A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Tibor Miseta
Date: 2020 Oct 25, 03:01 -0700
Dear NavList members,
I joined this community a year ago, and among other sources I learned a lot from here. Thank you for sharing your invaluable knowledge and experience here!
I am just a celnav hobbyist (completly landlocked :-( ) and recently started to be interested in "longitude by lunars". I developed my skills and knowledge (mainly thank to Frank's lunars pages and Henning Umland's book), and can peform quite good lunar distance observations.
On Friday I went out for practicing and beside the Moon both Mars and Jupiter were visible. Mars was roughly 75° on the left side of the Moon while Jupiter 12° on the right. Just for fun I measured them one after the other and got two acceptable results for my watch error. (My watch was on porpuse off from UTC by roughly two minutes.)
But then I had an idea, averaged the two results, and it was a big surprise: I almost received exactly my actual watch error (with only one second difference).
I thought that the biggest error in the measurement is with my vision of matching the limb with the star/planet. This may be systematic and with a near limb - far limb comparison this error may be cancelled out. And other systematic errors might be canceleld out as well: like the error in determining the Moon's semidiameter, parallax in azimuth caused by the oblateness of the Earth, delays in the measurement of time.
Is this (almost) simultaneous near limb - far limb lunar observation is really a good (and may be known for others) method for fine adjustment of longitude by lunars? Or is it just a coincidence and I was extraterrestially :-D lucky last Friday night? (Since then it's cloudy, I couldn't make a control measurement.) And if so, what level of accuracy may be expected or usual?
I attach my calculation for reference.