Astronomical constant

An astronomical constant is any of several physical constants used in astronomy. Formal sets of constants, along with recommended values, have been defined by the International Astronomical Union (IAU) several times: in 1964[1] and in 1976[2] (with an update in 1994[3]). In 2009 the IAU adopted a new current set, and recognizing that new observations and techniques continuously provide better values for these constants, they decided[4] to not fix these values, but have the Working Group on Numerical Standards continuously maintain a set of Current Best Estimates.[5] The set of constants is widely reproduced in publications such as the Astronomical Almanac of the United States Naval Observatory and HM Nautical Almanac Office.

Besides the IAU list of units and constants, also the International Earth Rotation and Reference Systems Service defines constants relevant to the orientation and rotation of the Earth, in its technical notes.[6]

The IAU system of constants defines a system of astronomical units for length, mass and time (in fact, several such systems), and also includes constants such as the speed of light and the constant of gravitation which allow transformations between astronomical units and SI units. Slightly different values for the constants are obtained depending on the frame of reference used. Values quoted in barycentric dynamical time (TDB) or equivalent time scales such as the Teph of the Jet Propulsion Laboratory ephemerides represent the mean values that would be measured by an observer on the Earth's surface (strictly, on the surface of the geoid) over a long period of time. The IAU also recommends values in SI units, which are the values which would be measured (in proper length and proper time) by an observer at the barycentre of the Solar System: these are obtained by the following transformations:[3]

Astronomical system of units

The astronomical unit of time is a time interval of one day (D) of 86400 seconds. The astronomical unit of mass is the mass of the Sun (S). The astronomical unit of length is that length (A) for which the Gaussian gravitational constant (k) takes the value 0.01720209895 when the units of measurement are the astronomical units of length, mass and time.[2]

Table of astronomical constants

QuantitySymbolValueRelative
uncertainty
Ref.
Defining constants
Gaussian gravitational constantk0.01720209895 A3/2S1/2D1defined[2]
Speed of lightc299792458 ms1defined[7]
Mean ratio of the TT second to the TCG second1 LG1 6.969290134×1010defined[8]
Mean ratio of the TCB second to the TDB second1 LB1 1.55051976772×108defined[9]
Primary constants
Mean ratio of the TCB second to the TCG second1 LC1 1.48082686741×1081.4×109[8]
Light-time for unit distanceτA499.0047863852 s4.0×1011[10][11]
Equatorial radius for Earthae6.3781366×106 m1.6×108[11]
Potential of the geoidW06.26368560×107 m2s28.0×109[11]
Dynamical form-factor for EarthJ20.00108263599.2×108[11]
Flattening factor for Earth1/ƒ0.0033528197
= 1/298.25642
3.4×108[11]
Geocentric gravitational constantGE3.986004391×1014 m3s22.0×109[10]
Constant of gravitationG6.67428×1011 m3kg1s21.0×104[12]
Ratio of mass of Moon to mass of Earthμ0.0123000383
= 1/81.30056
4.0×108[10][11]
General precession in longitude, per Julian century, at standard epoch 2000ρ5028.796195″*[13]
Obliquity of the ecliptic, at standard epoch 2000ε23° 26′ 21.406″*[13]
Derived constants
Constant of nutation, at standard epoch 2000N9.2052331″*[14]
Unit distance = AA149597870691 m4.0×1011[10][11]
Solar parallax = arcsin(ae/A)π8.7941433″1.6×108[2]
Constant of aberration, at standard epoch 2000κ20.49552″[2]
Heliocentric gravitational constant = A3k2/D2GS1.3272440×1020 m3s23.8×1010[11]
Ratio of mass of Sun to mass of Earth = (GS)/(GE)S/E332946.050895[10]
Ratio of mass of Sun to mass of (Earth + Moon)(S/E)
(1 + μ)
328900.561400[10]
Mass of Sun = (GS)/GS1.98855×1030 kg1.0×104[2]
System of planetary masses: Ratios of mass of Sun to mass of planet[10]
Mercury6023600
Venus408523.71
Earth + Moon328900.561400
Mars3098708
Jupiter1047.3486
Saturn3497.898
Uranus22902.98
Neptune19412.24
Pluto135200000
Other constants (outside the formal IAU System)
Parsec = A/tan(1")pc3.08567758128×1016 m4.0×1011[15]
Light-year = 365.25cDly9.4607304725808×1015 mdefined[15]
Hubble constantH070.1 kms1Mpc10.019[16]
Solar luminosityL3.939×1026 W
= 2.107×1015 SD1
variable,
±0.1%
[17]
Notes

* The theories of precession and nutation have advanced since 1976, and these also affect the definition of the ecliptic. The values here are appropriate for the older theories, but additional constants are required for current models.

† The definitions of these derived constants have been taken from the references cited, but the values have been recalculated to take account of the more precise values of the primary constants cited in the table.

References

  1. Resolution No.4 of the XIIth General Assembly of the International Astronomical Union, Hamburg, 1964.
  2. Resolution No. 1 on the recommendations of Commission 4 on ephemerides in the XVIth General Assembly of the International Astronomical Union, Grenoble, 1976.
  3. Standish, E. M. (1995), "Report of the IAU WGAS Sub-group on Numerical Standards", in Appenzeller, I. (ed.), Highlights of Astronomy (PDF), Dordrecht: Kluwer
  4. Resolution B2 of the XXVIIth General Assembly of the International Astronomical Union, Rio de Janeiro, 2009.
  5. IAU Division I Working Group on Numerical Standards for Fundamental Astronomy and Astronomical Constants: Current Best Estimates (CBEs) Archived 2016-08-26 at the Wayback Machine
  6. Gérard Petit; Brian Luzum, eds. (2010). "Table 1.1: IERS numerical standards" (PDF). IERS technical note no. 36: General definitions and numerical standards. International Earth Rotation and Reference Systems Service. For complete document see Gérard Petit; Brian Luzum, eds. (2010). IERS Conventions (2010): IERS technical note no. 36. International Earth Rotation and Reference Systems Service. ISBN 978-3-89888-989-6.
  7. International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), pp. 112–13, ISBN 92-822-2213-6, archived (PDF) from the original on 2017-08-14.
  8. Resolutions Nos. B1.5 and B1.9 of the XXIVth General Assembly of the International Astronomical Union, Manchester, 2000.
  9. Resolution 3 of the XXVIth General Assembly of the International Astronomical Union, Prague, 2006.
  10. Standish, E. M. (1998), JPL Planetary and Lunar Ephemerides, DE405/LE405 (PDF), JPL IOM 312.F-98-048, archived from the original (PDF) on February 20, 2012
  11. McCarthy, Dennis D.; Petit, Gérard, eds. (2004), "IERS Conventions (2003)", IERS Technical Note No. 32, Frankfurt: Bundesamts für Kartographie und Geodäsie, ISBN 3-89888-884-3
  12. IAU2012
  13. Resolution 1 Archived 2020-04-06 at the Wayback Machine of the XXVIth General Assembly of the International Astronomical Union, Prague, 2006.
  14. Resolution No. B1.6 of the XXIVth General Assembly of the International Astronomical Union, Manchester, 2000.
  15. The IAU and astronomical units, International Astronomical Union
  16. How Fast is the Universe Expanding?, NASA, 2008
  17. Noedlinger, Peter D., "Solar Mass Loss, the Astronomical Unit, and the Scale of the Solar System", Celest. Mech. Dyn. Astron., arXiv:0801.3807, Bibcode:2008arXiv0801.3807N
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