150 research outputs found
The astronomical units
The IAU-1976 System of astronomical constants includes three astronomical
units (i.e. for time, mass and length). This paper reports on the status of the
astronomical unit of length (ua) and mass (MSun) within the context of the
recent IAU Resolutions on reference systems and the use of modern observations
in the solar system. We especially look at a possible re-definition of the ua
as an astronomical unit of length defined trough a fixed relation to the SI
metre by a defining number.Comment: 2 pages, to be published in the Proceedings of the "Journees 2008
Systemes de reference spatio-temporels
Temporal variations of the gravity field and Earth precession-nutation
International audienceDue to the accuracy now reached by space geodetic techniques, and also considering some modelisations, the temporal variations of some Earth Gravity Field coefficients can be determined. They are due to Earth oceanic and solid tides, as well as geophysical reservoirs masses displacements. They can be related to the variations in the Earth's orientation parameters (through the inertia tensor). Then, we can try to improve our knowledge of the Earth Rotation with those space measurements of the Gravity variations. We have undertaken such a study, using data obtained with the combination of space geodetic techniques. In particular, we use CHAMP data that are more sensitive to such variations and that complete the ones already accumulated (for example with Starlette and LAGEOS I). In this first approach, we focus on the Earth precession nutation, trying to refine it by taking into account the temporal variations of the Earth dynamical flattening. The goal is mainly to understand how Geodesy can influence this field of science. Like this, we will be able to compare our computation with up to date determinations of precession nutation
Precession, nutation, and space geodetic determination of the Earth's variable gravity field
Precession and nutation of the Earth depend on the Earth's dynamical
flattening, H, which is closely related to the second degree zonal coefficient,
J2 of the geopotential. A small secular decrease as well as seasonal variations
of this coefficient have been detected by precise measurements of artificial
satellites (Nerem et al. 1993; Cazenave et al. 1995) which have to be taken
into account for modelling precession and nutation at a microarcsecond accuracy
in order to be in agreement with the accuracy of current VLBI determinations of
the Earth orientation parameters. However, the large uncertainties in the
theoretical models for these J2 variations (for example a recent change in the
observed secular trend) is one of the most important causes of why the accuracy
of the precession-nutation models is limited (Williams 1994; Capitaine et al.
2003). We have investigated in this paper how the use of the variations of J2
observed by space geodetic techniques can influence the theoretical expressions
for precession and nutation. We have used time series of J2 obtained by the
"Groupe de Recherches en G\'eod\'esie spatiale" (GRGS) from the precise orbit
determination of several artificial satellites from 1985 to 2002 to evaluate
the effect of the corresponding constant, secular and periodic parts of H and
we have discussed the best way of taking the observed variations into account.
We have concluded that, although a realistic estimation of the J2 rate must
rely not only on space geodetic observations over a limited period but also on
other kinds of observations, the monitoring of periodic variations in J2 could
be used for predicting the effects on the periodic part of the
precession-nutation motion
Units of relativistic time scales and associated quantities
This note suggests nomenclature for dealing with the units of various
astronomical quantities that are used with the relativistic time scales TT,
TDB, TCB and TCG. It is suggested to avoid wordings like "TDB units" and "TT
units" and avoid contrasting them to "SI units". The quantities intended for
use with TCG, TCB, TT or TDB should be called "TCG-compatible",
"TCB-compatible", "TT-compatible" or "TDB-compatible", respectively. The names
of the units second and meter for numerical values of all these quantities
should be used with out any adjectives. This suggestion comes from a special
discussion forum created within IAU Commission 52 "Relativity in Fundamental
Astronomy"
A First Assessment of the Corrections for the Consistency of the IAU2000 and IAU2006 Precession-Nutation Models
The Earth precession-nutation model endorsed by resolutions of each the International Astronomical Union and the International Union of Geodesy and Geophysics is composed of two theories developed independently, namely IAU2006 precession and IAU2000A nutation. The IAU2006 precession was adopted to supersede the precession part of the IAU 2000A precession-nutation model and tried to get the new precession theory dynamically consistent with the IAU2000A nutation. However, full consistency was not reached, and slight adjustments of the IAU2000A nutation amplitudes at the micro arcsecond level were required to ensure consistency. The first set of formulae for these corrections derived by Capitaine et al. (Astrophys 432(1):355–367, 2005), which was not included in IAU2006 but provided in some standards and software for computing nutations. Later, Escapa et al. showed that a few additional terms of the same order of magnitude have to be added to the 2005 expressions to get complete dynamical consistency between the official precession and nutation models. In 2018 Escapa and Capitaine made a joint review of the problem and proposed three alternative ways of nutation model and its parameters to achieve consistency to certain different extents, although no estimation of their respective effects could be worked out to illustrate the proposals. Here we present some preliminary results on the assessment of the effects of each of the three sets of corrections suggested by Escapa and Capitaine (Proceedings of the Journées, des Systémes de Référence et de la Rotation Terrestre: Furthering our Knowledge of Earth Rotation, Alicante, 2018) by testing them in conjunction with the conventional celestial pole offsets given in the IERS EOP14C04 time series.The four first authors were partially supported by Spanish Project AYA2016-79775-P (AEI/FEDER, UE)
The influence of Galactic aberration on precession parameters determined from VLBI observations
The influence of proper motions of sources due to Galactic aberration on
precession models based on VLBI data is determined. Comparisons of the linear
trends in the coordinates of the celestial pole obtained with and without
taking into account Galactic aberration indicate that this effect can reach 20
as per century, which is important for modern precession models. It is
also shown that correcting for Galactic aberration influences the derived
parameters of low-frequency nutation terms. It is therefore necessary to
correct for Galactic aberration in the reduction of modern astrometric
observations
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