, while others find lightweight aluminum models less tiring to use. Plastic sextants are very light but not easy to use in strong winds. If you're stuck between purchasing a lightweight sextant versus a heavier one, remember that you won't have to hold it for as long once you get more proficient in using it.

The next factor you should consider is your scope. If you're primarily reading from the sun or moon, low power magnification or even zero magnification should be fine and gives you a wide field of view, making it easier to take readings. If you're going to be using the stars, a 3.5 x 40 scope is ideal, giving a 3.5 magnification that makes it easy to find stars in calm or pitching seas.

Finally, we come to budgetary considerations. A sextant is a vital piece of equipment at sea, so it's best to get the best you can afford. Above all, speak to a trusted, reputable seller who can advise you on the best professional sextant for your needs. When considering where to buy a sextant, reputation and trust are critical. At American Nautical, we are here to help if you're looking for a marine sextant for sale and can advise you on the best models for your needs. So contact us today and talk to one of our expert representatives.