3) Kepler's third law states that the periodic time square (T2) of a planet's orbit around the Sun is directly proportional to the cube half the length of the largest axis of the planet's orbit (r ^ 3) . Where M is in kilograms, r is in meters, T is in seconds This law is known as the law of Periods. The square of the time period of the planet is directly proportional to the cube of the semi major axis of its orbit. T^ 2 propto a^ 3 That means the time 'T 'is directly proportional to the cube of the semi major axis i. e. 'a'. Let us derive the equation of Kepler's 3rd law. Let us suppose M = mass of the Sun in (kg),) r ( in (meters) and T are in seconds. T ^ 2 = (4Pi ^ 2)/((6.673)(10 ^ - 11) * M) * r ^ 3 Find half the length of the largest axis of the planet's orbit: Where M=6*10^ 24 kg and T = 5175 seconds