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centuries ago by Halley, that great Englishman who first told us a comet would come back, and was so proud that an Englishman should have been the first to predict it. He also made an even greater announcement, namely, that the stars were moving. From the beginnings of intelligence in man up till then it had been believed that the stars were fixed; they were called "fixed stars," and the name still survives. Yet Halley pointed out that when old observations were compared with those made in his day, one could not but conclude that some stars had actually moved. He chose especially Sirius, Aldebaran and Arcturus, which had moved since the time of Ptolemy more than the Moon's diameter in one direction, while Betelgeuse, the brightest star in Orion, had moved nearly twice that distance in the opposite direction.

"What shall we say, then?" he wrote. "It is scarce credible that the Antients could be deceived in so plain a matter, three observers confirming each other. Again, these stars being the most conspicuous in Heaven, are in all probability the nearest to the Earth, and if they have any particular motion of their own it is most likely to be perceived in them."

He seems almost apologetic: for he knew how slow people are to accept a new idea, however plain the evidence may be. After so many centuries of believing the stars to be fixed, it was very hard to unhitch the ideas. "Again, these stars being the most conspicuous in Heaven, are in all probability nearest to the Earth." We have already seen that this is not quite true, but it is nearly true, and the