Time error

Greg Roberts (grr@da.saao.ac.za)
Wed, 22 Jan 1997 08:23:51 +0200

Greetings all

Ted Molczan queried an observation of mine on 20 Jan 1997 as follows:

>Greg Roberts wrote:

>>Observation on 20th Jan 1997 from my usual site of 96072A as follows:
>>#24680 96072A 19h52m59.3s UT RA 11h47.7m,Dec -40d19' (J2000)
>>There may be a slight timing error-I had no time signals for about 10 minutes
>>either side of the observation so had to stop the one stopwatch against my
>>other stopwatch which I use as a "clock". This earlier had an error of about
>>1.5 seconds and I have corrected the time above for this.

>Your position appears to be very accurate, as usual, but the time is 
>about 2.9 s early, relative my 97018.858 elset, as judged from this 
>ephemeris:
 
>  TIME      %I   Mv     AZ  EL    R.A.    DEC   FE   VANG  RANGE   ALT
>--------    --  ----   ---  --   -----  ------  --   ----  -----  -----
>19:52:57    59   6.4   129  14   11:51  -40:50   4   0.17   2004    764
>19:52:58    59   6.4   129  14   11:51  -40:44   4   0.17   1999    764
>19:52:59    59   6.4   129  14   11:50  -40:38   4   0.17   1994    763
>19:53:00    60   6.4   129  14   11:49  -40:32   4   0.17   1990    763
>19:53:01    60   6.4   129  14   11:48  -40:26   4   0.17   1985    762
>19:53:02    60   6.4   129  14   11:48  -40:20   4   0.18   1981    762
>19:53:03    60   6.4   128  14   11:47  -40:14   4   0.18   1976    762
>19:53:04    60   6.4   128  14   11:46  -40:08   4   0.18   1971    761


>I would have to multiply the decay rate over the past 2 days by a factor 
>of about 3, to account for this, but that is virtually impossible, since 
>the sun has remained quiet during that period. 

>The difference also is well outside the uncertainty in the mean motion.

>One possible explanation - could you have applied the 1.5 s correction 
>in the wrong direction, i.e subtracted, instead of added?


YES , Ted, I am an idiot - I did SUBTRACT so the CORRECT TIME should be
3 seconds later, ie 19h 53m 02.3s UT. Thanks for pointing it out - now I
can understand why they retire old crocks like me when they get too old!


>>#24681 96072B -- looked for it but not seen.Shadow entry was predicted for 8
>>degrees and I cannot access this elevation from my site using my 5 inch
>>tracking telescope so I tried 7x50 binoculars. 

>The situation may have been even worse. My ephemeris for your site indicates
>shadow entry (mid-point of penumbra) at 6 deg. It would have reached the
>umbra at about 7 deg.

Interesting. I have been using several prediction programs for this current
series of tracking and the one I have used mainly is SKYMAP510 and the map
shows shadow entry at 8 degrees. Similarly for the observation last week
when you stated that the observation was in shadow but again SKYMAP510 showed
it still illuminated. I havent checked my own prediction program which does
shadow entry but I do recall that to fit actual observations of shadow
entry I went another 0.9 degrees into shadow for the actual "cut-off" point.
This was not a figure of my choosing- I think it originally appeared in
MEMOIRS OF THE BRITISH ASTRONOMICAL ASSOCIATION Artificial Satellites where
there was an article on SOLAR ILLUMINATION, by, if my memory is correct,
G.E.Taylor . I can dig up full details if anyone interested.

Last night cloudy so no observations.

Cheers
Greg