How To Work A Sextant


I trust that most of you are somewhat familiar with the triangular shape of the sextant… (similiar in shape to its illustrated forrunner) The sextant is held by its handle in the right hand. At the base of the upper angle will be the index mirror…along with the base of the index arm…adjusting the index arm along the lower base of the triangle called the arc, with its scale of degree markings, is what it is all about in taking a sight.

Holding the sextant perpendicular to the horizon, the closest object to your view will be the telescope. Before looking through the telescope, make sure your index arm and its scale along the arc reads zero.

Now look through the telescope at the horizon. What you will see is a somewhat broken image… one half the image is reflected through a mirror, the other half is just transparent glass. On a good instrument they should align perfectly when the arc reads zero or you will have to adjust for index error. What I mean is this. Your right hand is holding the sextant perpendicular to horizon, the left hand is holding the index arm release screw. By squeezing the screw you adjust the arc angle…If you have a sextant handy try it. Notice as you are looking at one half the horizon through the window, as you move the arc handle screw, the mirrored portion of the horizon will be out of whack with the window; so, if it was out of whack at zero to begin with…you will need to adjust the arc to bring the horizon into congruency. For instance if it lines up at two degrees note the two-degree index error. This will subsequently be corrected off your sextant angle "HS".

With the index arc at zero, raise the sextant and sight the target, in this case the sun. Before you become blinded you will notice that at the upper point situated just in front of the index mirror there are filters. When you have sited the sun through the telescope, employing sufficient filtering, the sun should look like a small yellow disc. Obviously to site the sun you needed to look up and sight the sun raising the sextant as you would if you were sighting a duck through a gun sight flying through the air. When the sight is captured, slowly adjust the index arc releasing screw, bringing down the image toward the horizon, lowering the sextant slowly. You will notice that you cannot quite situate the lower limb on the horizon using the index arm screw. If you move your fingers outward you will find a fine tuner called a micrometer drum…. Slowly turn the drum till the sun is just touching the horizon on its lower limb. Then hit the button of your stopwatch. Congratulations you have just taken your sight.

Look where the arrow is pointing on the arc. Let's say it is between 22 and 23 degrees. Now look at the micrometer drum, this will indicate the minutes. To the right of the drum is another little scale called the vernier… this will indicate the seconds of your sighting. After a little practice it will become second nature to you, degrees, minutes and seconds of arc angle. Time adjusted through your stopwatch subtracted to adjust for the exact GMT the sight was taken to the second.

It may be worthwhile mentioning that I did have a 1933 British Husun that was offered to me from a retired ship's captain in the Norfolk shipping hall. I just got my third mates license, and came back from my first "night mates' 8 hr watch. He would not sell me the sextant, which amounted to no more than a gratuitous offering to its value, unless I purchased the stopwatch that went along with it. The point being it's of no value having a fine precision instrument unless you have the means of fixing the precise time of the fix. You don't sell them, you pass them on.

Tomas
Godspeed
Goodsailing

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