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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Back sight at Lake Ontario
From: Wolfgang Hasper
Date: 2015 Sep 15, 10:37 +0200
From: Wolfgang Hasper
Date: 2015 Sep 15, 10:37 +0200
Stan,
thanks for sharing this.
14.9' off does correspond well with the semi-diameter of the apparent sun- sure you have no flaw in the calculations?
If this were so it could actually be a very good observation... :o)
regards
Wolfgang
Gesendet: Montag, 14. September 2015 um 22:52 Uhr
Von: "Stan K" <NoReply_StanK@fer3.com>
An: wolfgang.hasper@web.de
Betreff: [NavList] Back sight at Lake Ontario
Von: "Stan K" <NoReply_StanK@fer3.com>
An: wolfgang.hasper@web.de
Betreff: [NavList] Back sight at Lake Ontario
We found ourselves at Hamlin State Park on Lake Ontario last week, and I happened to have an Astra IIIB (whole horizon mirror) with me, so it seemed a shame not to take a sight. It would have to be a back sight, with the lake to the north and the sun to the south. This venture was not specifically planned, and, looking back on it, I was lucky to be able to do it at all. The Astra IIIB has a 130º arc, so the sun would have to be at least 50º high. This limited the available times to about 1200 to about 1420 EDT. I don't recall what time we arrived, but by the time we got done looking around and walking the nature trail it was around 1330.
At the time I did not have a computer, tablet, or even a cell phone with a celestial app or internet access, so I tried to determine the altitude of the sun by bringing the sun down over the land to what appeared to be the horizon, that is when the sextant telescope appeared to be horizontal. I then set the sextant to 180º minus the altitude I estimated and scanned the horizon in the direction of my shadow. I could not find the sun. I adjusted the sextant up and down a little and scanned again. Still no luck. As it was extremely hot that day, and the sweat was dripping in my eyes, I almost gave up, but I decided to try one more thing. I lied down on the ground, aimed the sextant at the sun behind me, and brought it down to the horizon. (Of course I got strange looks from the passersby entering the nature trail, and even from my wife and friends.)
Ultimately I did an upper limb sight with a sextant altitude of 128º00.8', with an estimated height of eye of 10 feet (not right at the water). The time was 135132 EDT, and the index error was zero. Intercept calculated to be 14.9 nm, a pretty poor sight.
I think one of my mistakes was doing an upper limb back sight, which appears as the lower limb in the sextant. When you rock the sextant with a back sight, the sextant is vertical when the body is at the top of the arc, not the bottom, so I had some trouble getting things right. With 20:20 hindsight, meaning I looked at books, a lower limb back sight, with the sun below the horizon, is what is recommended.
Anyway, it was the only back sight I have done since the early '90s, and the only one I have ever done with a whole horizon mirror. Not a very good sight, but at least now I can say I took a sight at Lake Ontario.
Below are a couple of photos. The one with me and the sextant is posed, not really taking a sight, but you can get an idea of the extent of the lake. Makes Long Island Sound look like a pond The other one is me kissing ash. (Apparently there is a problem with the emerald ash borer in New York State, and all the ash trees are tagged.)
Stan
