# Astronomical Constants

By on in Astronomy

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Introduction to Astronomy Series

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.

QuantitySymbolValueRelative
uncertainty
Defining constants
Gaussian gravitational constantk0.017 202 098 95 A3/2S−1/2D−1defined
Speed of lightc299 792 458 m s−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 distanceιA499.004 786 3852 s4.0×10−11
Equatorial radius for Earthae6.378 1366×106 m1.6×10−8
Potential of the geoidW06.263 685 60×107 m2 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 constantGE3.986 004 391×1014 m3 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 2000ε23° 26′ 21.406″*
Derived constants
Constant of nutation, at standard epoch 2000N9.205 2331″*
Unit distance = AA149 597 870 691 m4.0×10−11
Solar parallax = arcsin(ae/A)π8.794 1433″1.6×10−8
Constant of aberration, at standard epoch 2000κ20.495 52″
Heliocentric gravitational constant = A3k2/D2GS1.327 2440×1020 m3 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)/GS1.9818×1030 kg1.0×10−4
System of planetary masses: Ratios of mass of Sun to mass of planet
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)
Parsec = A/tan(1")pc3.085 677 581 28×1016 m4.0×10−11
Light-year = 365.25cDly9.460 730 472 5808×1015 mdefined
Hubble constantH070.1 km s−1 Mpc−10.019
Solar luminosityL3.939×1026 W
= 2.107×10−15S D−1
variable,
±0.1%

Last updated on: Tuesday 16th January 2018