http://calgary.rasc.ca/algol_minima.htm The eclipses come farther apart in time when the Earth is moving away from Algol. Therefore, the light is passing the Earth more slowly. Imagine you are standing still and that you have a line of people running past you at a constant speed and they are equally spaced apart from each other. If you are standing still and then start walking in the same direction that they're running, there will be more time in between each person reaching you and they are passing you more slowly. If you believe that the speed of light is constant relative to all observers then you will need to explain why the above reasoning doesn't apply to light.