Three-dimensional spacetime diagrams for problem about angles.

The first figure shows a lightray traveling along the mast of a moving boat, seen from the reference frame of the boat, and the second shows the same lightray as seen by an external observer. The worldsheet of the mast is represented by the green rectangle, and the heavy line along the lightcone represents the lightray. The projections into two spacetime dimensions (front) and two purely spatial dimensions (top) are also shown. Horizontal lines represent the mast at several different instants of time.

You can rotate the figures.