- Introduction to Astronomy
- The Celestial Sphere - Right Ascension and Declination
- What is Angular Size?
- What is the Milky Way Galaxy?
- The Astronomical Magnitude Scale
- Sidereal Time, Civil Time and Solar Time
- Equinoxes and Solstices
- Parallax, Distance and Parsecs
- Luminosity and Flux of Stars
- Kepler's Laws of Planetary Motion
- What Are Lagrange Points?
- Glossary of Astronomy & Photographic Terms
- Astronomical Constants - Some Useful Constants for Astronomy

## Defining constants

Quantity | Symbol | Value | Relative uncertainty |
---|---|---|---|

Gaussian gravitational constant | k | 0.017,202,098,95 A^{3/2},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 - L_{G} | 1 - 6.969,290,134×10^{-10} | defined |

Mean ratio of the TCB second to the TDB second | 1 - L_{B} | 1 - 1.550,519,767,72×10^{-8} | defined |

## Primary constants

Quantity | Symbol | Value | Relative uncertainty |
---|---|---|---|

Mean ratio of the TCB second to the TCG second | 1 - L_{C} | 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 | a_{e} | 6.378,1366×10^{6}m | 1.6×10^{-8} |

Potential of the geoid | W_{0} | 6.263,685,60×10^{7}m^{2},s^{-2} | 8.0×10^{-9} |

Dynamical form-factor for Earth | J_{2} | 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 | 3.986,004,391×10^{14}m^{3},s^{-2} | 2.0×10^{-9} |

Constant of gravitation | G | 6.673,84×10^{-11} m^{3},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 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(a_{e}/A) | p_{?} | 8.794,1433? | 1.6×10^{-8} |

Constant of aberration, at standard epoch 2000 | κ | 20.495,52? | |

Heliocentric gravitational constant = A^{3}k^{2}/D^{2} | GS | 1.327,2440×10^{20}m^{3},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 | 1.9818×10^{30}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 constants (outside the formal IAU System)

Quantity | Symbol | Value | Relative uncertainty |
---|---|---|---|

Parsec = A/tan(1") | pc | 3.085,677,581,28×10^{16} m | 4.0×10^{-11} |

Light-year = 365.25cD | ly | 9.460,730,472,5808×10^{15}m | defined |

Hubble constant | H_{0} | 70.1 km,s^{-1},Mpc^{-1} | 0.019 |

Solar luminosity | L_{?} | 3.939×10^{26} W= 2.107×10 | variable, ±0.1% |