Eight Minutes and a Half

“It takes about eight minutes and a half for the light from the Sun to reach us, therefore when we look at a beautiful sunset and we see the Sun at the horizon… the Sun is actually not there anymore, it’s already below the horizon! The real sunset happened eight and a half minutes earlier! Similarly, at sunrise, the Sun seems to be at the horizon, but is already up in the sky, due to the same time delay.”
I first heard this statement from a friend, a former colleague in physics teaching who found it in different publications, ranging from physics textbooks, general science books and astronomical magazines.
At first, the quoted statement seems perfectly reasonable. After all if something happens on the Sun, for example if new sunspots were to develop on its surface or if for some strange reason the Sun suddenly were to turn green or purple, we would actually observe these events with a time delay of approximately eight and a half minutes.
Or one can think of apparently similar situations, such as observing the lights of a moving car at night, with the car suddenly turning around a corner and disappearing from sight. The time delay for the light to reach us would be practically minimal, but the car is effectively already around the corner when its last light reaches us.
A quick informal poll of some of my students revealed that most of them tend to agree with the statement, for reasons similar to those I just mentioned. However, a closer inspection of the statement reveals the real hidden issue: a problem of rotating vs. inertial reference systems. In this paper I will analyze this problem and describe a simple experimental activity which can also be used as a demonstration of the peculiar kinematical effects of rotating systems.