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Sky watch: Neptune and its relative years

Discovery is a funny thing. Sometimes it occurs accidentally and sometimes after a conscious search.

Take the planet Neptune, for example, which was discovered just "one year" ago.

Neptune was found after the planet Uranus didn't keep to the precise path astronomers had expected.

An English astronomer, John Couch Adams, calculated that the motion of Uranus was affected by another world that tugged gravitationally on it. Adams figured out where this new planet might appear; but no one in England bothered to look for it.

The same was true in France where Urbain Le Verrier independently made the same calculations.

But Le Verrier showed his calculations to the German astronomer Johann Galle who found the new planet -- eventually named Neptune -- on his very first night of searching!

That was in September 1846 . . . exactly one year ago. Consider this: the definition of the word "year," like so many other things, is relative.

We define a "year" as the time the Earth requires to orbit the sun once. We'd be more correct to speak of this as an "Earth-year," since each planet requires a different time to orbit our star. Mercury lies so close to the sun that only 0.24 Earth-years are needed for it to complete one orbit. For a hypothetical Mercurian, this would be "one year."

Mars, on the other hand, requires 1.88 Earth-years to make one circuit: one "Mars-year."

Neptune is so distant that it takes 165 Earth years to orbit the sun just once, and 165 years from 1846 is 2011.