CelNav One - Introduction

[HenryC]

CelNav One edited 9/14/10

A sextant is an optical-mechanical device used to measure the angle between distant objects.  It operates with two mirrors arranged in such a way that one can be pointed at each object, and both mirror images fused together so they can be inspected simultaneously by the human eye.  In navigation, a sextant is generally used to bring together the image of some celestial body, such as star, moon, sun or planet, and an image of the horizon.  (There are other navigational applications too, which I will not get into here).  When the body and horizon image are superimposed, the angular distance between the two can then be read off a graduated micrometer and vernier scale. Most sextants use a telescope to magnify the combined images, although this is not necessary and some navigators even prefer to work without one.  Sextants also have have some sort of semi-opaque sun filters used to protect the eyes when observing the sun.  WARNING: Be very careful when observing the sun, especially through a telescope.  Unfiltered sunlight, off the orb itself or reflected off the water, can cause severe and permanent blindness.  Some modern sextants also have a battery and a source of illumination to read the angle off the scale in dim light.  I use a flashlight and bifocals.

Sextants also have adjusting screws and wrenches which can be used to tilt and parallelize the mirrors and calibrate the zero point on the scale.  Your sextant should come with an instruction booklet which explains how to adjust and calibrate your instrument, as well as tests and procedures to check that it is properly adjusted. All sextants are essentially identical in design, but there are minor differences in how these adjustments are carried out. I recommend highly "The Sextant Handbook", 2nd Edition, by Bruce Bauer, International Marine, Camden, Maine, 04843. Everything you will ever need to know about the use, care, adjustment, variations and history of this device is there--and it is fun to read!

For the time being, let us take a closer look at the basic features of the sextant.  You will note it has a handle, so that it can be gripped in the right hand. The handle is on the right side of the sextant.  The rest of the sextant consists of a triangular "frame", with a semicircular arc at the  bottom of it with numbers engraved on it (called the "limb").  There is also a moveable piece free to pivot about a point at the top of the frame opposite the limb  (the "index arm"), which is clearly meant to move to different points along the limb so you can measure different angles off the scale there.  The index arm has a quick-release latch and a micrometer drum at the limb (opposite the pivot end of the index arm) which allows you to make a precise measurement.  There is a mirror (the "index mirror") attached to the index arm so that it moves with it, and another fixed to the frame, (the "horizon glass").  You will note the horizon glass is half mirrored, half clear.  The idea is that light from the celestial body will hit the index mirror (which you can adjust by swinging the index arm back and forth), reflect to the horizon mirror, and then reflect off it to the telescope or your eye at the back end.  Meanwhile, the horizon is also visible simultaneously through the clear half of the horizon glass. This allows you to see both the celestial body AND the horizon through the telescope at the same time.  You can bring them into perfect contact by twisting the knob of the micrometer. 

At that moment when body and horizon are just touching, you note the exact time to the second, (it helps if you have an assistant/recorder working with you).  You then read the altitude in degrees, minutes and tenths off the limb, knob and vernier and write it all down.  This is an "observation", or a "shot". Keeping horizon and body in the narrow field of the telescope while twisting the micrometer knob on the rolling deck of a small craft is where the skill part comes in.  It's a lot like archery from horseback while riding at a gallop.

I can write a whole chapter on sextant technique, and how to develop it, and I may do so later on, but suffice it to say that only repetition makes perfect. I suggest you practice at the beach a lot before you even go near a boat.  Try to be completely at home and familiar with your instrument before you start making observations.  You'll have enough trouble getting into a rhythm of acquiring, setting up, and shooting a target while at the same time worrying about holding a bobbing image in the telescope field. You can also use your sextant at home, using a roof top or telephone line as a horizon.  It won't give you valid readings, but it will get you used to manipulating the instrument, acquiring and holding a sight picture.  It's a lot like marksmanship. At any rate, it's not as hard as it sounds, but you won't be happy with your first few tries, either.  Still, there are tricks and shortcuts, too.  We'll get to them later.

