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c = 2.997 924 58 x 10^{8} m s^{−1} 
c 
Status: 
 Natural defining constant.
 IAU 2009 adopted constant.
 Adopted CBE 2009 August 10.
 A constant fixed by convention.

Notes: 
 The speed of light is an (unchanged) defining value for the IAU
and the IERS. The reference has been updated from CODATA 1998
to CODATA 2006 to reflect the use of the value of this
constant by the current set of CODATA constants.

References: 
 CODATA 2006,
physics.nist.gov/cuu/Constants
 Mohr, P. J. and Taylor, B. N., 2000,
"CODATA recommended values of the fundamental physical constants: 1998,"
Rev. Mod. Phys., 72, pp. 351495.
 Mohr, P. J., Taylor, B. N., and Newell, D. B., 2008,
"The CODATA recommended values of the fundamental physical constants: 2006,"
Rev. Mod. Phys., 80, pp. 633730.

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1−d(TT)/d(TCG) 
L_{G} = 6.969 290 134 x 10^{−10} 
L_{G} 
Status: 
 Auxiliary defining constant.
 IAU 2009 adopted constant.
 Adopted CBE 2009 August 10.
 A constant fixed by convention

Notes: 
 The value for L_{G} is such that the mean rate of
TT is close to the mean rate of the proper time of an observer located
on the rotating geoid.
 It is specified in IAU 2000 Resolution B1.9 as a defining constant.

References: 
 International Astronomical Union (IAU), 2000,
"Proceedings of the TwentyFourth General Assembly,"
Transactions of the IAU, XXIVB, pp. 3457.
 Petit, G., 2000,
"Report of the BIPM/IAU Joint Committee on relativity for spacetime
reference systems and metrology," in
Proc. of IAU Colloquium 180,
Johnston, K. J., McCarthy, D. D., Luzum, B. J., Kaplan, G. H. (eds.),
U.S. Naval Observatory, Washington, D.C., pp. 275282.

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1−d(TDB)/d(TCB) 
L_{B} = 1.550 519 768 x 10^{−8} 
L_{B} 
Status: 
 Auxiliary defining constant.
 IAU 2009 adopted constant.
 Adopted CBE 2009 August 10.
 A constant fixed by convention

Notes: 
 L_{B} is specified in
IAU 2006 Resolution B3 as a defining constant.
 The fixed value for L_{B} was derived in 2005 from
the expression
L_{B} =
L_{C} +
L_{G} −
L_{C} x L_{G},
where L_{G} is given in
IAU Resolution B1.9 (2000) and L_{C}
has been determined (Irwin and Fukushima, 1999) using the JPL ephemeris
DE405. When using the DE405, the defining L_{B} value
effectively eliminates a linear drift between TDB and TT, evaluated at
the geocenter.
 When realizing TCB using other ephemerides, the difference between TDB and
TT, evaluated at the geocenter, may include some linear drift, not expected
to exceed 1 ns per year.

References: 
 International Astronomical Union (IAU), 2006
"Proceedings of the TwentySixth General Assembly,"
Transactions of the IAU, XXVIB.
 Irwin, A. and Fukushima, T., 1999,
"A numerical time ephemeris of the Earth,"
Astron. Astrophys., 348, pp. 642652.

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TDB − TCB at T_{0} 
TDB_{0} = −6.55 x 10^{−5} s 
TDB_{0} 
Status: 
 Auxiliary defining constant.
 IAU 2009 adopted constant.
 Adopted CBE 2009 August 10.
 A constant fixed by convention

Notes: 
 TDB − TCB, the difference between TDB and TCB at
T_{0} (2443144.5003725) is specified in IAU 2006 Resolution B3
as a defining constant.
 TDB = TCB − L_{B} x
(JD_{TCB} − T_{0})
x 86400 + TDB_{0},
where T_{0} = 2443144.5003725.
 The value for TDB_{0} is chosen to provide consistency with the
TDB−TT formula of Fairhead and Bretagnon (1990).

References: 
 International Astronomical Union (IAU), 2006
"Proceedings of the TwentySixth General Assembly,"
Transactions of the IAU, XXVIB.
 Fairhead, L. and Bretagnon, P., 1990,
"An Analytical Formula for the Time Transformation TBTT,"
Astron. Astrophys., 229, pp. 240247.

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Astronomical unit 
au = 1.495 978 707 00 x 10^{11} m 
au 
Status: 
 Auxiliary defining constant.
 Adopted 2012 August 30, IAU GA 2012 Resolution B2
 A constant fixed by convention.

Notes: 
 The IAU GA 2012 Resolution B2
fixes the value of the astronomical unit (au) in kilometres.
The definition states that:
 The astronomical unit is to be used with all time
scales TCB, TDB, TCG, TT, etc.,
 the Gaussian gravitational constant k be deleted from the
system of astronomical constants,
 the value of the solar mass parameter (previously called the
heliocentric gravitational constant), GMS, be determined
observationally in SI units,
 the unique symbol au be used for the astronomical unit.
 This value is the IAU 2009 value adopted 2009 August and it was
taken from Pitjeva and Standish (2009) and is an average of
recent estimates for the au defined by
k.

References: 
 The IAU GA 2012 Resolution B2
 Pitjeva, E.V. and Standish, E.M., 2009,
"Proposals for the masses of the three largest asteroids,
the MoonEarth mass ratio and the astronomical unit,"
Celest. Mech. Dyn. Astr., 103, pp. 365372,
doi: 10.1007/s1056900992038.

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Earth Rotation Angle at J2000.0
Rate of advance of Earth Rotation Angle 
θ_{0} = 0.779 057 273 2640 revolutions
dθ/dUT1 = 1.002 737 811 911 354 48 revolutions UT1day^{−1} 
θ_{0}
dθ/dUT1 
Status: 
 Auxiliary defining constants.
 IAU 2009 adopted constant.
 Adopted CBE 2009 August 10.
 Costants fixed by convention

Notes: 
 These values are specified in
IAU 2000 Resolution B1.8 as defining constants.
 They come from the expression
θ(UT1) = 2π (0.779 057 273 2640 + 1.002 737 811 911 354 48
(Julian UT1 date − 2451545.0))

References: 
 International Astronomical Union (IAU), 2000,
"Proceedings of the TwentyFourth General Assembly,"
Transactions of the IAU, XXIVB, pp. 3457.
 Capitaine, N., Guinot, B., and McCarthy, D.D., 2000,
"Definition of the Celestial Ephemeris Origin and of UT1
in the International Celestial Reference Frame,"
Astron. Astrophys., 355, pp.398405.

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