The Sextant

The lucky ones who can handle and see this timeless nautical object are now few.

icaptain, the sextant What is and how it works., ship point, stars
Brass Sextant

First of all, because even those on board no longer use it, unless imposed by the shipping company to maintain the high standard of training of its crews (such as MSC), or as training of the students desired by the captain or the officers on board in case of long and uncomplicated sailings.

Already in other articles we have explained that today, the advent of Gps has completely revolutionized the going to sea and in particular the control of the position of the ship, but in our view, what makes the difference between a True Commander and a improvised one, is precisely the being able to lead a ship to its destination even in absence of on-board electronics.

Computers are not foolproof and being able to calculate the position of the ship by observing the stars in cases of prolonged blackouts or GPS errors or similar when you are 1500 miles away from the nearest port could make the difference between life and m Orte.

Let us remember that GPS as well as the Russian GLONAS of the same name is a system managed by the Department of Defense and the current satellites are still able to deprecate the signal for civilian use for reasons of national security as happened during the events of September 11, 2001.

That is why we advise all present and future sailors not to neglect this fascinating and still fundamental discipline in their professional training.

So let’s go back to the Sextant, that is, the instrument without which it is not possible to calculate the ship point by observing the stars.

This tool is essential to measure the elevation of a star over the horizon. From this measure it is possible, through a series of calculations and the use of the Nautical Ephemera, to draw a Place of Position (place of the points on which the ship can be found) known as the Straight of Height.

As in coastal navigation, the use of multiple locations allows you to calculate the ship point.

The name of the instrument is associated with the width of the graduated scale that is on the same equal to 1/6 of Lap Angle i.e. 60 degrees.

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Main parts of a marine sextant

Reduced to its essential elements, the instrument consists of:

  • A metal frame with a width of at least 60 degrees (45 degrees in the first sixes, also referred to as octants i.e. 1/8 of a turn angle, 80 degrees in the most modern ones)
  • Of a revolving alidada whose position allows the reading of the height of Astro observed on the Horizon.
  • A nonio or micrometer needed to make measurements with a tenth of the precision first arc.
  • A telescope to focus the star to be observed.
  • A Mirror and a Mirror/Glass.
  • A series of filters to dim the light of some stars especially Sun and Moon.
  • A menagerie system so as not to accidentally move the alidada.

The flap of the frame is graded twice as much as the angular sector of the hooping: a sextant with a frame with a width of 60 degrees will have a graduated flap from -5 to about 120 degrees; this subdivision starts with negative values for errors that the tool can present with the passing of time and which will be the subject of the next article.

This breakdown is twice the size of the frame due to the optical principle of the same: if a ray of light undergoes two reflections in the same plane, the angle between its first reflection and its last direction is equal to twice the sharp angle formed by the reflective surfaces,it seems complex but let’s trust, it works…

The reflections are made using the mirror and the mirror/glass present on the frame.

Without going further we try to understand practically how it should be used.

You place the instrument at 0 degrees (we’ll see in another article how to calculate the possible index error) you aim with the telescope the desired star, you press on the menagerie system to keep the alidada free, at this point you slowly rotate the frame holding stops the alidada. In this way the image we were looking at splits: one remains in the real position, the other begins to move; when we arrive with the reflected image to lap the horizon we stop, make some possible adjustment with the ninth and read the value reported by the instrument: in this way we will have obtained the height of the object on the horizon.

In the next article, we’ll see how to properly correct this height called Instrumental Height (hi) to get the true height of the Observed Astro.