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Re: FW: A noon sight conundrum
From: Paul Hirose
Date: 2003 Nov 24, 22:55 -0800
From: Paul Hirose
Date: 2003 Nov 24, 22:55 -0800
Kieran Kelly wrote: > > Position : S 21d 48.3' E 132d 40.0' > Date: July 20, 2002 > > The USNO computer gave me the following results: > GMT Hc Zn > 3h 23m 36s 47d 26.6' 357.3d > 3h 15m 48s 47d 29.2' 359.9d > 3h 15m 47s 47d 29.2 360.0d > 3h 15m 40s 47d 29.2' 360.0d > 3h 15m 38s 47d 29.2' 0.0d > 3h 15m 35s 47d 29.2' 0.0d > 3h 15m 34s 47d 29.2' 0.0d > 3h 15m 29s 47d 29.2' 0.0d > 3h 15m 28s 47d 29.2' 0.1d > 3h 07m 42s 47d 26.6' 2.7d I generated the following table with the USNO MICA program. Time scale is UT1, the same as the USNO online sight reduction program, so the results can be compared. Note that my table shows zenith distance, not altitude. The minimum zenith distance (highest altitude) seen below, 42 30 54.2, corresponds to altitude 47 29 05.8. Sun Apparent Topocentric Positions Local Zenith and True North Location: E132?40'00", S21?48'18", 0m (Longitude referred to Greenwich meridian) Date Time Zenith Azimuth Distance (UT1) Distance (E of N) to Object h m s ? ' " ? ' " AU 2002 Jul 20 03:15:35.0 42 30 54.3 0 01 25.4 1.016087417 2002 Jul 20 03:15:36.0 42 30 54.3 0 01 04.6 1.016087416 2002 Jul 20 03:15:37.0 42 30 54.3 0 00 43.9 1.016087415 2002 Jul 20 03:15:38.0 42 30 54.3 0 00 23.1 1.016087414 2002 Jul 20 03:15:39.0 42 30 54.3 0 00 02.3 1.016087413 2002 Jul 20 03:15:40.0 42 30 54.3 359 59 41.6 1.016087412 2002 Jul 20 03:15:41.0 42 30 54.2 359 59 20.8 1.016087412 2002 Jul 20 03:15:42.0 42 30 54.2 359 59 00.1 1.016087411 2002 Jul 20 03:15:43.0 42 30 54.2 359 58 39.3 1.016087410 2002 Jul 20 03:15:44.0 42 30 54.2 359 58 18.5 1.016087409 2002 Jul 20 03:15:45.0 42 30 54.2 359 57 57.8 1.016087408 2002 Jul 20 03:15:46.0 42 30 54.2 359 57 37.0 1.016087407 2002 Jul 20 03:15:47.0 42 30 54.2 359 57 16.3 1.016087406 2002 Jul 20 03:15:48.0 42 30 54.2 359 56 55.5 1.016087405 2002 Jul 20 03:15:49.0 42 30 54.3 359 56 34.7 1.016087404 2002 Jul 20 03:15:50.0 42 30 54.3 359 56 14.0 1.016087404 There is a slight error in this computation: MICA assumes Terrestrial Time was 68.1 seconds ahead of UT1 on this date. The actual value was 64.4, however. "Delta T", as this offset is called, is interpolated from a table which is hard coded into MICA. The table has delta T increasing 1 second per year. That's too much. We haven't had a leap second since the end of 1998. Unfortunately, there's no way to input a value into MICA manually, or substitute an updated table. I obtained the true delta T of 64.4 seconds from the instructions here: http://maia.usno.navy.mil/ (just above the section on IERS Bulletin B), and the Bulletin B from September 2002: http://hpiers.obspm.fr/iers/bul/bulb/bulletinb.175 I'm not sure what effect the incorrect delta T has on the meridian passage computation.