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IERS Message No. 135 November 05, 2008
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Joint Discussion 6: Time and Astronomy
XXVII IAU GENERAL ASSEMBLY, Rio de Janeiro
Joint Discussion 6: Time and Astronomy
Thursday 6 & Friday 7 - August 2009
Coordinating IAU division:
I: Fundamental Astronomy
X: Radio Astronomy
Participating Commissions:
31: Time
4: Ephemerides
7: Celestial Mechanics and Dynamical Astronomy
8: Astrometry
19: Rotation of the Earth
52: Relativity in Fundamental Astronomy
40: Radio Astronomy
Scientific Organizing Committee:
Pascale Defraigne (Belgium, Chair)
Aleksander Brzezinski (Poland, Co-chair, president Com 19)
Richard Manchester (Australia)
Sergei Klioner (Germany, President Com 52)
Gambis Daniel (France)
Rendong Nan (China, President Com 40)
Demetrios Matsakis (US)
Michael Kramer (UK)
Yury Ilyasov (Russia)
Gerard Petit (BIPM)
Contact Person:
Pascale Defraigne
Royal Observatory of Belgium
Avenue Circulaire, 3
B-1180 Brussels
phone: +32-2-373 02 60
fax: +32-2-374 98 22
e-mail: p.defraigne@oma.be
URL: http://www.astro.oma.be/IAU/COM31/jd6.php
Preliminary scientific program summary:
The objective of this JD is to introduce the astronomical community to
the state of the art in the aspects of time, its use for astronomy, and
the contributions from astronomy. The JD will provide a forum for
discussion of recent work on these topics, and will be organized in
three sessions:
Earth Rotation and Time: an overview of UT1 determination as well as UT1
modeling and prediction.
Atomic Time Scales: the present realizations and performance of atomic
time scales and time transfer techniques.
Pulsar Timing and its Applications: recent developments in precision
pulsar timing and its application to time scales, planetary ephemerides,
detection of gravitational waves and tests of gravitational theories.
Draft Program:
Earth Rotation and Time
- UT1/LOD realization and accuracy
- Geophysical impact on UT1
- Short- and long-term variations of UT1
- Use of UT1 for astro-geodetic studies
- UT1 prediction
Atomic Time scales
- Stability of TAI, TT(BIPM)
- Impact of new frequency standards on SI time scales
- GNSS time scales
- Time transfer
- Recent Developments with Leap Seconds
Pulsar Timing and its Applications
- Techniques of precision pulsar timing
- Pulsar time scales
- Pulsar tests of gravitational theories
- Pulsar Timing Arrays and detection of gravitational waves
- Improving solar-system ephemeredes
- Pulsar Timing Noise
Abstract submission:
The abstracts must be submitted before March 1st, 2009,
via the website of the IAU General Assembly:
http://www.astronomy2009.com.br/
sub-menu "Instructions for Presenters"
Detailed scientific rationale:
Responsibility for the definition of time scales left the astronomical
community some 40 years ago when, in 1967, the second became defined by
an atomic transition in the International System of units SI and when
TAI was defined as the primary international time scale in 1971. Atomic
time is now 107 times more stable than the Earth rotation and some 104
times more stable than the planetary orbital motions that were used to
define time until 1967.
But time still interacts with astronomy in many ways: as the independent
variable for the description of all dynamical systems, its stability
allows one to study these systems and their perturbations. Time is
therefore of major importance for astronomers, with time scales based on
the SI second for practical applications and coordinate time scales for
theoretical developments. Precise timing of the rotational and orbital
periods of pulsars has the potential to contribute to the long-term
stability of International Atomic Time (TAI), thereby returning some
aspects of time keeping to astronomy. Furthermore, since observational
techniques rely on the measurement of the time of propagation of
electromagnetic signals, astronomy provides an important testing ground
for relativity.
Although universal time is not used any more to provide the official
time, it still reflects the Earth rotation and dictates the leap seconds
and hence UTC, the international reference for time and frequency. The
future of leap seconds is presently still a subject of debate led by the
ITU-R Special Rapporteur Group. In parallel, the knowledge of UT1 is
needed for any coordinate transformation between terrestrial and
celestial reference systems. Together with the atomic time, UT1 serves
precise orbit determination for both space research and Earth studies.
The recent measurements of multiple space geodetic techniques,
individually or in combination, provide high precision monitoring of LOD
and UT1, and allow predictions as needed for real time applications.
This improved accuracy on the UT1/LOD measurements also brings a new set
of constraints for geophysical models.
The present realizations of the Terrestrial Time TT are the
International Atomic Time TAI provided by the BIPM on a monthly basis,
and TT(BIPM) based on the reprocessing using all primary frequency
standard measurements. TT(BIPM) is presently the best reference to
estimate the long-term stability of any other time scale, and in
particular the long-term stability of pulsars. Thanks to the recent
development of new frequency standards, and to the parallel progress in
time scale algorithms and in time transfer techniques, the present
atomic time scale stabilities and accuracies has been significantly
improved.
Pulsars are incredibly stable clocks with a stability that rivals the
best atomic clocks. Many pulsars are members of a binary system,
enabling very precise measurement of the gravitational interactions
between massive bodies and tests of gravitational theories. Such
measurements have already given the most precise verification of
Einstein's general relativity in the strong-field case. Future
observations will reveal higher-order gravitational perturbations,
providing even more stringent tests of gravitational theories.
Recent developments of "pulsar timing arrays", in which a large sample
of millisecond pulsars are regularly timed, have the potential to
establish a time scale which is more stable than the best atomic
timescales over long time intervals and hence to investigate the
stability of these time scales. They also are sensitive to errors or
omissions in the planetary ephemeredes used to model the Earth’s motion
and hence to improve those ephemeredes. Finally, they have the potential
to make a direct detection of gravitational waves from astronomical
sources. Such measurements would open up a new window on the Universe
and provide important constraints on models for the early Universe and
galaxy evolution.
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