The eighth planet from the Sun, Neptune was the first planet located
through mathematical predictions rather than through regular observations
of the sky. When Uranus
didn't travel exactly as astronomers expected
it to, two mathematicians, working independently of each other, proposed
the position and mass of another, as yet unknown planet that could
account for Uranus' orbit. Although "the establishment" ignored the predictions,
a young astronomer decided to look for the predicted planet.
Thus, Neptune was discovered in 1846. Seventeen days later, its largest
, was also discovered.
Nearly 4.5 billion kilometers from the Sun, Neptune orbits the Sun once
every 165 years, and therefore it has not quite made a full circle around
the Sun since it was discovered. It is invisible to the naked eye because
of its extreme distance from Earth. Interestingly, due to Pluto
elliptical orbit, Neptune is actually the farthest planet from the Sun for a
20-year period out of every 248 Earth years.
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