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On 2015-04-09 22:21, Sean C wrote:
> However, I'm still not sure about the "/////" result. If you look back to my 
first post on the subject, you will see the result for the following day was:
>
>        Date         Begin      Rise  Az.   Transit Alt.    Set  Az.     End
>       (UT1)         Civil                                              Civil
>                    Twilight                                           Twilight
>                       h  m     h  m   °      h  m  °       h  m   °      h  m
> 2015 Apr 09 (Thu)   10:14    10:40  80     17:08 61S     23:35 280     00:01
>
> This must mean that the end of civil twilight at this location occurs at 1 
minute into the next day UT1 (Apr. 10th), right? So, shouldn't MICA have 
displayed either "00:00" or "00:01"...or even "?????" (Phenomenon is 
indeterminate.) for the UT1 calculation on Apr. 8th? As it is, it seems to 
suggest that the Earth stops rotating briefly. Although that would obviously 
be absurd.

You have a point there. I didn't notice that. If we compute a set of
zenith distances near the end of Apr 8 (UT) at 1-min intervals, the best
choice for end of civil twilight is 0001 UT on the next day:

                                   Sun

                         Apparent Topocentric Positions
                           Local Zenith and True North

                                  Newport News

                 Location:  W 76°28'36.0", N37°03'18.0",    10m

                   (Longitude referred to Greenwich meridian)


    Date        Time          Zenith          Azimuth         Distance
         (UT1)               Distance        (E of N)        to Object
              h  m   s        °  '   "        °  '   "           AU
2015 Apr 08 23:59:00.0      95 38 53.4     283 39 52.2     1.001341745
2015 Apr 09 00:00:00.0      95 50 30.6     283 49 12.6     1.001342091
2015 Apr 09 00:01:00.0      96 02 07.4     283 58 33.7     1.001342437

In fact, the times for end of civil twilight ca. 2400 UT, computed as
above to the nearest 10 sec, are:

Apr 7 23:59:50 UT
Apr 8 00:00:50 next day
Apr 9 00:01:40 next day

So the end of evening civil twilight on April 7 happens seconds before
the end of the UT day. But look at April 8. As the day begins, we're
barely past the end of civil twilight, and it doesn't occur again until
about a minute into the next day.

Thus MICA is absolutely right, and my earlier explanation was wrong. The
phenomenon doesn't occur on the 8th. The times in the rise/set table for
Apr 9 UT are all on that date. That means the time for end of twilight
at the far right applies to the sunset on the 8th, but the sunset time
to its left is for the 9th!

   

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