Astronomical Constants
v.20240327
Units |
IAU Standards |
Derived and older constants |
Other Constants and Formulas |
Precession |
Mean periods |
the Earth |
the Moon |
the Sun |
Units of Length |
the Milky Way |
the Universe |
References |
The units meter (m), second (s) and kilogram (kg) are the units of length, time and mass in the International System of Units (SI) (ref.[1] ).
The astronomical units of length, time and mass are (ref.[11] ):
A = astronomical unit of length; approximately equal to the mean radius of the orbit of the Earth.
D = astronomical unit of time; equal to one day of 86400 SI seconds.
S = astronomical unit of mass; equal to the mass of the Sun.
The International Astronomical Union (IAU) maintains a system of definitions and current best estimates of astronomical constants. See the Transactions (ref.[2] ) and Resolutions (ref.[3] ). Below is a selection of best values as known at the end of 2021 (ref.[4] ). For astrophysical use fixed rounded nominal values have been established (ref.[5] ).
speed of light
c = 299 792 458 m/s
astronomical unit
au = A = 149 597 870 700 m
rotation angle of the Earth on J2000.0 UT1
θ0 = 0.779 057 273 2640 rev.
change in rotation angle of the Earth
dθ/dUT1 = 1.002 737 811 911 354 48 rev./UT1-day
Newtonion constant of gravitation
G = 6.674 28 (±67) ×10–11 m3 /kg/s2
newer value from CODATA 2018:
6,674 30 (±15) ×10–11 m3 /kg/s2
solar mass parameter (heliocentric gravitational constant) GMS
TDB compatible value:
= A3 k2 /D2 = 1.327 124 400 41×1020 m3 /s2
equatorial radius of the Earth
ae = 6 378 136.6 m
dynamical form factor of the Earth (zero-frequency tide model)
J2 = 0.001 082 6359
secular change of J2 per century
dJ2 = −3.0×10–9 /cy
geocentric gravitational constant GME
TT compatible value:
GME = 3,986 004 415×1014 m3 /s2
TDB compatible value:
= 3,986 004 356×1014 m3 /s2
potential of the geoid
W0 = 62 636 856.0 J/kg = m2 /s2
nominal mean angular velocity of the Earth
TT compatible value:
ω = 7.292 115×10–5 rad/s
mass ratio of the Moon to the Earth
μ = MM /ME = 0.012 300 0371
Derived and older constants
Gaussian gravitation constant
k = 0,017 202 098 95
light time for 1 a.u.
τA = A/c = 499.004 783 84 s
ratio of mass of the Sun to the Earth
(GMS )/(GME ) = S/E = 332 946.0487
ratio of the mass of the Earth to the Moon
ME /MM = 1/μ = 81.300 5678
ratio of the mass of the Sun to the combined mass of Earth and Moon
(S/E)/(1+μ) = 328 900.5596
Solar mass
(GMS )/G = S = 1.9884×1030 kg
Earth mass
(GME )/G = E = 5.9722×1024 kg
standard acceleration of gravity
gn = (GME )/(RE 2 )= 9.806 65 m/s2
solar parallax
arcsin(ae /A) = π = 8.794 143"
constant of aberration for epoch J2000.0
κ = 20.495 52"
polar flattening factor of the Earth (zero-frequency tide model)
f = 0.003 352 8197 = 1/298.256 42
In the formulae below, T is the time passed since J2000.0 (= JD 2 451 545.0 TDB) measured in Julian centuries of 36 525 days.
