Charon (moon)
Any final doubts were erased when Pluto and Charon entered a five-year period of mutual eclipses between 1985 and 1990. This happens when the Pluto-Charon orbital plane is edge-on as seen from Earth, which only happens at twice in Pluto's 248-year orbital period. It was very lucky that one of these intervals happened to occur so soon after Charon's discovery.