Illustrating the space-time coordinates of the events associated with the apparent and the actual position of a light source

Abstract

We present a space-time diagram that displays in true values the space- time coordinates of events associated with the apparent and actual positions of a point like source moving with constant velocity. We use it in order to construct the actual shape of a moving luminous profile or to determine the relationship between apparent and actual distances. We show that the simple fact that light propagates at finite speed has important consequences like "length contraction" or "length dilation" the effects being sensitive against the "approaching" and the "receding" character of the source relative to an observer located at the origin of the inertial reference frame