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Paper: Changes in Reference Systems and Time
Volume: 338, Astrometry in the Age of the Next Generation of Large Telescopes
Page: 349
Authors: Seidelmann, P.K.
Abstract: The International Celestial Reference System (ICRS) has been introduced as recommended by the International Astronomical Union (IAU). This is a fixed reference system that is epoch independent. This is based on Earth kinematics rather than the solar system dynamics. It is implemented by a reference frame, the International Celestial Reference Frame (ICRF), determined from distant radio sources, that are not moving. The optical implementation is based on those Hipparcos Catalog stars which do not have known problems, and has been oriented to the ICRF.

Since most observations are made from the surface of the Earth, which has many motions, it is still necessary to use a moving, time dependent reference frame. There is a new precession-nutation model, IAU 2000A, with submilliarcsecond accuracies. Based on the new precession-nutation model, the Celestial Intermediate Pole (CIP) has been introduced to replace the Celestial Ephemeris Pole. A new fiducial point, the Celestial Intermediate Origin (CIO), based on the non-rotating origin, has been introduced as a possible replacement for the equinox. Since observations now permit the determination of sub-daily periodic motions in the Earth's pole, arbitrarily all periodic motions with periods less than two days are considered polar motion.

Some additional changes are the introduction of geodesic precession and nutation in the precession-nutation model, the detection of free core nutation, although that is only represented in observational data and not included in the precession-nutation model. There are continuing discussions concerning the definitions and terminology for some aspects of the new system.

Significantly, there is a proposal to redefine UTC such that there would no longer be leap seconds, but only leap hours. This means that the difference between UTC and UT1 would grow continuously and software and applications would require the introduction of both UTC and UT1 as separate quantities.

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