Astronomical constants:
An astronomical constant is a physical constant used in astronomy. Formal sets of constants, along with recommended values, have been defined by the International Astronomical Union (IAU) several times: in 1964^{} and in 1976^{} (with an update in 1994^{}). 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^{} to not fix these values, but have the Working Group on Numerical Standards continuously maintain a set of Current Best Estimates.^{} 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.^{}
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 T_{eph} 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:^{}
Table of astronomical constants:
Quantity  Symbol  Value  Relative uncertainty 


Defining constants  
Gaussian gravitational constant  k  0.017 202 098 95 A^{3/2} S^{−1/2} D^{−1}  defined  
Speed of light  c  299 792 458 m s^{−1}  defined  
Mean ratio of the TT second to the TCG second  1 − L_{G}  1 − 6.969 290 134×10^{−10}  defined  
Mean ratio of the TCB second to the TDB second  1 − L_{B}  1 − 1.550 519 767 72×10^{−8}  defined  
Primary constants  
Mean ratio of the TCB second to the TCG second  1 − L_{C}  1 − 1.480 826 867 41×10^{−8}  1.4×10^{−9}  
Lighttime for unit distance  τ_{A}  499.004 786 3852 s  4.0×10^{−11}  
Equatorial radius for Earth  a_{e}  6.378 1366×10^{6} m  1.6×10^{−8}  
Potential of the geoid  W_{0}  6.263 685 60×10^{7} m^{2} s^{−2}  8.0×10^{−9}  
Dynamical formfactor for Earth  J_{2}  0.001 082 6359  9.2×10^{−8}  
Flattening factor for Earth  1/ƒ  0.003 352 8197 = 1/298.256 42 
3.4×10^{−8}  
Geocentric gravitational constant  GE  3.986 004 391×10^{14} m^{3} s^{−2}  2.0×10^{−9}  
Constant of gravitation  G  6.674 28×10^{−11} m^{3} kg^{−1} s^{−2}  1.0×10^{−4}  
Ratio of mass of Moon to mass of Earth  μ  0.012 300 0383 = 1/81.300 56 
4.0×10^{−8}  
General precession in longitude, per Julian century, at standard epoch 2000  ρ  5028.796 195″  *  
Obliquity of the ecliptic, at standard epoch 2000  ε  23° 26′ 21.406″  *  
Derived constants  
Constant of nutation, at standard epoch 2000  N  9.205 2331″  *  
Unit distance = cτ_{A}  A  149 597 870 691 m  4.0×10^{−11}  
Solar parallax = arcsin(a_{e}/A)  π_{☉}  8.794 1433″  1.6×10^{−8}  
Constant of aberration, at standard epoch 2000  κ  20.495 52″  
Heliocentric gravitational constant = A^{3}k^{2}/D^{2}  GS  1.327 2440×10^{20} m^{3} s^{−2}  3.8×10^{−10}  
Ratio of mass of Sun to mass of Earth = (GS)/(GE)  S/E  332 946.050 895  
Ratio of mass of Sun to mass of (Earth + Moon)  (S/E) (1 + μ) 
328 900.561 400  
Mass of Sun = (GS)/G  S  1.98855×10^{30} kg  1.0×10^{−4}  
System of planetary masses: Ratios of mass of Sun to mass of planet^{}  
Mercury  6 023 600  
Venus  408 523.71  
Earth + Moon  328 900.561 400  
Mars  3 098 708  
Jupiter  1047.3486  
Saturn  3497.898  
Uranus  22 902.98  
Neptune  19 412.24  
Pluto  135 200 000  
Other constants (outside the formal IAU System)  
Parsec = A/tan(1″)  pc  3.085 677 581 28×10^{16} m  4.0×10^{−11}  
Lightyear = 365.25cD  ly  9.460 730 472 5808×10^{15} m  defined  
Hubble constant  H_{0}  70.1 km s^{−1} Mpc^{−1}  0.019  
Solar luminosity  L_{☉}  3.939×10^{26} W = 2.107×10^{−15} S D^{−1} 
variable, ±0.1% 