A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bob Goethe
Date: 2019 Feb 4, 10:31 -0800
If you can find a good, real-world example, then great. But it takes little more time to reverse engineer a set of sights than it would to reduce them from an actual set of sextant observations. Assuming I was doing this and wanted my fix to be related to my house, I would do this:
1. Look at my phone and come up with my GPS position. Use this as my dead reckoning position on my sight reduction worksheet.
2. Select a time that I am going to suppose is my time of observation. Use the Nautical Almanac and Pub. 249 to come up with an assumed position and azimuth of celestial object.
3. Decide where you WANT your "fix" to be. It can be as near or far from your dead reckoning position as you wish. Draw a line of position at right angles to your LOP.
4. Measure what the intercept is for this LOP.
5. From this, you know what your Ho (height observed) would have to be to come up with the intercept you determined in step 4.
6. Work backwards to come up with a main correction for refraction. Give yourself - if desired - some sort of index error and dip, and out of this you come up with what your actual observed sextant altitude would be if you were working from an actual sight.
Do this with 3 LOPs and you can make your cocked hat as close or far from equilateral as you wish.
I have done this before with students, where I picked a DR that was NOT near my house, but out somewhere on the ocean. I picked a date for my supposed sights where I could emphasize d or v corrections, if that was on my lesson plan to teach. I have used this approach to teach star/planet sights, or maybe instances where the sun and moon were in the sky together. I use Stellarium or www.heavens-above.com to select dates and times where objects that I want are all properly visible.