Kepler's constant
Kepler’s constant is the constant of proportionality between T2 and a3 in Kepler’s third law of planetary motion.
We can find a formula for K from Newton’s Law of universal gravitation. The result in IS units is
K= 4π/GM,
where π is the usual number, G is the constant of Newton’s universal law of gravitation, and M is the mass of the object being orbited around (for example, M is the mass of the Sun in the Solar system). Since π and G are universal constants, K depends only on M.