The well defined eclipse shadow seen on Jupiter does indeed prove the eclipsing body subtends the same angle in the sky as the Sun, a similar sharp shadow has been imaged on planet Uranus during an eclipse by its satellite Ariel proving both bodies have the same angular dimension, that eclipses on planet Mars never produce sharp shadows is because neither of its moons has sufficient angular width to cover the disc of the Sun, likewise if the eclipsing bodies on Jupiter and Uranus were smaller or larger their shadows would never be in focus. Edit.