We were taught at school that a stone which is thrown forward in a horizontal direction from the edge of a cliff will describe a curved path which is a parabola. The path is in fact an ellipse—but that is not the only thing we were taught at school which turned out to be erroneous in after life. This error arises from the elementary assumption that all vertical lines are parallel; but the vertical in fact is the direction of a plumb line, and all such lines converge towards the centre of the Earth. With a small initial velocity the stone will describe a small ellipse which intersects the surface of the Earth, but with greater speeds the ellipse becomes larger, until it finally clears the Earth completely and returns to the thrower. Although the stone is now circling the Earth, it is still ‘falling’ freely in a gravitational field, and from this point of view all such orbits are referred to as ‘free orbits’, the stone being in a state of ‘free fall’.