The Sextant – a Celestial Navigation Tool

What is a Sextant and How Do You Use It?

Before the invention of satellite GPS tracking, gyro compasses and marine radar, vessels had to rely on manual navigation tools to be able to identify their position in the ocean. One of these tools (which has been in use since the early 1700s) is the sextant.

The sextant is an instrument that uses reflecting mirrors to determine the distance between a celestial object (sun, moon, planet, star) and the horizon, in order to calculate the latitudinal position of the vessel.

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You may have heard of the sextant, or have even seen an example of one in a maritime museum somewhere. Despite their museum status, they are far from an obsolete tool! Being able to use a sextant is a valuable skill that anyone spending a lot of time on the water should learn.

What Was a Sextant Originally Used for and Is It Still Used Today?

The first version of the sextant was introduced in the 1730s by John Hadley and Thomas Godfrey, although the origins of the tool were later discovered in an unpublished work of Isaac Newton’s from the late 1600s. The sextant became a hugely important tool during explorative voyages of the 18th century as mariners were able to more accurately pinpoint their position and distance from their destination.

Anyone who spends time on the water knows that having good and reliable navigational tools are vital. The majority of recreational boaters can be confident that the GPS, VHF radio and radar equipment on their yacht, along with a basic understanding of marine charts and weather will be sufficient for any journey they make.

However, for crews of larger ships and tankers or sailors that make ocean crossings, using a sextant is still relevant as it is considered to be the ultimate fail-safe option. It doesn’t rely on any kind of power to work and can be used in the day or at night. This makes it a useful emergency backup navigation tool.

How Does a Sextant Work? Step by Step Guide

A sextant is made up of the following parts:

• A frame in the shape of 1/6th of a circle.
• A pair of mirrors – an index mirror and a horizon mirror.
• An arm which moves the mirrors – called the Index Arm.
• A sighting telescope.
• Sun shades.
• A micrometer gauge.

To take a reading — otherwise known as a sighting — from the sextant, take the following steps:

Prepare the Sextant for Navigation

1. Remove the sextant from its case ensuring you hold it correctly:

• Use your left hand to hold the frame and your right hand to hold the handle. The arc should be at the bottom with the eyepiece pointing towards you. Use your left hand to operate the shades and index arm.

2. When you look through a sextant, you see a reflection of the horizon that is split in half vertically.  For an accurate sighting, the two halves of the horizon should show as a straight line, therefore the first step is to make the following corrections to your instrument. 

• Perpendicularity Error – Occurs when the index mirror is not perpendicular to the plane of the instrument resulting in the arc looking broken or stepped.
• Side Error – Occurs when the horizon mirror is not perpendicular to the instrument resulting in the horizon looking stepped.
• Index Error – When the index mirror and horizon mirror are not parallel. When the instrument is set to 0o resulting in a horizon that isn’t in line.

Any errors can be corrected by making adjustments using screws and gauges on the instrument. For guidance on making these adjustments see this guide from Casual Navigation.

Take the Sighting

3. Set the micrometer scale to 0o then locate the celestial body you are using for your measuring point through the scope. If you are looking at the sun or another bright object, use the shades to protect your eyes. The body should be in the centre of your image.

4. Move the entire instrument until you see the horizon line in half of your image. At the same time, move the index arm using the clamp, so the celestial body remains in the centre.

5. Using the micrometer, adjust the image until the celestial body appears to be resting on the horizon.

6. To confirm you have the correct sight, rock the sextant from side to side, so the body moves in a curve across your view. Adjust again if necessary.

7. Read and note the measurement on scale arc (degrees) and micrometer drum (minutes).  Note the exact time of the reading in hours, minutes and seconds. 

Calculate your position

8. Once you have your reading there are further adjustments that are required. These are:

• Dip: An adjustment to allow for how high your eye line is above sea level.
• Refraction: Allows for distortion from bending of light rays.
• Semi-Diameter: When using the sun or moon, this adjustment makes the measurement as if from the centre of that body.
• Parallax: Adjustment for fluctuations in the celestial body’s distance from earth.

The information required for these adjustments can be found in the Nautical Almanac, an essential tool for sextant use.

9. Once all the adjustments have been applied, you have your true altitude. Subtract this number from 90 to find your latitude.

Full explanation of the process, adjustments and calculations required can be found on wikihow. As a process it requires a lot of practice and patience. Taking lessons from someone who already understands the process is a good starting point. Before you know it, you’ll feel perfectly comfortable using a sextant for navigation.

Get a Yacht with Modern Navigation Tools from Van Isle Marina

If using the sextant seems like it might be a step too far for you, no need to worry! Today’s modern yachts come loaded with all sorts of reliable navigational tools that are easy to use and understand. 

The Yacht Sales Team at Van Isle Marina will be glad to help you find a new boat with the latest satellite GPS and radar gear. 

Contact us at our state-of-the-art Sidney, BC marina today for more information.