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    Lunars on eclipse day, IE surprise and time brackets
    From: Ed Popko
    Date: 2019 Jan 24, 06:58 -0800

    Like many NavListers, I really enjoyed the recent Lunar eclipse. While waiting for the event, I decided to take some lunar-star shots. Aldebaran was the best choice. The results were far better than I deserve. But I learned something about sextant Index Error I had not experienced before.

    Overall lunars conditions excellent (does not get better):
    - perfect viewing conditions, very dark, no wind
    - cold (but nowhere as bad as back in New York that day which were near zero degrees F)
    - GMT, location and night lighting from Frank's vary convenient GPS Anti Spoof Pro smartphone app.
    - stable car hood shooting position, physically quite comfortable
    - Lunar reduction, Frank's web Lunars Calculator
    - Lunar-body time and Geocentric Distances from Frank's web Predicted Geocentric Lunar Distances

    The shooting experience had an unexpected surprise - my sextant Index Error before and after the shooting changed at lot, at least a lot by the exacting lunars standards. The day before, temps were mild and I zeroed out my Astra IIIB's index error by sighting bright stars.  But by lunar eclipse day, January 21st, temperatures started to drop and just prior to the eclipse had dropped almost 35 degrees in Central Florida.

    I did not leave my sextant outside to bring it up to temperature, instead, I went out prior to the eclipse and just started taking lunar shots of Aldebaran. By chance, at the end of the series, I re-took shots of a star at zero index to see if the Index Error had changed and was quite surprised to see it had shifted to 0.9'off (IC would be +0.9').  I did not expect the aluminum Astra IIIB to change that much. In taking lunars, knowing your IE is critical and .9 is a BIG error if you don't compensate for it.

    Below are the shots I took.  The first one is so far off, I rejected it. Perhaps I was adapting my eyes or just getting physically comfortable (I was using my car's hood as a work bench).

    KP Lat 28° 41.1'N  Lon 082° 18.2'W
    WE = 00"  (Using Franks GPS Anti Spoof Pro for time and location)
    IE= 0.9' off   IC=0.9' on

                         Near Limb    Cleared
    Sight  GMT      LD               LD        Error
     1.  01:54:35   49° 26.1'   48° 58.7'   2.6' <-- rejected
     2.  01:57:32   49° 25.3'   48° 58.3'   0.2'
     3.  02.00:02   49° 26.6'   48° 59.9'   0.4'
     4.  02:01:33   49° 27.3'   49° 00.8'   0.3'
     5.  02:03:30   49° 27.7'   49° 01.5'  -0.3'
     6.  02:05:00   49° 28.9'   49° 02.9'   0.2'
     7.  02:06:40   49° 29.4'   49° 03.6'  -0.1'
     8.  02:08:28   49° 30.7'   49° 05.2'   0.3'
     9.  02:09:49   49° 31.0'   49° 05.7'  -0.1'
    10.  02:11:22  49° 31.8'   49° 06.7'  -0.1'

                               Predicted         Cleared     Lunars               
               GMT       Geocentric         Lunar       Computed   Computed vs
    Sight   Time      Lunar Distance  Distance   Time            Actual GMT
    *       01:00:0     48 21.5'                                        
    1.      01:54:35                             48° 58.7'     01:58:44     04min 09sec fast <-- rejected
    *       02:00:00    48° 59.5'                                        

    3.      02:00:02                             48° 59.9'       02:00:33       31sec fast
    5.      02:03:30                             49° 01.5'       02:03:04       26sec slow
    7.      02:06:40                             49° 03.6'        02:06:23      17sec slow                           
    *       03:00:00    49° 37.6'                                        
    * from web Predicted Geocentric Lunar Distances app

    - Meatal sextants can have big change in IE due to temperature changes. Best to recheck the IE after the shoot
    - Franks GPS Anti Spoof Pro is a huge help in eliminating time and location errors. Also produces
      enough light to serve during note taking and sextant reading

    - Lunars Calculator web app eaves no excuses for not trying lunars
    - Steady comfortable viewing counts for a lot.


    When the actual GMT time of your lunar observation is very near the time bracketed by an almanac or Frank's predictor, is it more accurate to use the next time bracket either earlier or later?  An example makes my question clearer. In the LUNARS TIME BRACKET table above, sight 3 is only a few seconds away from a time bracket 02:00:00. Is it more accurate to interpolate sight 3 between the 01:00:00 - 03:00:00 bracket and not the 02:00:00-03:00:00 bracket? I suspect rounding errors etc. would have less impact.


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