This interactive demonstration shows what would happen if the speed of light were not constant, but rather increased or decreased by the speed of the light source. In a binary star system, the smaller star orbiting the larger star is the light source of interest. Use the sliders to choose the distance of the binary star system from Earth and the angular velocity of the smaller star. Then click Start to see what happens. To play around with different distances and angular velocities, reset the animation and click Start again. Notice that at sufficiently high angular velocity, light emitted later in the orbit of the star can arrive at Earth before the light emitted earlier in its orbit. (This is not observed, providing evidence that the speed of light is not affected by the speed of the light source.)