Discovery of Charon 

Charon is the only satellite of Pluto and was discovered in 1978. The discovery was fortunate because its orbital plane was just about to move edge-on toward the inner solar system, which would have made it almost impossible to observe.

However, immediately after the discovery, scientists were able to utilize the eclipses of the two bodies and investigate the brightness of the edge of Charon over Pluto (and vice versa) and construct a map of the dark and light regions on both bodies.