Sources: [19a] , [20] , [21] .
annual general precession
pA = 50.287 961 95" + 0.022 108 696" ×T
annual precession in right ascension
m = 3.074 773 605s + 0.001 855 4463s×T
annual precession in declination
n = 20.041 919 03" − 0.008 589 868" ×T
obliquity of the ecliptic
εA = 84 381.406" − 46.836 769"×T
Source: [18a]
mean solar day expressed in mean sidereal time:
1.002 737 909 344 99d + 59.0107d×10-12 ×T
= 24h03m56.555 367s + 0.000 0510s×T
mean sidereal day expressed in mean solar time:
0.997 269 566 334 86d − 58.6888d×10-12 ×T
= 23h56m04.090 531s − 0.000 0507s×T
sidereal rotation period of the Earth expressed in mean solar time:
dUT1/dθ =
0,997 269 663 237 157d
= 23h56m04.098 904s
Source: [36a]
sidereal month (from fixed star to same fixed star):
27.321 661 554d + 0.000 000 216d×T
= 27d07h43m11.558s + 0.019s×T
anomalistic month (from perigee to perigee):
27.554 549 886d − 0.000 001 007d×T
= 27d13h18m33.110s − 0.087s×T
tropical month (from aequinox to same aequinox):
27.321 582 252d + 0.000 000 182d×T
= 27d07h43m04.707s + 0.016s×T
draconitic month (from node to same node):
27.212 220 815d + 0.000 000 414d×T
= 27d05h05m35.878s + 0.036s×T
synodic month (from a phase to the same phase):
29.530 588 861d + 0.000 000 252d×T
= 29d12h44m02.878s + 0.022s×T
365.25d
= 365d06h00m00.000s
sidereal year (from fixed star to same fixed star):
365.256 362 95d + 0.000 000 11d×T
= 365d06h09m09.759s + 0.010s×T
anomalistic year (from apside to same apside):
365.259 635 77d + 0.000 003 12d×T
= 365d06h13m52.531s + 0.270s×T
tropical year (mean, from aequinox to same aequinox):
365.242 190 42d − 0.000 006 15d×T
= 365d05h48m45.252s − 0.531s×T
ecliptic year (from lunar node to same lunar node):
346.620 074 49d + 0.000 032 38d×T
= 346d14h52m54.436s + 2.798s×T
period of the lunar nodes:
6 793.476 501d + 0.012 400d×T
= 18.600 years
period of the lunar apsides:
3 233.605 425d + 0.016 894d×T
= 8.853 years
beat period of anomalistic and synodic months
= 411.78443d
14 synodic = 15 anomalistic months
= 413.4d
223 synodic = 242 draconitic = 239 anomalistic months = 16 Full Moon Cycles
= 6585.3d = 18y + 11d
235 synodic months = 19 years
= 6939.7d
251 synodic = 269 anomalistic months = 18 Full Moon Cycles
= 7412.2d = 20y + 107d
(WGS-1984/EGM-1996 [16] ,[22b] )
aequatorial radius
a = 6 378 137 m
nominal
RN eE = 6.3781×106 m
aequatorial circumference
2πa = 40 075 017 m
flattening
f = 1/298.257 223 563
polar radius
b = (1-f)a = 6 356 752.31 m
nominal
RN pE = 6.3568×106 m
polar circumference
π×{3(a+b)−√[(3a+b)×(a+3b)]} = 40 007 862 m
volume
π×4/3×(a2 b) = 1.0832×1021 m3
geocentric gravitational constant
GM = 3 986 004.418 (±8)×108 m3 /s2
nominal
(GM)N E = 3 986 004 ×108 m3 /s2
original (for GPS):
3 986 005 ×108 m3 /s2
(nominal) mass
(GM)N E /G = 5.9722×1024 kg
nominal mean angular velocity
ω = 7 292 115×10−11 rad/s
geopotential coefficient (C2,0 )
derived:
−484.166 774 985×10–6
original (defining):
−484.166 85 ×10–6
dynamical form factor (J2 ) from GRS80
108 263×10–8
average density
5 513 kg/m3
gravitational acceleration (in mgal = 10-5 m/s2 ):
g(φ) = 978 032.677 14 + 5 185.960×sin2 (φ) − 5.736×sin2 (2φ) − 0.3086×h
in which: φ = geodetic latitude on the WGS-84 ellipsoid; h = altitude above the ellipsoid in meters.
escape velocity
√(2GM/a) = 11.18 km/s