When you observe a moon orbiting around its planet, the distance to that moon from Earth will be slightly larger when it is in the far part of the orbit than when it is in the near part. Similarly, since the Earth orbits the Sun, the distance between the Earth and an object varies over a year as well. The Roemer delay is the classical light travel time across the Earth’s orbit, and is also used to describe the light travel time across other objects orbits (e.g. pulsars in binary systems).

The effect is named after Ole Rømer, a Danish astronomer from the late 17th century, who first noticed it by observing the Galilean moons from different parts of the Earth’s orbit – and used it to estimate the speed of light. Today, it is mostly important for observations of binary orbits and tracking of spacecraft.