Astronomical Constants - Useful Constants for AstronomyThe IAU system of constants defines a system of astronomical units for length, mass and time, and also includes astronomical constants.
Introduction to Astronomy
This article is part of a series of articles. Please use the links below to navigate between the articles.
- Astronomy for Beginners - Complete Guide
- What are Right Ascension (RA) and Declination (Dec)?
- What is Angular Size in Astronomy?
- Magnitude Scale and Distance Modulus in Astronomy
- Sidereal Time, Civil Time and Solar Time
- What Are The Equinoxes and Solstices About?
- How Do We Measure Distance in Space Using Parallax and Parsecs
- Brightness, Luminosity and Flux of Stars Explained
- Kepler's Laws of Planetary Motion Explained
- What Are Lagrange Points?
- List of Astronomy Equations with Workings
- Glossary of Astronomy & Photographic Terms
- Astronomical Constants - Useful Constants for Astronomy
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.
Defining Astronomical Constants
| Quantity | Symbol | Value | Relative uncertainty |
|---|---|---|---|
| Gaussian gravitational constant | k | 0.017,202,098,95 A3/2,S-1/2,D-1 | defined |
| Speed of light | c | 299,792,458 ms-1 | defined |
| Mean ratio of the TT second to the TCG second | 1 - LG | 1 - 6.969,290,134×10-10 | defined |
| Mean ratio of the TCB second to the TDB second | 1 - LB | 1 - 1.550,519,767,72×10-8 | defined |
Primary Astronomical Constants
| Quantity | Symbol | Value | Relative uncertainty |
|---|---|---|---|
| Mean ratio of the TCB second to the TCG second | 1 - LC | 1 - 1.480,826,867,41×10-8 | 1.4×10-9 |
| Light-time for unit distance | ?A | 499.004,786,3852 s | 4.0×10-11 |
| Equatorial radius for Earth | ae | 6.378,1366×106 m | 1.6×10-8 |
| Potential of the geoid | W0 | 6.263,685,60×107 m2,s-2 | 8.0×10-9 |
| Dynamical form-factor for Earth | J2 | 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 | 3.986,004,391×1014 m3,s-2 | 2.0×10-9 |
| Constant of gravitation | G | 6.673,84×10-11 m3,kg-1,s-2 | 1.2×10-4 |
| 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 | 5029.796,195? | * |
| Obliquity of the ecliptic, at standard epoch 2000 | ε | 23ε 26' 21.406? | * |
Derived Astronomical Constants
| Quantity | Symbol | Value | Relative uncertainty |
|---|---|---|---|
| Constant of nutation, at standard epoch 2000 | N | 9.205,2331? | * |
| Unit distance = c?A | A | 149,597,870,691 m | 4.0×10-11 |
| Solar parallax = arcsin(ae/A) | p? | 8.794,1433? | 1.6×10-8 |
| Constant of aberration, at standard epoch 2000 | κ | 20.495,52? | |
| Heliocentric gravitational constant = A3k2/D2 | GS | 1.327,2440×1020 m3,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 | 1.9818×1030 kg | 1.0×10-4 |
System of planetary masses & Ratios of mass of Sun to mass of planet
| Planet | Ratios of mass | ||
|---|---|---|---|
| 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 Astronomical Constants (outside the formal IAU System)
| Quantity | Symbol | Value | Relative uncertainty |
|---|---|---|---|
| Parsec = A/tan(1") | pc | 3.085,677,581,28×1016 m | 4.0×10-11 |
| Light-year = 365.25cD | ly | 9.460,730,472,5808×1015 m | defined |
| Hubble constant | H0 | 70.1 km,s-1,Mpc-1 | 0.019 |
| Solar luminosity | L? | 3.939×1026 W = 2.107×10-15S,D-1 | variable, ±0.1% |
