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/node216.html | 110 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 110 insertions(+) create mode 100644 src/slalib/sun67.htx/node216.html (limited to 'src/slalib/sun67.htx/node216.html') diff --git a/src/slalib/sun67.htx/node216.html b/src/slalib/sun67.htx/node216.html new file mode 100644 index 0000000..ab75104 --- /dev/null +++ b/src/slalib/sun67.htx/node216.html @@ -0,0 +1,110 @@ + + + + +The Hipparcos Catalogue and the ICRS + + + + + + + + + + + + +
+ +next + +up + +previous +
+ Next: Timescales +
+Up: EXPLANATION AND EXAMPLES +
+ Previous: Efficiency considerations +

+

+

+The Hipparcos Catalogue and the ICRS +

+With effect from the beginning of 1998, the IAU adopted a new +reference system to replace FK5 J2000. The new system, called the +International Celestial Reference System (ICRS), differs profoundly +from all predecessors in that the link with solar-system dynamics +was broken; the ICRS axes are defined in terms of the directions +of a set of extragalactic sources, not in terms of the mean equator and +equinox at a given reference epoch. Although the ICRS and FK5 coordinates +of any given object are almost the same, the orientation of the new frame +was essentially arbitrary, and the close match to FK5 J2000 was contrived +purely for reasons of continuity and convenience. +

+A distinction is made between the reference system (the ICRS) +and frame (ICRF). The ICRS is the set of prescriptions and +conventions together with the modelling required to define, at any +time, a triad of axes. The ICRF is a practical realization, and +currently consists of a catalogue of equatorial coordinates for 608 +extragalactic radio sources observed by VLBI. +

+The best optical realization of the ICRF currently available is the +Hipparcos catalogue. The extragalactic sources were not directly +observable by the Hipparcos satellite and so the link from Hipparcos +to ICRF was established through a variety of indirect techniques: VLBI and +conventional interferometry of radio stars, photographic astrometry +and so on. The Hipparcos frame is aligned to the ICRF to within about +0.5 mas and 0.5 mas/year (at epoch 1991.25). +

+The Hipparcos catalogue includes all of the FK5 stars, which has enabled +the orientation and spin of the latter to be studied. At epoch J2000, +the misalignment of the FK5 frame with respect to Hipparcos +(and hence ICRS) are about 32 mas and 1 mas/year respectively. +Consequently, for many practical purposes, including pointing +telescopes, the IAU 1976-1982 conventions on reference frames and +Earth orientation remain adequate and there is no need to change to +Hipparcos coordinates, new precession/nutation models and so on. +However, for the most exacting astrometric applications, SLALIB +provides some support for Hipparcos coordinates in the form of +four new routines: +sla_FK52H and +sla_H2FK5, +which transform FK5 positions and proper motions to the Hipparcos frame +and vice versa, and +sla_FK5HZ and +sla_HFK5Z, +where the transformations are for stars whose Hipparcos proper motion is +zero. +

+Further information on the ICRS can be found in the paper by M.Feissel +and F.Mignard, Astron.Astrophys. 331, L33-L36 (1988). +

+

+ +next + +up + +previous +
+ Next: Timescales +
+Up: EXPLANATION AND EXAMPLES +
+ Previous: Efficiency considerations +

+

+

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