source: branches/FACT++_part_filenames/pal/palDt.c

Last change on this file was 18347, checked in by tbretz, 9 years ago
File size: 3.5 KB
Line 
1/*
2*+
3* Name:
4* palDt
5
6* Purpose:
7* Estimate the offset between dynamical time and UT
8
9* Language:
10* Starlink ANSI C
11
12* Type of Module:
13* Library routine
14
15* Invocation:
16* double palDt( double epoch );
17
18* Arguments:
19* epoch = double (Given)
20* Julian epoch (e.g. 1850.0)
21
22* Returned Value:
23* palDt = double
24* Rough estimate of ET-UT (after 1984, TT-UT) at the
25* given epoch, in seconds.
26
27* Description:
28* Estimate the offset between dynamical time and Universal Time
29* for a given historical epoch.
30
31* Authors:
32* TIMJ: Tim Jenness (JAC, Hawaii)
33* PTW: Patrick T. Wallace
34* {enter_new_authors_here}
35
36* Notes:
37* - Depending on the epoch, one of three parabolic approximations
38* is used:
39*
40* before 979 Stephenson & Morrison's 390 BC to AD 948 model
41* 979 to 1708 Stephenson & Morrison's 948 to 1600 model
42* after 1708 McCarthy & Babcock's post-1650 model
43*
44* The breakpoints are chosen to ensure continuity: they occur
45* at places where the adjacent models give the same answer as
46* each other.
47* - The accuracy is modest, with errors of up to 20 sec during
48* the interval since 1650, rising to perhaps 30 min by 1000 BC.
49* Comparatively accurate values from AD 1600 are tabulated in
50* the Astronomical Almanac (see section K8 of the 1995 AA).
51* - The use of double-precision for both argument and result is
52* purely for compatibility with other SLALIB time routines.
53* - The models used are based on a lunar tidal acceleration value
54* of -26.00 arcsec per century.
55*
56* See Also:
57* Explanatory Supplement to the Astronomical Almanac,
58* ed P.K.Seidelmann, University Science Books (1992),
59* section 2.553, p83. This contains references to
60* the Stephenson & Morrison and McCarthy & Babcock
61* papers.
62
63* History:
64* 2012-03-08 (TIMJ):
65* Initial version with documentation from SLA/F.
66* Adapted with permission from the Fortran SLALIB library.
67* {enter_further_changes_here}
68
69* Copyright:
70* Copyright (C) 1995 Rutherford Appleton Laboratory
71* Copyright (C) 2012 Science and Technology Facilities Council.
72* All Rights Reserved.
73
74* Licence:
75* This program is free software; you can redistribute it and/or
76* modify it under the terms of the GNU General Public License as
77* published by the Free Software Foundation; either version 3 of
78* the License, or (at your option) any later version.
79*
80* This program is distributed in the hope that it will be
81* useful, but WITHOUT ANY WARRANTY; without even the implied
82* warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
83* PURPOSE. See the GNU General Public License for more details.
84*
85* You should have received a copy of the GNU General Public License
86* along with this program; if not, write to the Free Software
87* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
88* MA 02110-1301, USA.
89
90* Bugs:
91* {note_any_bugs_here}
92*-
93*/
94
95#include "pal.h"
96
97double palDt ( double epoch ) {
98
99 double t,w,s;
100
101 /* Centuries since 1800 */
102 t = (epoch - 1800.0) / 100.0;
103
104 /* Select model */
105 if ( epoch >= 1708.185161980887 ) {
106
107 /* Post-1708: use McCarthy & Babcock */
108 w = t - 0.19;
109 s = 5.156 + 13.3066 * w * w;
110
111 } else if ( epoch >= 979.0258204760233 ) {
112
113 /* 978-1708: use Stephenson & Morrison's 948-1600 model */
114 s = 25.5 * t * t;
115
116 } else {
117
118 /* Pre-979: use Stephenson & Morrison's 390 BC to AD 948 model */
119 s = 1360.0 + (320.0 + 44.3*t) * t;
120
121 }
122
123 return s;
124
125}
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