The constants in this table are based originally on the set of constants adopted by the International Astronomical Union (IAU) in 1976. Updates have been made when new data were available (Refs. 1-8), with the latest updates being 2021. The left-most column of the table gives the most current reference for the cited value. All values are given in SI units; thus, masses are expressed in kilograms and distances in meters or kilometers as appropriate.
The astronomical unit of time is a time interval of one day (1 d) equal to 86400 s. An interval of 36525 d is one Julian century (1 cy). See Refs. 5-7 for an explanation of the way in which the various distance units used in astronomy are related.
Data on the Sun, the Earth, and other planets in the solar system are found in “Properties of the Solar System” in this section. Column definitions for the table are as follows.
Column heading | Definition |
Quantity | Name of astronomical constant |
Symbol | Constant symbol |
Value | Value of astronomical constant |
Uncertainty | Uncertainty in value of constant |
Units | Units for value and uncertainty |
Ref. | Reference |
Quantity | Symbol | Value | Uncertainty | Units | Ref. |
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Defining constants | |||||
Astronomical unit | au | 149 597 870 700 | exact | m | 1 |
Speed of light | c | 299 792 458 | exact | m s–1 | 1 |
Primary constants | |||||
Light-time for unit distance (1 au) | τA | 499.004 783 836 ... | exact | s | 2 |
Equatorial radius of Earth | ae = 6 378 136.6 m ± 0.1 | ae = 6 378 136.6 m ± 0.1 | |||
Polar radius of Earth | ap = 6 356 752 m | ap = 6 356 752 m | |||
Dynamical form factor for Earth | J2 = 0.001 082 635 9 ± 1 × 10-10 | J2 = 0.001 082 635 9 ± 1 × 10-10 | |||
Geocentric gravitational constant | GE | 3.986 004 418 × 1014 | ± 8 × 105 | m3 s–2 | 2 |
Constant of gravitation | G | 6.674 28 × 10–11 | ± 000 067 × 10–11 | m3 kg–1 s–2 | 2 |
Ratio of mass of moon to that of Earth | µ = 0.012 300 0371 ± 4 × 10-10 | µ = 0.012 300 0371 ± 4 × 10-10 | |||
Ratio of mass of Earth to that of the moon | 1/µ = 81.300 568 ± 3 × 10-6 | 1/µ = 81.300 568 ± 3 × 10-6 | |||
General precession in longitude, per Julian century, at standard epoch J2000 | ρ = 5028.796 195 arcsec/Julian Century | ρ = 5028.796 195 arcsec/Julian Century | |||
Obliquity of the ecliptic at standard epoch J2000 | ε | 23°26′21″.448 | 2 | ||
Derived constants | |||||
Constant of nutation at standard epoch J2000 | N | 9″.2052 331 | 2 | ||
Solar parallax (π0 = arcsin(ae/ua)) | π0 | 8″.794 143 | 2 | ||
Constant of aberration for standard epoch J2000 | κ | 20″.495 51 | 2 | ||
Flattening factor for Earth | ƒ = 1/298.256 42 = 0.003352820 | ƒ = 1/298.256 42 = 0.003352820 | |||
Heliocentric gravitational constant (GS = A3k2/D2) | GS | 1.327 124 400 18 × 1020 | ± 8 × 109 | m3 s–2 | 3 |
GS | 1.327 124 400 42 × 1020 | ± 10 × 109 | m3 s–2 | 4 | |
Derived constants | |||||
Ratio of mass of sun to that of Earth (S/E) = (GS)/(GE)) | S/E = 332 946.048 7 | S/E = 332 946.048 7 |