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# Astronomical Constants – Some Useful Constants for Astronomy

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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

## 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
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%

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