Neptune Photo from Voyager 2 – NASAThe planet Uranus was observed by scientists as early as 1690. For over 100 years astronomers watched this “last planet” in the solar system with wonder. The problem was that the orbit of the new planet did not follow the rules. The eccentricity of Uranus’ orbit told the astronomers that there was something very large and forceful that was having a real impact on what Uranus did and how it behaved. As time went by, better telescopes were built. Astronomers had watched Uranus long enough to know where the great influence was, so they turned their instruments to that part of the sky and were astounded to see still another planet–Neptune! The orbit of Neptune was established and studied, and it too did not quite obey the rules, so astronomers turned their most powerful instruments even further out and discovered Pluto. Most modern discoveries in astronomy are related to influence. We know where to look and what to look for because we see the influence of an object long before we see the object itself.
