Author Topic: CelNav Nine--Sextant Technique  (Read 903 times)

Offline HenryC

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CelNav Nine--Sextant Technique
« on: October 18, 2010, 10:06:23 AM »
Now that you've had the opportunity to digest the celestial navigation articles and decide whether or not you really want to get into it, I've saved the best for last.  Just how do you use a sextant, anyway?  What do you see when you look through it? Each line of position begins with an observation, or a "shot".  You actually look through the sextant at a star or other heavenly body and measure its altitude above the horizon.  The previous chapters in the CelNav series tell you what to do with this information.  This one will tell you how to acquire it.

Take the sextant out of its box and grab it by the handle with your right hand.  The side of the sextant where the handle is the right side.  All the controls and moving parts are on the other side, the left side.  Hold the sextant so you can see the left side and note the index arm, which pivots at the top.  At its other end is the micrometer knob and the quick release.  Depress the release and move the index arm back and forth so that the pointer points to the numbers engraved on the sextant arc, or limb.  Now move the arm so the pointer is roughly 0.  The micrometer knob can now be twisted so the the micrometer knob also reads about 0.  The sextant is now on its zero point, twisting the knob allows you to move the pointer back and forth along the limb slowly for fine adjustments; the quick release allows you to move the index arm quickly along the limb.

Bring the sextant up to your eye and look through the telescope at the horizon.  Since the index is set to zero (more or less), you should see the horizon through the scope as a line, but if you look closely, you will see the line is really two parallel lines, there are two horizons!  This is because you are looking at two images, the one of the horizon through the horizon glass (the half-silvered glass at the front of the sextant) and another reflected from the index mirror mounted on the index arm.  If the sextant is set exactly to 0, and the sextant is adjusted perfectly for index error, these two images merge; but generally, the two images are not exactly lined up.  However, by twisting the knob and matching up the two horizon images perfectly you can set the sextant to exactly 0 degrees.  Now look at the limb and the micrometer to get your reading.  It should be zero, but it is usually off a little bit, the so-called "index error". 

In the instruction book that came with the instrument it will show you the little set-screw that is used to remove this error, but in practice, it is usually very difficult to eliminate the error entirely, you tend to over-do the correction every time you even breathe on the screw, it is very sensitive! So just get it as close as you can and just write down the remaining  index correction to be added or subtracted to each reading.  If you can get it to within a minute or two of arc (one minute = 1/60th of a degree) that is more than good enough.  Incidentally, even though the micrometer is usually marked to tenths of a minute, it is very difficult to read a sextant to better than half a minute of arc, so it is better to check your index error several times, then average the results.  It is also a good idea to check the error before and after each sight, just in case the sextant is bumped, or the frame warps slightly due to thermal expansion.  Remember, no matter how carefully you handle your instrument, it is used outdoors on a violently moving platform, so it takes rough usage!  The more expensive instruments are more resistant to loss of adjustment. There are several other mechanical adjustments I won't go into here, so read the manual that came with the instrument.

Your first practice shot of a real celestial object should be of the moon or a star.  The sun is easier, but it involves  flipping the shades to protect your eyes, and this should be done only after you have gained some familiarity with the instrument.  Remember, you can go blind by looking at the sun through a telescope without eye protection even for a moment.  Your sextant is equipped with multiple filters for every conceivable illumination condition, but flipping them around takes some practice--and patience.  You will note each mirror has filters, the index mirror set protects your eye from direct sunlight, the filters on the horizon glass are there to protect it from reflection off the water.

Taking a shot involves seeing the celestial body through the telescope and then "bringing down" its image to the image of the horizon.  When the two are just touching, note the exact time and read the sextant off the micrometer to get the angle.  Apply your index, dip and refraction corrections (and if applicable, your semidiameter and parallax corrections) to get the corrected reading. 

Needless to say, this "bringing down" part is not easy, the body and the index and horizon glasses must be perfectly lined up vertically, and that vertical line must be exactly perpendicular to the horizon, and the telescope must be aimed at the horizon.  Its like patting your head and rubbing your belly, while riding a bicycle. Doing this at home is easy, at the beach it takes some practice.  Doing it on a moving boat takes a bit more.  The process is further complicated by the fact you are looking at very tiny pieces of sky and horizon, all that is visible through the telescope.  Many navigators simply remove the telescope from the sextant (there is a quick-release for exactly this reason), and prefer to work with the naked eye.  As you learn the ropes, however, you will find the 'scope allows for more precise work on fainter stars.

Getting the star and the horizon simultaneously in the sextant telescope field is difficult, particularly for stars nearer the zenith; here's an old trick. Set the index arm to roughly 0°; now hold the sextant upside down in your left hand and sight directly at the star. Once you have it in the field, move the index arm and bring the horizon TO the star. Now that you have the index set to the approximate angle, turn the sextant around correctly and take your reading using the right hand to hold the instrument, and the left to tweak the micrometer knob. Second, to ensure that the sextant is perpendicular to the horizon the instant the sighting is marked (not easy on a rolling ship, or pitching small craft), it is recommended that the operator swing the sextant from side to side while turning the micrometer and bring the star to meet the horizon. What the operator will see is the star describing an arc that gets closer and closer to the horizon, just kissing it at one point, which will happen only when the sextant is perpendicular. Incidentally, this arc is extremely flat for a star near the zenith and difficult to judge, another good reason to avoid overhead shots whenever possible. Needless to say, this should all be thoroughly practiced well before you go to sea.

Good luck.
« Last Edit: January 10, 2017, 11:32:25 AM by HenryC »