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Celestial Navigation Part IV – Publication 249 and The Universal Plotting Sheet

Celestial is coastal navigation on a galactic scale; the sextant essentially an advanced hand-bearing compass. Imagine coming across a bell buoy in mid-ocean. Your radar tells you the buoy is a mile away. Your instruments are dead and for now you have no idea of the buoy’s compass bearing. What are your possible positions? The answer is simple – if plotted on a chart, you would draw a circle around that buoy, with a one-mile radius – you could be anywhere on that circle of position. If given a compass bearing to that buoy, you now have the simplest of fixes – a distance and a range. Keep this in mind.

There are three figures needed to enter the sight reduction tables (Pub. 249) – AP Latitude, Declination (north or south), and LHA. Each whole degree of AP latitude has several corresponding pages in Pub. 249. Find the correct page, based on declination, which is shown as a range north or south. Now find your exact declination, and move down that column to the corresponding LHA. You’ll find three numbers – Hc, the calculated sextant angle; d, which in this case is the declination factor, and Z, azimuth (the angle to the GP from geographic north). On the back page of Pub. 249, you will find corrections for the d number. This correction is applied to the Hc.

Transfer your Ho reading from section one. The intercept is calculated from the difference between Ho and Hc. Always subtract the lower number from the higher one so your answer is always a positive number. The intercept represents the difference between your actual sextant reading, and the imaginary sextant reading that was taken from the AP and calculated in Pub. 249. This then is how you derive position (or rather a line of position, LOP) – if your actual reading (Ho) is larger than the imaginary reading (Hc), then you must be closer to the sun, as it’s a wider angle. Conversely, a Ho lower than Hc would mean the opposite, that you are further away and the sun appears lower on the horizon to you. This towards the sun or away from the sun (from the AP) distinction is imperative when moving onto the plotting sheet.

The Universal Plotting Sheet

From the sight reduction form, record four important bits of info in an upper corner of the plotting sheet – DR position, AP, the azimuth and the intercept (towards or away). Begin by setting up the plotting sheet for the correct latitude – note that lines of longitude converge towards the poles, so that the distance between them changes dramatically as you head north or south of the equator. The plotting sheet accounts for this with the scale on the bottom right corner.

First plot your DR position, and mark it with the traditional dot and half-circle to indicate as such. Then plot your AP, with the dot and full-circle, as it’s in fact a known position (whereas DR is just an educated guess). The azimuth is plotted as a dotted line drawn through the AP. Label the correct end with a small smiley sun (be careful not to plot the reciprocal bearing!). The intercept then is the final step. Starting at the AP, measure the intercept towards or away from the sun – your little smiley picture – and make a mark. The intercept, as mentioned, is the difference between your sextant reading and the calculated reading. The beauty of measuring in arc degrees is that the degrees and minutes off the sextant exactly correspond to degrees and minutes of latitude, and therefore distance. An intercept of 24′ away means that your sextant altitude was taken 24 nautical miles furtherfrom the sun than the calculated altitude. Finally, plot your LOP through the intercept and at an angle exactly 90º to the azimuth, as a solid line.

What this LOP represents is merely a tiny tangent to a much larger circle of position around the GP of the sun. And since an LOP is not a position, a second sight must be taken on a second celestial body, or on the same one at a later time in the day, just as you can take a running fix on a lighthouse ashore as you sail along a coast.

Celestial then requires a big commitment to rely on as a sole means of navigation. The navigator will typically follow a pattern when taking sights, and is always prepared ahead of time. At dawn and dusk, during twilight when the brightest stars are out and their remains a visible horizon, the navigator will be ready with his sextant, and already know, by examining the books, in which direction to look and for which stars. At these times he’ll get the most accurate fixes, as likely three, four or even five stars and/or planets can be shot within minutes of each other. As the sun rises, he will be ready around 10am, to get a good morning sight. He will advance and cross his am LOP with the noon sight and further advance the noon sight to cross with another sight, taken sometime in the afternoon. At dusk he will repeat the procedure with the evening stars. Safe landfalls are dependent upon the navigators accuracy, which over the course of a long offshore passage, may not be truly known for weeks. When an island finally appears over the horizon when and where it should, only then can the navigator relax.

Editor’s note: Publication 249, Sight Reduction Tables for Air Navigation, was used throughout our series on celestial navigation.  This book can be used for air or marine navigation and is recommended as easier to use than the marine sight reduction tables.

Andy Schell is a professional yacht captain. He contributes regularly to All at Sea and several other sailing publications, and is Chief Editor of the annual Yacht Essentials Portbook. He and his wife Mia recently completed an Atlantic crossing to Ireland via Newfoundland on their yawl Arcturus. Find them online at fathersonsailing.com

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