Astronomy Constants and Units Reference
Look up common astronomical constants and defined units, see their values and understand when each one belongs in a calculation.
QuantitySymbolValueStatusTypical use
Speed of light in vacuumc299,792,458 m/sExact SI defined constantLight-travel calculations and relativity.
Astronomical unitau149,597,870,700 mExact IAU definitionDistances within planetary systems.
Julian yeara365.25 days = 31,557,600 sDefined durationLight-year and astronomical time calculations.
Light-yearly9.4607304725808 × 10¹⁵ mDerived from c × Julian yearInterstellar distance.
Parsecpc≈ 3.085677581 × 10¹⁶ mDefined through one arcsecond of parallax geometryStellar and galactic distance.
Gravitational constantG6.67430 × 10⁻¹¹ m³ kg⁻¹ s⁻²CODATA measured constantOrbital mechanics and gravity.
Solar massM☉≈ 1.9884 × 10³⁰ kgReference astronomical massStars, black holes and orbital systems.
Earth mean radiusR⊕≈ 6,371.0 kmReference planetary radiusPlanetary scale comparisons.
Defined units follow SI and IAU definitions. Measured constants should be checked against the current CODATA recommended values when precision scientific work requires the latest uncertainty.