From d54fe7c1f704a63824c5bfa0ece65245572e9b27 Mon Sep 17 00:00:00 2001 From: Joseph Hunkeler Date: Wed, 4 Mar 2015 21:21:30 -0500 Subject: Initial commit --- src/slalib/sun67.htx/node73.html | 131 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 131 insertions(+) create mode 100644 src/slalib/sun67.htx/node73.html (limited to 'src/slalib/sun67.htx/node73.html') diff --git a/src/slalib/sun67.htx/node73.html b/src/slalib/sun67.htx/node73.html new file mode 100644 index 0000000..fca730c --- /dev/null +++ b/src/slalib/sun67.htx/node73.html @@ -0,0 +1,131 @@ + + + + +SLA_DT - Approximate ET minus UT + + + + + + + + + + + + +
+ +next + +up + +previous +
+ Next: SLA_DTF2D - Hour,Min,Sec to Days +
+Up: SUBPROGRAM SPECIFICATIONS +
+ Previous: SLA_DSEP - Angle Between 2 Points on Sphere +

+

+

SLA_DT - Approximate ET minus UT +   +

+
+
ACTION: +
Estimate $\Delta$T, the offset between dynamical time +and Universal Time, for a given historical epoch. +

CALL: +
D = sla_DT (EPOCH) +

+

+
GIVEN: +
+
+ + + + + +
EPOCHD(Julian) epoch (e.g. 1850D0)
+

+
RETURNED: +
+
+ + + + + +
sla_DTDapproximate ET-UT (after 1984, TT-UT1) in seconds
+

+
NOTES: +
+
1. +
Depending on the epoch, one of three parabolic approximations +is used: +

+before AD 979 		 Stephenson & Morrison's 390 BC to AD 948 model
+		 AD 979 to AD 1708 		 Stephenson & Morrison's AD 948 to AD 1600 model
+		 after AD 1708 		 McCarthy & Babcock's post-1650 model
+
+ The breakpoints are chosen to ensure continuity: they occur + at places where the adjacent models give the same answer as + each other. +
2. +
The accuracy is modest, with errors of up to $20^{\rm s}$ during + the interval since 1650, rising to perhaps $30^{\rm m}$ by 1000 BC. Comparatively accurate values from AD 1600 + are tabulated in + the Astronomical Almanac (see section K8 of the 1995 + edition). +
3. +
The use of DOUBLE PRECISION for both argument and result is + simply for compatibility with other SLALIB time routines. +
4. +
The models used are based on a lunar tidal acceleration value + of + $-26\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.00$ per century. +
+

+
REFERENCE: +
Seidelmann, P.K. (ed), 1992. Explanatory +Supplement to the Astronomical Almanac, ISBN 0-935702-68-7. + This contains references to the papers by Stephenson & Morrison + and by McCarthy & Babcock which describe the models used here. +
+
+ +next + +up + +previous +
+ Next: SLA_DTF2D - Hour,Min,Sec to Days +
+Up: SUBPROGRAM SPECIFICATIONS +
+ Previous: SLA_DSEP - Angle Between 2 Points on Sphere +

+

+

+SLALIB --- Positional Astronomy Library
Starlink User Note 67
P. T. Wallace
12 October 1999
E-mail:ptw@star.rl.ac.uk +
+ + -- cgit