An experienced celestial navigator, Henry Cordova explains the
preparation for and execution of a morning celestial fix.
The navigator's workday starts the night before. In a comfortable and well-lit workspace, settle down with the Nautical Almanac, your Star Finder and a hot cup of coffee (or if you're on a yacht, a rum toddy!). Determine your DR position for the following sunrise as closely as practicable, but don't worry if you're off a bit; a close position is good enough. Once you've determined roughly where you'll be, use your Almanac to calculate the Universal Time (UT) of Nautical Twilight for that location. This is especially critical in the tropics, and especially so around the Equinoxes in late March and September where the time between First Light and sunrise can be quite brief. The whole point of this exercise is to plan your work ahead of time because when you're actually shooting stars the clock will be ticking away like a time bomb, eating away at the window in which you can work. It might be a good idea to calculate UT of sunrise for that DR location the following day as well. Keep in mind that in high latitudes, twilight can last a long time (sometimes, all day long), which might in turn affect your time estimate to select your DR position.
Once these preliminaries are taken care of, use the DR lat/long you just determined and the Almanac in order to set the Star Finder, and pick as many stars as possible to shoot at twilight the following morning. Make sure you pick more stars than you can practically shoot, because the next day may be partly cloudy, some stars may be obscured, and the more options you have the better. Remember, two stars can give you a good fix, but you have no way of knowing how good that fix is. Any fool can make two lines cross! If you blow one sight on a three star fix, you'll know it, but you won't which one is in error. If you get three good sights and one bad one, you'll quickly be able to identify the bad one, if your other sights are all good. Five shots or more is ideal, that way you can afford to make more than one mistake and still recover. But you can't have too many Lines Of Position, so identify as many potential stars and planets as you can, and the moon, if available. It's better to have too many than not enough. The stars should be evenly spaced around the sky, not bunched up, and they should be more than twenty degrees from both the zenith and the horizon to avoid awkward observing angles and extreme atmospheric refraction.
On one clean sheet of paper, write the name of each star and its predicted azimuth and elevation from the star finder, plus plenty of additional space to write your observations the following day. Don't record your data on a separate sight reduction form for each star or you and your assistant will wind up fumbling around in the dark (usually in a stiff wind) with a bunch of unruly sheets of paper. Make one observing list and fasten it well to a clipboard so you can control it. Even, better, use a notebook so you'll have a permanent record. One more hint: make sure the magnitude, or brightness of each star is copied from the Almanac onto your list (to help identification) and remember, the brighter the star, the smaller the magnitude. To summarize, your list should have the star name, magnitude, azimuth, and altitude (in that order) for each body you plan to observe that morning, plus plenty of space to write in the time and altitude of the observation. One line of text per body works best, with blank lines between to help keep them all straight.
Incidentally, some navigators like to shoot more than one sight of each body, and average the values, but I believe that procedure is in itself prone to error. I suggest you shoot as many sights of each one as you have time for, but don't bother reducing them all, keep the extra shots in reserve in case you come up with a sighting that seems really bad. Keep this all in mind when you are writing up your observing list for the following morning. That one piece of paper is going to be the raw data your position will depend on and it has to be NEAT! You are now ready to get a good night's sleep, and make sure the morning watch has instructions to wake you and your assistant well before First Light, so you can be ready to work as soon as there is enough light to see the horizon.
Some navigators can work well without an assistant, others, particularly those who wear reading glasses as I do, find it very difficult. Needless to say, if you are on your own, you should have your procedure worked out and rehearsed ahead of time. I will assume that the navigator in our discussion has an assistant who has been thoroughly briefed in his duties. The navigator and assistant get comfortable (on a small boat, I suggest the sextant operator sits leaning against the mast, the recorder huddles in the companionway or cockpit), and wait for the horizon to appear as the sky starts to gradually brighten. By the way, this is a good time to get your night vision back after your well-lit breakfast below, and to quick-locate the stars on your list in the sky by having the recorder call out the azimuths and elevations. This is also time to check index error and sextant adjustment on a bright star (although it will be too late to do anything about the latter if you note a problem!). Make sure the recorder writes down the index error and makes sure the correction is written with the sign necessary to make the correction. For example, if you have to subtract 1.3 minutes from the reading to get a correct value, the error should be written as -1.3'. The sign of the error should tell you whether to add or subtract it to the raw reading. This seems obvious, but people get it wrong every time.
The horizon will be first visible near where the sun will be rising, so it is those stars nearest the sun which should be observed first. Remember, as the sun approaches, the horizon will become easier to see and the stars harder to see, so you will want to work progressively further away from the sun. Your assistant/recorder should be sharp enough to give you the stars closest to the sun first. If the navigator was sharp they would have been written down in that order on the observing list! The recorder calls out the star information, for example, "Deneb, magnitude 1.3, azimuth 280, elevation 025." The sextant operator brings the star down and calls out as he swings the instrument "Deneb, standby, standby...MARK!" The recorder notes the exact instant of time from a watch after which the observer carefully reads out the sextant reading "zero-two-niner degrees, one-fiyiv point two." The recorder will read back what is written while the observer re-checks the sextant reading. Later on, when the sights are being reduced, how well the observed elevation agrees with the predicted will give a quick check of whether the right star was observed. The team now moves on to the next star, and so forth. With a little practice a good team will be able to knock off about a star a minute. If the moon or sun is observed, make sure the recorder notes upper or lower limb. And don't forget to determine dip correction, based on the height of eye for the navigator at the time of the observation.
Although this is not the place to discuss sextant technique, there are two old hints of which I should remind the reader. First, getting the star and the horizon simultaneously in the sextant telescope field is difficult, particularly near 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 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. 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.
By the time the sun gets high enough to make all the stars invisible, our navigation team should have all the stars they had ready the night before in the bag. However, if a star was inadvertently observed which was not on the observing agenda, or if all you could get was a brief glimpse and a quick shot through a hole in an overcast, remember you can still use your Star Finder and have a good chance of identifying it, and maybe salvaging an LOP from your session.
The navigator is now ready to go below and reduce the sights, and plot them on a plotting sheet: you're not likely to carry all the charts you need on a small boat, especially for mid-ocean coverage, so you should make sure you know how to construct a plotting sheet, even without the pre-printed forms. A maneuvering board will do in a pinch, but even a blank sheet of paper can be pressed into service if you know how to lay it out. By the time the fix is plotted and the new DR laid out, it should be just about time for your morning sunline!
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