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<H2><A NAME="SECTION00056000000000000000">
Epoch</A>
</H2>
The word ``epoch'' just means a moment in time, and can be supplied
in a variety of forms, using different calendar systems and timescales.
<P>
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 (<I>e.g.</I> 1987.5).
<P>
The older system,
<I>Besselian epoch</I>, is defined in such a way that its units are
tropical years of about 365.2422 days and its timescale is the
obsolete <I>Ephemeris Time</I>.
The start of the Besselian year is the moment
when the ecliptic longitude of the mean Sun is
<IMG WIDTH="34" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img167.gif"
ALT="$280^\circ$">; this happens near the start of the
calendar year (which is why <IMG WIDTH="34" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img167.gif"
ALT="$280^\circ$"> was chosen).
<P>
The new system, <I>Julian epoch</I>, 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.
<P>
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).
<P>
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:
<P><PRE>
DOUBLE PRECISION sla_EPJ,sla_EPB2D
:
WRITE (*,'(1X,''J'',F10.5)') sla_EPJ(sla_EPB2D(1950D0))
</PRE>
<P>(The answer is J1949.99979.)
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<ADDRESS>
<I>SLALIB --- Positional Astronomy Library<BR>Starlink User Note 67<BR>P. T. Wallace<BR>12 October 1999<BR>E-mail:ptw@star.rl.ac.uk</I>
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