As the sun rises, he will be ready around 10am, to get a good morning sight. 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. 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. The navigator will typically follow a pattern when taking sights, and is always prepared ahead of time. Mastering Celestial Navigation with Publication 249 and the Universal Plotting SheetĬelestial then requires a big commitment to rely on as a sole means of navigation. 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. What this LOP represents is merely a tiny tangent to a much larger circle of position around the GP of the sun. Finally, plot your LOP through the intercept and at an angle exactly 90º to the azimuth, as a solid line. An intercept of 24′ away means that your sextant altitude was taken 24 nautical miles furtherfrom the sun than the calculated altitude. 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. The intercept, as mentioned, is the difference between your sextant reading and the calculated reading. Starting at the AP, measure the intercept towards or away from the sun – your little smiley picture – and make a mark. Label the correct end with a small smiley sun (be careful not to plot the reciprocal bearing!). The azimuth is plotted as a dotted line drawn through the AP. 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). Navigating the Seas: A Guide to Marine Navigation and Chart Readingįirst plot your DR position, and mark it with the traditional dot and half-circle to indicate as such. The plotting sheet accounts for this with the scale on the bottom right corner. 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. This correction is applied to the Hc.įrom 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). 249, you will find corrections for the d number. 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). Now find your exact declination, and move down that column to the corresponding LHA. Find the correct page, based on declination, which is shown as a range north or south. Each whole degree of AP latitude has several corresponding pages in Pub. 249) – AP Latitude, Declination (north or south), and LHA. There are three figures needed to enter the sight reduction tables (Pub. Publication 249: The Sight Reduction Tables If given a compass bearing to that buoy, you now have the simplest of fixes – a distance and a range. 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. Your instruments are dead and for now you have no idea of the buoy’s compass bearing. Your radar tells you the buoy is a mile away. Imagine coming across a bell buoy in mid-ocean. Celestial is coastal navigation on a galactic scale the sextant essentially an advanced hand-bearing compass.
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