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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

QuantitySymbolValueRelative
uncertainty
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

QuantitySymbolValueRelative
uncertainty
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
3.4×10-8
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
4.0×10-8
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

QuantitySymbolValueRelative
uncertainty
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)
328,900.561,400
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
Mercury6,023,600
Venus408,523.71
Earth + Moon328,900.561,400
Mars3,098,708
Jupiter1047.3486
Saturn3497.898
Uranus22,902.98
Neptune19,412.24
Pluto135,200,000

Other constants (outside the formal IAU System)

QuantitySymbolValueRelative
uncertainty
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

variable,

±0.1%

Tutorial Series

This post is part of the series Introduction to Astronomy. Use the links below to advance to the next tutorial in the couse, or go back and see the previous in the tutorial series.

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