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Navigation with the Sextant

Blessed is the navigator who knows where he is!


So if today the vast majority of us trust our GPS point, that little red arrow that shines on our favorite chartplotter or plotter, many would not wouldn't know how to be without it! A faulty connection, an update that crashes the tracker or any kind of breakage can quickly happen. When coastal navigation, taking out your paper map and taking bearings to find your way can be relatively simple, but on the high seas it's a whole different story!


This is where sextant navigation becomes important. Accustomed to the facilities brought to us by technological revolutions, we sometimes forget that GPS was only available to the public in the 2000s!


Astronomical navigation aboard Venus

Share the mystery of astronomical navigation on board


We took the test during our crossing of the Pacific Ocean and we were surprised by the results, so back to the sailing not so long ago , astronomical navigation!

Astronomical navigation with a sextant

The observation of the stars as a means of navigation is, contrary to what we might think, an effective and fairly precise science for initiates. Furthermore, it was not invented recently to compensate for possible technical failures but was the essential companion of many sailors in the history of humanity before the advent of current technologies.

Indeed, since humanity was old enough to leave traces allowing it to retrace its history, it seems that it has always asked itself the question of its position and by extension, its place in the universe.

Well before knowing the real nature of the stars and observable planets, man was already using them to navigate, position himself, know the time or even, to determine the circumference of the terrestrial globe.


The sextant

Invented in 1731, the sextant is an improved form of the astrolabe, the instrument formerly used by the Byzantines and Greeks, but that's another story.

The principle of the sextant is simple: it allows you to obtain the angular height of a star located above the horizon.

It is in this angle that the secret to finding your position lies. The body that is mainly used is the Sun although the moon and the stars can also be used.

There are two main methods for calculating your position using the sun: the meridian and the height line. Each method has its advantages and disadvantages.


The chaise longue

The main advantage of this method is its simplicity. Only three elements are necessary to find your position using the meridian, a sextant, a watch and the nautical ephemeris for the current year. Ephemeris are tables intended to accurately predict the position of the stars and give, among other things, the exact time of the zenith in Greenwich.


The main disadvantage of this method is that it must be carried out at a certain time of the day: at the zenith of the sun at our position, which is generally around noon local time. Why is this a disadvantage? Because if the sky is overcast or if a cloud passes in front of the sun at that time then it's a failure, no position for today! Which is still annoying!


In short, here is where our contact details come from using this method:

Longitude is the time difference between the zenith at Greenwich (which is taken from the ephemeris) and the zenith measured at our position using the sextant.

The latitude is given by the value of the angular height of the sun at the zenith measured with the sextant and corrected using the ephemeris.

And that's it, here you are with your position!



The height line

The main advantage of the height line method is that, unlike the meridian, you can take measurements at almost any time of the day! On a cloudy day, as soon as the sun shows its face you can be on the lookout to take its angular height and that's it!

On the other hand here the ephemeris and a watch will not be sufficient, you must also have other navigation tables like the HO 249, take several measurements throughout the day, know our average heading and the distance traveled between each of these measurements.

If the measurement itself with the sextant is often faster in this method than in the previous one, the time spent at the chart table doing calculations is however significantly more long.

But better than not knowing where you are!


Precision in astronomical navigation

It's surprising how precise astronomical navigation can be.

Completely new to the subject, we tested it several times during our various crossings of the Pacific and Atlantic Oceans and we were surprised by the precision of our positions compared to those given by our GPS. In fact, the majority of easily avoidable errors come from calculation errors because with so many tables and figures it is easy to get lost. The other source of error comes from measuring the angle of the sun with the horizon at the given time! Add waves and the rolling of the boat and that’s enough to add a few miles of error!


The errors of our astronomical position in relation to the GPS were approximately 20 miles during our first measurements and ended up with less than 3 miles of error in our last ones. And 3 nautical miles in an ocean isn't much and the occasions where you need such precision will be very rare, because at that distance you should already be able to see land!


So with that, to your sextant and see you soon!!

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