Aball is thrown horizontally from the top of a cliff with initial speed v0 at t=0.?

A ball is thrown horizontally from the top of a cliff with initial speed v0 (t = 0). At any moment, its direction of motion makes an angle theta to the horizontal. Derive a formula for theta as a function of time, t, as the ball follows a projectile's path. Express answer in terms of g, t, and v0.