Now let’s move on to the final experiment I want to discuss in this section: the behaviour of gyroscopes in orbit around the Earth. A gyroscope is essentially a spinning top, whose central axis is allowed to point in any direction. According to Einstein’s theory of gravity, there should be two new efects that can be observed when a gyroscope is put in orbit around the Earth. The first of these is a change in direction of the gyroscope’s axis of rotation, as it orbits the Earth. This efect, known as geodetic precession, is due to the curvature of space-time around the Earth. The second efect is known as frame-dragging, and is due to the rotation of the Earth efectively pulling space around with it as it rotates. This is an entirely new type of gravitational interaction, and so it is of great interest to see if it can be confirmed experimentally.