A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Antoine Couëtte
Date: 2010 Nov 9, 22:57 -0800
Dear Greg and Dear Frank,
In reply to Greg's post ,[ NavList # 13999] dated 22 Sep 2010 - which I have extensively reproduced at the bottom of the current post - , my comments would be as follows :
The least minimum geocentric distance between Jupiter and Moon centers happened at a UT time (very) close from 22 Sep 2010 04h53m49s7.
Accordingly, almost 2 hours earlier - which is the time of your Plath sextant near limb observations - and from your location (N34d10.4m W119d13.8m) the apparent (i.e. sextant measured) distance between Jupiter and Moon near limb kept varying extremely slowly. My computations (TT-UT=67seconds of time) show that between 02h48m16.0sUT and 02h58m16.0sUT - i.e. a 10 minute period centered around your 02h53m16.0sUT near limb sextant observation - their apparent such distance varied by only 0.194 arc minute.
Accordingly their apparent distance changed by 0.1 arc minute with every 309 seconds of time. IN THIS VERY SPECIAL SITUATION, every 0.1 arc minute observational error on their angular apparent topocentric separation (i.e. the lunar distance itself you are attempting to measure with your sextant) translates into an error of 309 seconds of time for the intended and related UT determination.
IN OTHER WORDS, IN THIS VERY SPECIAL CONFIGURATION, EVERY SUCH 0.1 ARC MINUTE OBSERVATIONAL ERROR TRANSLATES INTO 77 ARC MINUTES (OVER ONE FULL DEGREE) IN LONGITUDE DETERMINATION.
Since you mentioned that your results were cleared by Frank's Reed program, I have used it. Here are the results I have obtained with Frank's on-line computer (http://www.historicalatlas.com/lunars/lunars_v4.html). I reproduce them extensively so that anybody can double-check them.
From Position N34d10.4m W119d13.8m on Sep 29, 2010 at UT time 02h53m16.0s you observed a Jupiter-Moon near limb distance of 5d32.4m. I have assumed standard atmospheric pressure (i.e. 29.92 InHg) and temperature. In order to see how Frank's computer "reacts" to this highly special planetary configuration, I have varied the sextant heights from your observed value of 5d32.4m up up to 5d32.8m by 0.1 arc minute increments, every else being equal. Here are the results I am getting from Frank's on-line computer.
5d32.4m Error in Lunar 0 arc minute approximate error in longitude 0d01.4m
5d32.5m Error in Lunar 0 arc minute approximate error in longitude 0d01.3m
5d32.6m Error in Lunar 0.1 arc minute approximate error in longitude 0d03.9m
5d32.7m Error in Lunar 0.2 arc minute approximate error in longitude 0d06.5m
5d32.8m Error in Lunar 0.3 arc minute approximate error in longitude 0d09.1m
Clearly, the approximate errors in longitude yielded by the on-line computer are highly underestimated. For a 0.1 arc minute error they indicate a longitude error of 2.6 arc minutes in Longitude, which is almost 30 times too low.
Certainly, we could just regret that - since one is totally misled by the highly underestimated published errors in longitude - (almost) nothing here warns the casual user about this VERY SPECIAL CONFIGURATION. The only partial cues which could lead the experienced user to (maybe ?) exercise some caution would be to compare both the difference in Azimuth (given by its cosine (0.99904) - here Jupiter is in Azimuth 101.6 and the Moon in Azimuth 099.1 - and their given altitudes with the fact that the Observer's latitude is fairly distant from the Equator.
Certainly this very special planetary configuration should prevent from treating it as a "conventional lunar" and I am sure that you have already addressed this a few times earlier Frank, but I am certainly surprised to see that in this very special case - which might be so easily overlooked and treated as a casual case - your longitude error ratio is so far off from reality, by a factor of 30.
Of course, all my comments stand if and only if my computations are not in error and certainly I will listen to any and all feedback here.
Thank you for your Kind Attention. Your views on this matter will be most welcome.
Antoine M. "Kermit" Couëtte
THIS IS GREG's REFERENCED POST :
This evening the eastern sky was clear for a near/far Jupiter Luner DSLR camera trial and a near/far Jupiter Lunar using a 4x40 scoped Plath as a check.
10MP DSLR 50mm lens 1/200 sec. ISO 1600 F4 infinite focus
9/23/2010 UT 3:20:40 Lat.34° 10.4'N Lon.119° 13.8'W
Near Limb 5°31.4' clearing 45.3' longitude (-1.5')
Far Limb 6° 3.4' clearing 15.7' longitude ( 0.5')
cleared by Reed online program
Plath 4x40 scope
9/23/2010 UT 2:53:16 Lat.34° 10.4'N Lon.119° 13.8'W
Near Limb 5° 32.4' clearing 2.2' longitude (-0.1')
9/23/2010 UT 3:35:6
Far Limb 6° 3.0' clearing 7.6' longitude (-0.3')
cleared by Reed online program
The sextant is still the champion instrument when it comes to taking Lunars but the DSLR camera with a 50mm lens put in a respectable showing.
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com