Though many had seen Uranus before, none of them had noticed its contrasting movement from other distant stars. Even when William Herschel noticed its
movement, his first inclination was that Uranus was a comet. It wasn't until he received assistance from several other astronomers that Uranus was determined to be a planet. They were able to identify its planetary status because of years of acquired knowledge relating to physical law, and specifically the motions of planets and comets.
One cannot simply observe an event and hope to explain it. I could view an explosion, but without knowing the factors leading up to the blast, I would only know the effect. However, given the location, individuals involved, objects from the scene, and samples of debris, an expert could likely determine the cause. The experts determination would only be possible because of prior experience and compiled and analyzed data. As we learn more about the causes of the events in the universe, it continually makes it easier for us to predict and understand movements of strange and far off areas.