# Physical Constants

This chapter describes macros for the values of physical constants, such as the speed of light, c, and gravitational constant, G. The values are available in different unit systems, including the standard MKS system (meters, kilograms, seconds) and the CGS system (centimeters, grams, seconds), which is commonly used in Astronomy.

The definitions of constants in the MKS system are available in the file ``gsl_const_mks.h'`. The constants in the CGS system are defined in ``gsl_const_cgs.h'`. Dimensionless constants, such as the fine structure constant, which are pure numbers are defined in ``gsl_const_num.h'`.

- Fundamental Constants
- Astronomy and Astrophysics
- Atomic and Nuclear Physics
- Measurement of Time
- Imperial Units
- Nautical Units
- Printers Units
- Volume
- Mass and Weight
- Thermal Energy and Power
- Pressure
- Viscosity
- Light and Illumination
- Radioactivity
- Force and Energy
- Prefixes
- Physical Constant Examples
- Physical Constant References and Further Reading

The full list of constants is described briefly below. Consult the header files themselves for the values of the constants used in the library.