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Astronomical Applications Department, U.S. Naval Observatory - NOVAS 2006 Overview (Page 2)

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Astronomical Applications Department, U.S. Naval Observatory - NOVAS 2006 Overview
(7) But the real purpose of EQXRA was for use by CIOLOC, which was formerly called
CEORAI. The purpose of CIOLOC is to "locate" the CIO and it can now do that in two
ways; an extra output argument, K, indicates which way was used. If the file of CIO
right ascension values is available (nominally called `CIO_RA.TXT') then CIOLOC will
provide the GCRS right ascension of the CIO, and will set K to 1. If the file is not
available then CIOLOC will provide the arc on the instantaneous equator from the
equinox to the CIO, and will set K to 2. To do the hard work, CIOLOC calls either
CIORD (for K=1) or EQXRA (for K=2). Note that the old and somewhat ad hoc series
approximation that was embedded in CEORA is now gone.

(8) Within NOVAS, the old CEORA was always used together with CEOBAS. With the
change of names, the pairing is now CIOLOC and CIOBAS. CIOBAS now takes an
extra input argument, K, passed from CIOLOC. CIOBAS has been altered to work with
either kind of CIO location parameter provided by CIOLOC. That is, extra code was
added to handle the K=2 case. The output basis vectors from CIOBAS are, however, the
same (to within a few microarcseconds) in either case.

(9) Some INQUIRE statements were added to CIORD, which reads the file of CIO right
ascensions, to check on the existence of the file before attempting to open it. A new error
message and error code were added, to be used if the file is not found. Also, now,
CIORD regards a call with TJD=0.D0 and NVALS=1 as a special call that simply checks
on the existence of the file of CIO right ascensions and returns IERR=0 if it is present
and IERR=4 if not. Nothing else is done in that case, and an error message is not
displayed if IERR=4. This is how CIOLOC determines which way it will use to locate
the CIO.

(10) A thorough scrub of the comments was done to make sure that "ICRS" was being
used appropriately. In about half the cases it was changed to "GCRS". The name of
subroutine ICRSEQ was changed to GCRSEQ.

(11) The double-Julian date argument to subroutine GEOPOS (TT and TDB) has been
changed to a single TT Julian date argument. Internally, GEOPOS now sets TDB=TT,
which is plenty good enough for what it does. Errors caused by neglecting polar motion
are much larger than the errors caused by this time approximation, but neither is
important.

(12) Major changes to the output of PLACE: The output parameters are now all
contained in an array called SKYPOS. The unit vector toward the object comprises the
first three elements of SKYPOS, followed by the RA (in hours) in element 4, dec (in
degrees) in element 5, and true distance (for planets, in AU) in element 6. Element 7 now
contains the radial velocity (in km/sec), which is new (see below).

(13) Subroutines PLACES and MPSTAR have been changed to accommodate the new
output array SKYPOS in PLACE. There are 4 calls to PLACE in PLACES and 1 call to
PLACE in MPSTAR.

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