Join Date: Aug 2006
Location: Arlington, VA
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The GP is not your DR or assumed position. It's the Ground Point of the celestial body under observation. That is, it's the point on earth which at the precise time of the sight lies directly under the celestial body you're using.
After you've determined the true sextant altitude of that celestial body (using your sextant and applying corrections), and you've determined the calculated altitude for that celestial body at the time of the sight, you compare the two and come up with a difference in minutes or nautical miles (nm).....say 8nm.
Now, you draw a line FROM your DR or assumed position TOWARDS or AWAY from the GP of the celestial body, using the calculated Azimuth. Then, you draw a perpendicular to that line and you have a line (part of a huge circle) on which the boat is somewhere located.
Here are the steps as listed on the BBC website:
"You would actually proceed as follows:
1. Measure the true altitude of the Sun.
2. Determine the tabulated altitude of your DR position.
3. Take the difference between them (in minutes, which is also nautical miles). Let's say it is eight miles.
4. Plot your dead reckoning position on the chart.
5. Draw a line towards (or away from) the Sun's geographical position using the azimuth from the DR position.
6. Measure off eight miles, or whatever your difference was.
7. At this point draw a line at right angles. This is a small part of the boat's position circle."
Last edited by btrayfors; 02-22-2010 at 08:41 PM.