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Sextant accuracy with short distance to horizon
From: Dan Allen
Date: 2001 Jun 21, 7:51 AM
From: Dan Allen
Date: 2001 Jun 21, 7:51 AM
BACKGROUND I was out sailing yesterday through the San Juan islands and in the Strait of Juan de Fuca. It was a gorgeous day aboard a friend's Nauticat 44 and we took out an Astra IIIb sextant to work on sites from a rolling ship--the swells at the east end of the Strait kept us rolling +/- 20 degrees either way for about an hour. Anyway, we wanted to measure our use of sextant accuracy so we did this: we would record our exact GPS position, exact time, and the angle of our sun shot and then calculate our position via an HP-48 program of mine to see how close we came to the truth, the truth being approximated to within 12 feet by the GPS position using a Garmin GPS III. My friend hadn't used his sextant much before so his first shots were off by 300 miles, but quickly they got to within 5 miles of our actual position, a great improvement. I took one shot myself and got to within 4 miles. Later we did the noon shot and his results were about 68 miles off for a bunch of shots. These were taken in Guemes channel with a local horizon less than a mile away. QUESTION This led me to think more about possible inaccuracies of using a horizon that is very close rather than naturally far away. That is, typically you use a horizon that is at the limit of your vision, perhaps 20 miles away. Our local horizon was within a mile. I don't remember reading much about any errors with regard to this. Do any on this list have anything to add about possible errors of using a very near horizon? I plan to go back and reread the usual suspects on the topic, but I thought it may be of interest to others. Simply put, is there a formula or correction to give greater accuracy to sextant shots taken using a very close horizon? If one knows the exact distance to the local horizon, can this be applied to the shot to give a more accurate reading of the angle of a celestial body? Or, is there a rule of thumb that says if the local horizon is closer than X miles, one should use an artificial horizon instead due to big inaccuracies that cannot be corrected for? Thanks in advance, Dan Daniel K. Allen danallen@nwlink.com http://www.nwlink.com/~danallen/ Navigate | Calculate | Sail