After you have shot your star, what have you got?  First, you'll have to apply corrections.  Your sextant may have a small amount of "Index" error which has to be added or subtracted to your reading.  This is due to the fact that your instrument may not read "zero" when it is reading a zero angle. When you point both the horizon and index mirrors at the horizon and fuse the horizons into a single line, the reading should be exactly  0 deg 0'.0.  It usually isn't.  There is an adjustment screw for this, but sometimes it is difficult to get the last bit of error out so just write down your residual index error and apply it to each reading as a correction. You can check it before and after each session, especially if the instrument is handled roughly. You also have to correct for distortion by the atmosphere, the "Refraction", which is theoretically zero for a star overhead but gets progressively worse nearer the horizon.  The refraction correction may also have to be corrected for extremes in temperature and barometric pressure.  If you're shooting the moon or the sun, you also have to make a correction for the "Semidiameter" of the disk, since the Almanac tables are for the center of the orb but you can only bring down to the horizon the top or bottom edge with the sextant.  You have to make a correction for "Dip", how high you are above the surface of the water (the distance to the horizon is further from the bridge of an aircraft carrier than from the deck of your sailboat). For the moon, you have to make a correction for parallax, since the moon is so close to the earth where you are on earth affects its position relative to the distant stars.  All these corrections are listed in tables in the Almanac.  They are either positive or negative minutes of arc. The sign of the correction is always what you add(+) or subtract(-) it from your reading. You add positive values to your reading, subtract negative ones, to get the corrected value.

We'll get more into all that later, but after you have made all your corrections to your Sextant Altitude (Hs), you will have your Observed Altitude (Ho). Later on, comparing your Ho to the Computed Altitude (Hc) (which is calculated from the Sight Reduction Tables for the body as it would be if seen from your Assumed Position), you will get your Intercept.  In other words, if you compare what you measure with your sextant (Ho) with what you expected to measure from your AP (Hc), the difference in minutes of arc is how far you are from the AP in nautical miles. To put it still another way, if you actually are located at your Assumed Position, then Ho = Hc and Hc - Ho = 0.  Is it all starting to make sense, now? Let's summarize.

Shooting the Sight

The basic drill is you go out and pick your celestial body, and shoot it with your sextant, noting the exact time and the sextant reading Hs. In actual practice, you shoot all the bodies at once, usually at twilight, when both stars and horizon are visible at the same time. Knowing how to locate and identify the stars will be covered in another chapter.

Almanac Look-up

Now go to the Almanac and make all the appropriate corrections to your reading and write the resulting angle Ho down.  For the instant of time you made that reading, look up the heavenly body and write down all the information there on that body for that time.

Sight Reduction Calculation

With the aid of the Almanac  information, now go to the Sight Reduction Tables and calculate what its Hc should have been if you were shooting from the Assumed Position. This is 90d minus the distance to the Geographical Position (GP )of the body from the AP in degrees. The Sight Reduction Tables will also give you an Azimuth (a fancy word for a bearing) to the GP. In other words, the Pepperday table tells you where on earth the spot directly UNDER the body, (it's GP), is located by giving you its bearing and range.  The Azimuth that comes out of the Pepperday will look like any other bearing, something between 0 and 360 degrees. Convert the Intercept Hc-Ho to minutes of arc, which is the same as nautical miles. The size of the Intercept will depend on how close your Assumed Position  was to your actual position. You don't know your actual position, but now you know how far away it is from the AP..

Plotting the LOP

The final step is to go where you plotted the AP on your chart and draw the Azimuth line, pointing to the body's GP. Mark off the Intercept on the Azimuth line;  if the Intercept is positive, that is, the computed was greater than the observed, draw it on the Azimuth on the side AWAY from the body. If it is negative, i.e.,  the computed is less than the observed,  plot it on the side of the AP TOWARD the body. This makes sense, doesn't it?  The bigger the Altitude, the closer you are to the body's GP.  Now draw a line through the Intercept at right angles to the Azimuth and that will be the LOP.  It is a tiny piece of the huge circle on the earth's surface where the body's altitude is Ho.  You are somewhere on that line.

Once you get really good at this and learn all the shortcuts; look up, sight reduction and plotting a five shot sextant session will take about an hour of work. To get an idea of how all this is actually done at sea, read the following:

http://www.qmss.com/article/morn_sight.html

Don't panic, we'll cover all this again, one step at a time, and I promise it will start coming together.