## ABSTRACT

Strictly speaking Kepler's laws of planetary motion apply to the motion of an isolated body round the Sun. But Kepler showed they hold to a good approximation for individual bodies in the many-body solar system. This tutorial shows how the use of Kepler's laws, together with conservation of energy and angular momentum, simplifies the solution of many orbit problems. For reference, Kepler's laws are (i) Planets move in ellipses with the Sun as a focus (ii) The area swept out per unit time is constant (iii)T α a^{3/2} (where T is the period of the orbit and a the semi-major axis of the ellipse.)