How do we know the sun's mass?
(Lansing State Journal, October 20,1993)
Kepler took careful measurements of the position of the planets over several years. From this, he discovered that the time it takes a planet to travel around the sun was related to its distance from the sun. The square of a planet's orbital period is proportional to the cube its distance to the sun.
Newton discovered the universal law of gravitation: Gravitational force between two objects is proportional to the masses of the objects and inversely proportional to the square of the distance between them. By combining Newton's Law and Kepler's Law algebraically, we can calculate the sun's mass from quantities we can measure more directly.
The actual mass of the sun is about 2*1020 kilograms.