File Coverage

palsrc/palDt.c

Criterion	Covered	Total	%
statement	6	9	66.6
branch	1	4	25.0
condition			n/a
subroutine			n/a
pod			n/a
total	7	13	53.8

line	stmt	bran	code
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
97	10		double palDt ( double epoch ) {
98
99			double t,w,s;
100
101			/* Centuries since 1800 */
102	10		t = (epoch - 1800.0) / 100.0;
103
104			/* Select model */
105	10	50	if ( epoch >= 1708.185161980887 ) {
106
107			/* Post-1708: use McCarthy & Babcock */
108	10		w = t - 0.19;
109	10		s = 5.156 + 13.3066 * w * w;
110
111	0	0	} else if ( epoch >= 979.0258204760233 ) {
112
113			/* 978-1708: use Stephenson & Morrison's 948-1600 model */
114	0		s = 25.5 * t * t;
115
116			} else {
117
118			/* Pre-979: use Stephenson & Morrison's 390 BC to AD 948 model */
119	0		s = 1360.0 + (320.0 + 44.3t) t;
120
121			}
122
123	10		return s;
124
125			}