Mars would be the planet to expect to show the greatest variation in apparent brightness as seen from Earth. None of the planets makes a large percentage change in its distance from the Sun. Mercury only makes a moderate change in its distance from Earth, which tends to be canceled by its phases. Mercury can hardly be seen except near Greatest Elongation, so it’s a never far from being a half-Mercury phase if visible. Venus makes a large fractional change only about 0.3 to 1.6 AU, but this is offset by the smaller fraction of illuminated surface, we see due to the phases. For Mars, the distance varies from 0.52 AU at opposition to 2.52 AU at conjunction. This is a factor of 5 in distance. Later it can be see that this corresponds to a factor
