Celestial Navigation in Practice

Fundamentals of one of the oldest seamen’s skills and arts.

Introduction

Celestial navigation, also known as astronavigation, is the ancient but still used practice of position fixing that uses “sights”, or angular measurements taken between a celestial body (e.g. the Sun, the Moon, a planet, or a star) and the visible horizon.

Before GPS became generally available in the early 1990s, astronavigation was the only method of position fixing available to yachtsmen on ocean passages. It was therefore a crucially important skill. Today satellite navigation is the primary means of position fixing for the vast majority of long-distance sailors. However, GPS is dependent on electric power and complex electronics, which cannot be repaired at sea. If your GPS fails for whatever reason (e.g. lightning), you can use astronavigation to navigate and is the only method of doing so that requires no electronics.

Sailing using just celestial objects is a really special experience and requires a completely different approach to navigation. When you are navigating out of sight of land you will not be able to pinpoint your position so accurately as with the GPS where you get an accuracy of 5 meters or so. On a small boat, you can expect an accuracy of 1 mile at most and only in favourable conditions. But the art of traditional navigation is not knowing where you are but where you could be. Your position is no longer a point, it is an area/circle. High accuracy is not required as long as there are no hazards in the vicinity - in the open ocean. It is far more important during landfall where you need to establish an actual position as soon as land comes into view (lighthouses and high land features).

What do you need

• Sextant to measure celestial body altitude above the horizon
• An accurate watch with a second’s precision. The best is a simple digital Casio watch with an LED light that you can buy for 18 Euros or so. Just make sure to check how many seconds it loses or gains every week or month.
• Nautical almanac and sight reduction tables
• Plotting sheets

Procedure

Getting position fix using celestial navigation is quite simple using tables, but there are a lot of detailed steps to be followed. We do not have a space for this here but there are a lot of publications and courses that can teach you this (see section below). From a very high-level perspective, these are the steps:

• Establish a Dead Reckoning (DR) position
• Take the sextant sight(s) and read the time
• Use nautical almanac and sight reduction tables to reduce the sight
• Draw position line(s) on the plotting sheets and read off your observed position (where the position circles cross)

Learning

Celestial navigation can be a very intimidating subject. It is indeed a complex topic but only if you want to understand every detail of it. Luckily enough, it is fairly easy if you just want to use it. You will have to master the use of the sextant and perform simple calculations with the help of a nautical almanac and sight reduction tables.

Books

Celestial Navigation - With the Sight Reduction Tables from Pub. No 249 - Dominique Prinet - very clear explanation on how celestial navigation works and how to use it without bogging down in theory

Courses

RYA Yachtmaster Ocean theory course - will teach you how to use a sextant, how to do the calculation using the tables, introduce you to worldwide meteorology, passage planning and communications for bluewater sailors

Buying a sextant

If you want to get good sights, you need a decent sextant. A very good article on that subject can be found on Celestaire.

My choice is to use metal sextant with a whole horizon mirror (e.g. Astra IIIB Deluxe Sextant). The whole horizon mirror is easier to use than the split mirror. The metal frame will make sure that you do not have to correct the sextant for errors very often.

I would recommend buying a new sextant and only consider the old one if someone very experienced in this subject can verify it for you. These devices are quite delicate and precise. If they have any flaws, you will not be able to get good sights.

Using a sextant

How to use a sextant (PDF).

This PDF includes the following:

• Introduction to Celestial Navigation
• General information on how sextant works
• How to use to take sights of sun, moon, planets and stars
• Sextant errors and corrections
• How to navigate without a GPS and Sextant, just with the use of a stick
• How to draw your own plotting sheets if you run out of them on the sea

Video on how a sextant works and how to use it

Required publications

Below you can find a list of publications required for the calculations:

Online publications

The Nautical Almanac and Sight Reduction tables are available online for free in PDF format: https://thenauticalalmanac.com/

The main advantage of using these tables, apart from the price, is that you can print the relevant pages for your trip without carrying all the books.

To have a complete set for navigation you need to download:

*what does not change
**what you need to update every year
***what you need to update every 5 years

Practice, Practice, Practice …

Like with every new skill the more you practice, the better you will be. For taking sights you do not necessarily have to be at sea away from land. You can practice at home by taking sights at arbitrary celestial objects, and if you can create artificial horizon, you can also establish your position. You can practice on a beach or anchor whenever you can see the horizon, and finally when you are sailing at sea. Taking sights when the boat is moving even in rough conditions is easier than you might think.

With lots of practice and patience, you should be getting sights accurate to a mile or so.

Fair Winds and Following Seas!

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