Illustration of the forward (upper) and reverse (lower) cases. The dashed circles represent uncertainty in the orbits and positions of the two planets. The beams transmitted by our respective civilizations can be made narrower, to be seen more easily over the transmitting civilization’s star, or broader, to signal more of the solar system at once (even if we do not know where planets are within it). If a planet’s position is known, then the optimum beamwidth must be exactly wide enough to encompass the uncertainty in its position.