Define Third law of time period …

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Kepler’s 3rd Law of Periods: 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 semimajor axis of its orbit. T² ∝ a³ That means the time ‘ t ‘ is directly proportional to the cube of the major axis. Let us derive the equation of Kepler’s 3rd law. Let us suppose, m = mass of the planet M = mass of the Sun v = velocity in the orbit So, there has to be a force of gravitation between the Sun and the planet. F = GmMr² Since it is moving in an elliptical orbit, there has to be a centripetal force. Fc = mv²r² Now, F = Fc ⇒ GMr = v² Also, v = circumferencetime = 2πrt Combining the above equations, we get ⇒ GMr = 4π²r²T² T² = 4π2r3)GM ⇒ T² ∝ r³