Three distinct observations suffice to determine the orbit of a planet completely, but it is well to have the three observations as far apart as possible so as to minimize the effects of minute but necessary errors of observation. 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. Examined in this way the tabulated observations of Flamsteed showed that he had unwittingly observed Uranus five distinct times; the first time in 1690, nearly a century before Herschel discovered its true nature. But more remarkable still, Le Monnier, of Paris, had observed it eight times in one month, cataloguing it each time as a different star. If only he had reduced and compared his observations, he would have anticipated Herschel by twelve years. As it was, he missed it. It was seen once by Bradley also. Altogether it had been seen twenty times.