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/node206.html | 125 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 125 insertions(+) create mode 100644 src/slalib/sun67.htx/node206.html (limited to 'src/slalib/sun67.htx/node206.html') diff --git a/src/slalib/sun67.htx/node206.html b/src/slalib/sun67.htx/node206.html new file mode 100644 index 0000000..9a94a3d --- /dev/null +++ b/src/slalib/sun67.htx/node206.html @@ -0,0 +1,125 @@ + + + + +Epoch + + + + + + + + + + + + +
+ +next + +up + +previous +
+ Next: Proper Motion +
+Up: EXPLANATION AND EXAMPLES +
+ Previous: Mean Places +

+

+

+Epoch +

+The word ``epoch'' just means a moment in time, and can be supplied +in a variety of forms, using different calendar systems and timescales. +

+For the purpose of specifying the epochs associated with the +mean place of a star, two conventions exist. Both sorts of epoch +superficially resemble years AD but are not tied to the civil +(Gregorian) calendar; to distinguish them from ordinary calendar-years +there is often +a ``.0'' suffix (as in ``1950.0''), although any other fractional +part is perfectly legal (e.g. 1987.5). +

+The older system, +Besselian epoch, is defined in such a way that its units are +tropical years of about 365.2422 days and its timescale is the +obsolete Ephemeris Time. +The start of the Besselian year is the moment +when the ecliptic longitude of the mean Sun is +$280^\circ$; this happens near the start of the +calendar year (which is why $280^\circ$ was chosen). +

+The new system, Julian epoch, was adopted as +part of the IAU 1976 revisions (about which more will be said +in due course) and came formally into use at the +beginning of 1984. It uses the Julian year of exactly +365.25 days; Julian epoch 2000 is defined to be 2000 January 1.5 in the +TT timescale. +

+For specifying mean places, various standard epochs are in use, the +most common ones being Besselian epoch 1950.0 and Julian epoch 2000.0. +To distinguish the two systems, Besselian epochs +are now prefixed ``B'' and Julian epochs are prefixed ``J''. +Epochs without an initial letter can be assumed to be Besselian +if before 1984.0, otherwise Julian. These details are supported by +the SLALIB routines +sla_DBJIN +(decodes numbers from a +character string, accepting an optional leading B or J), +sla_KBJ +(decides whether B or J depending on prefix or range) and +sla_EPCO +(converts one epoch to match another). +

+SLALIB has four routines for converting +Besselian and Julian epochs into other forms. +The functions +sla_EPB2D +and +sla_EPJ2D +convert Besselian and Julian epochs into MJD; the functions +sla_EPB +and +sla_EPJ +do the reverse. For example, to express B1950 as a Julian epoch: +

+            DOUBLE PRECISION sla_EPJ,sla_EPB2D
+             :
+            WRITE (*,'(1X,''J'',F10.5)') sla_EPJ(sla_EPB2D(1950D0))
+
+

(The answer is J1949.99979.) +

+

+ +next + +up + +previous +
+ Next: Proper Motion +
+Up: EXPLANATION AND EXAMPLES +
+ Previous: Mean Places +

+

+

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