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The IAU system of constants defines a system of astronomical units for length, mass and time, and also includes astronomical constants such as the speed of light and the constant of gravitation which allow transformations between astronomical units and SI units.
Introduction to Astronomy Series
  1. Introduction to Astronomy
  2. The Celestial Sphere - Right Ascension and Declination
  3. What is Angular Size?
  4. What is the Milky Way Galaxy?
  5. The Astronomical Magnitude Scale
  6. Sidereal Time, Civil Time and Solar Time
  7. Equinoxes and Solstices
  8. Parallax, Distance and Parsecs
  9. Luminosity and Flux of Stars
  10. Kepler's Laws of Planetary Motion
  11. What Are Lagrange Points?
  12. Glossary of Astronomy & Photographic Terms
  13. Astronomical Constants - Some Useful Constants for Astronomy

Defining constants

Gaussian gravitational constantk0.017,202,098,95 A3/2,S-1/2,D-1defined
Speed of lightc299,792,458 ms-1defined
Mean ratio of the TT second to the TCG second1 - LG1 - 6.969,290,134×10-10defined
Mean ratio of the TCB second to the TDB second1 - LB1 - 1.550,519,767,72×10-8defined

Primary constants

Mean ratio of the TCB second to the TCG second1 - LC1 - 1.480,826,867,41×10-81.4×10-9
Light-time for unit distance?A499.004,786,3852 s4.0×10-11
Equatorial radius for Earthae6.378,1366×106m1.6×10-8
Potential of the geoidW06.263,685,60×107m2,s-28.0×10-9
Dynamical form-factor for EarthJ20.001,082,63599.2×10-8
Flattening factor for Earth1/ƒ0.003,352,8197
= 1/298.256,42
Geocentric gravitational constantGE3.986,004,391×1014m3,s-22.0×10-9
Constant of gravitationG6.673,84×10-11 m3,kg-1,s-21.2×10-4
Ratio of mass of Moon to mass of Earthµ0.012,300,0383
= 1/81.300,56
General precession in longitude, per Julian century, at standard epoch 2000Ρ5029.796,195?*
Obliquity of the ecliptic, at standard epoch 2000ε23ε 26' 21.406?*

Derived constants

Constant of nutation, at standard epoch 2000N9.205,2331?*
Unit distance = c?AA149,597,870,691 m4.0×10-11
Solar parallax = arcsin(ae/A)p?8.794,1433?1.6×10-8
Constant of aberration, at standard epoch 2000κ20.495,52?
Heliocentric gravitational constant = A3k2/D2GS1.327,2440×1020m3,s-23.8×10-10
Ratio of mass of Sun to mass of Earth = (GS)/(GE)S/E332,946.050,895
Ratio of mass of Sun to mass of (Earth + Moon)(S/E)
(1 + μ)
Mass of Sun = (GS)/GS1.9818×1030kg1.0×10-4

System of planetary masses & Ratios of mass of Sun to mass of planet

PlanetRatios of mass
Earth + Moon328,900.561,400

Other constants (outside the formal IAU System)

Parsec = A/tan(1")pc3.085,677,581,28×1016 m4.0×10-11
Light-year = 365.25cDly9.460,730,472,5808×1015mdefined
Hubble constantH070.1 km,s-1,Mpc-10.019
Solar luminosityL?3.939×1026 W

= 2.107×10-15S,D-1



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