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When Uranus was found old records of stellar
observations were ransacked with the object of discovering whether it
had ever been unwittingly seen before. If seen, it had been thought, of
course, to be a star--for it shines like a star of the sixth magnitude,
and can therefore be just seen without a telescope if one knows
precisely where to look for it and if one has good sight--but if it had
been seen and catalogued as a star it would have moved from its place,
and the catalogue would by that entry be wrong. The thing to do,
therefore, was to examine all the catalogues for errors, to see whether
the stars entered there actually existed, or whether any were missing.
If a wrong entry were discovered, it might of course have been due to
some clerical error, though that is hardly probable considering the care
spent in making these records, or it might have been a tailless comet,
or possibly the newly found planet.
The next thing to do was to calculate backward, to see whether by any
possibility the planet could have been in that place at that time.


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