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No GPS? Who’s the guide ?

Did you know that the Ocean Globe Race in which we are going to compete doesn’t allow all types of geolocation system?

Indeed, we are not allowed to take a GPS with us to help us navigate. So, how are we going to find the way?

To answer that question, a lot of people would most likely say that a quick look at the sun or at the stars would help us navigate more or less. When you know where the sun rises, where it sets, and what way it takes in between, you’d have an idea of the direction you need to take. Nevertheless, it is quite obvious that this only approximation isn’t enough to manage sailing around the world! Indeed, to know where you need to go, you first have to make sure you know where you are!

Celestial navigation

It is obvious now that in order to prevent our captain to navigate straight until the North Pole, knowing our position in the first place is essential! Not allowed a GPS, we will rely on a much older-fashioned way to take us back where we started.

Watching the stars or the sun, is a much more accurate way to navigate than most people believe when you are prepared to it. Only, you have to watch closely. Moreover, it hasn’t been invented recently to respond to an emergency. It has indeed been used for a very long time in the sailing world before us.

As far as the history of humanity goes, it seems that we have always been keen on trying to answer the question of our origin, and by extension our location in the world.

Before humanity knew what a star was really made of, men already used them to find their way, know their position, deduce the time or even, calculate the earth’s circumference. And before going further in explaining the techniques that we are going to use to navigate, we thought that we ought to present you the astrolabe, an ingenious antique tool that takes a major place in the history of humanity’s geolocation.

The astrolabe, what is it ?

Figure 1 : Stéréographique Astrolabe . J.A. Linden, Heilbronn (Allemagne) © Royal Art and History Museum

An astrolabe is an elaborate inclinometer, and can be considered an analog calculator capable of working out several different kinds of problems in astronomy. The celestial archway is projected on the front side of the astrolabe in a similar way the globe is projected on a classic map. Sure the projection isn’t the result of the same calculations but still, the principle remains. It’s been invented by Hipparchus (greek astronomer c. 190 – c. 120 BC) and has been improved since according to its different purposes. The most common is the planispheric astrolabe.

Figure 2 :sides of an astrolabe

The one we are the most interested in among the different types of astrolabe is the nautical. It is a simpler version of the planispheric astrolabe and mainly allows the measurement of the celestial bodies. A simple calculation with a specific measure would give you instantly the latitude from which you made your observation. It has been developed by the Portuguese around the end of the fifteenth century.

Figure 3 : Nautical astrolabe. Considered to be one of the oldest known today. It has been made around the year 1500 and is made of very heavy bronze. Despite of the boat’s movement, its weight allowed it to keep steady even in rough seas when hanged as a plumb line. This astrolabe is currently kept in Gran Canaria within the collections of the « house-museum of Christopher Columbus ».

You might have guessed it now, the nautical astrolabe is actually the ancestor of the sextant. The sextant is indeed the tool that we will need out in the ocean to know our position!

Figure 4 : The sextant

Do you know that in French, there is an expression saying: « faire le point » that means you assess a specific situation, look at what you have achieved so far, if it meets your expectations or so. Literally, we could translate it as “to make the point”. And yet, it has been several centuries that sailors and world travellers use this sentence to actually point the boat on a map! and thus know… where they are! People adopted the sentence along the years to finally use it for another meaning but hey, not very different from the first one if you think about it…

So now, when actually comes the time to choose between a GPS or a sextant, what do you reckon the best choice would be?

Sextant vs GPS

Let’s see how different they are.

The working principle?

Even if there is a slight difference, we talk about triangulation for both of the two technics. Triangulation is the process of determining the location of a point by forming triangles to it from known points.

The GPS actually receives information from the satellites about the distance that separates them and their position. Therefore, it needs at least three of them to acquire a position (x, y, z).

In the case of astronavigation, we will also need to measure the angles with three separates celestial bodies (or the same at different times) to calculate our position. The only difference is that these bodies don’t communicate with us to give out their own position☺. But know that these positions, and thanks to humanity’s curiosity regarding its environment, we know them at a specific time

Figure 5 : Illustration triangulation GPS Figure 6: Illustration triangulation stars

(A fourth satellite is actually needed to lower the great imprecision due to the variable t (time). The receiver needs to synchronise its time with the atomic time of the satellites. The knowledge of the time needs to be very accurate to be able to get a reasonable precision.)

What about the accuracy?

A difference of a few hundred meters to the advantage of the GPS. Now, the question is, how accurate do we need the measurement to be when in the middle of the ocean?

And reliability?

Well, we know one thing for sure, at our life scale, a celestial body will never let us down!

Did you know ?

It is possible to create your own sextant with an A4 size paper?

That in the tenth century, a famous persian scientist had calculated the earth circumference with an astrolabe?



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