## 44 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 MKSA system (meters, kilograms, seconds, amperes) and the CGSM system (centimeters, grams, seconds, gauss), which is commonly used in Astronomy.

The definitions of constants in the MKSA system are available in the file `gsl_const_mksa.h`. The constants in the CGSM system are defined in `gsl_const_cgsm.h`. Dimensionless constants, such as the fine structure constant, which are pure numbers are defined in `gsl_const_num.h`.

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