A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2017 May 12, 00:48 -0700
Isn’t it strange that you sometimes stumble upon something quite by accident that should have been obvious to you years ago, but you never noticed, because you never needed to use it? Wednesday evening was our first clear evening for some time, so I decided to spend an hour in the “Astrovan” checking the accuracy of my Smith’s “2b and 2c pendulous reference periscopic sextants against Jupiter, which was one of the few bodies easily visible in moon glow and local light pollution. As the hour passed, I noticed that the difference between successive observations was tending towards zero as the azimuth approached 180, so I thought “What if I tried a simple mer-pass calculation?” The details were as follows:
Sextant: Smith’s Mk2c Serial No 148KHI/73 last serviced 5th June 1997 by Fenns of Farnborough
Time: 2hrs44min UTC
Hs 32° 33.3’ +1.0’ error on the 30° stop – 1.5’ refraction. Ho=32° 32.8’
Mer-pass Latitude = 90°-(Ho+declination S) = 90°-(32° 32.8' + 4° 19.1') = 52° 08.1’N
Actual Latitude = 53°10.2’N difference 2.1nm
For the mariners and astronomers +/- 3nm in the air and +/- 1nm on the ground would be considered very good for a freshly serviced aircraft sextant, so 2nm for one last serviced in 1997 and delivered by post via a number of dealers is quite good.
From the Air Almanac: The actual mer-pass time was:
21hrs 40min LHA Jupiter = GHA-LongW=359°55' – 000°32' = 359°23'
37’arc=2min 28secs time. Actual mer-pass time was 21hrs 42min 28secsUTC