Light-time correction occurs in principle during the observation of any moving object, because the speed of light is finite.
By convention, light-time correction is not applied to the positions of stars, because their motion and distance may not be known accurately.
An approximate light-time is calculated by dividing the object's geometric distance from Earth by the speed of light.
For planetary motions, a few (3–5) iterations are sufficient to match the accuracy of the underlying ephemerides.
He correctly deduced that this difference was caused by the appreciable time it took for light to travel from Jupiter to the observer on Earth.