There is an interaction between gravitational tidal forces and the instantaneous moment of inertia of any massive extended body. A mechanism that changes the altitude of a spacecraft orbiting a central mass by periodically changing its instantaneous moment of inertia tensor is discussed. This can be achieved most simply by connecting two bodies with a tether of adjustable length and does not rely on any chemical propellant. By using a series of reasonable approximations, a formula for the rate of change of height is given in terms of accessible data. This formula predicts that by suitably varying the length of a 50 km connecting tether a connected system can rise at about 300 m an hour in low earth orbit.