Edmund Halley 59 



Wilde, and, though not generally accepted, it shows that 

 Halley recognized that the study of terrestrial magnetism 

 could yield important information on the constitution of 

 the earth and that he looked upon the subject from a 

 wider point of view than that of its mere application to the 

 purposes of navigation. The observations he took in two 

 journeys specially undertaken for the purpose of determining 

 the magnetic declination in different parts of the world, are 

 invaluable to us as historical records. 



Halley's most important discoveries in astronomy were 

 the secular acceleration of the moon's mean motion, the 

 proper motion of the stars, and the periodicity of comets. 

 Comparing the dates at which certain total eclipses of the 

 sun had occurred, Halley could fix the times of the new 

 moon with sufficient accuracy to ascertain that the length 

 of the month was diminishing by about one-thirtieth of a 

 second per century. This implied that the moon's orbital 

 velocity is increasing and may be explained in accordance 

 with Newton's principles, partly as a result of an indirect 

 effect on the earth's orbit round the sun due to the attrac- 

 tion of planets, and partly by friction between the tides and 

 the solid parts of the earth, which increases the length of 

 the day, and indirectly reacts on the moon. 



In all three of the discoveries mentioned, Halley made 

 extensive use of old records; it was by comparing the 

 observed distances of well-known stars from the ecliptic 

 with the observations of the Greek astronomers, that he 

 discovered their independent motions, and, similarly by 

 calculating the orbits of comets observed in previous 

 centuries, he found that some of them pursued nearly 

 identical paths. He concluded that though these were regis- 

 tered each time as new intruders into the solar systems, 

 they might only be reappearances of the same body. As an 

 example, he took the comet which had been observed at 

 intervals of about seventy-six years, and had last been seen 

 in 1682. He predicted that it would be seen again in 1758. 

 Halley did not live to see his prophecy come true : the 

 comet was actually observed on Christmas Day of that 

 year, and is now recognized as a permanent member of the 

 Solar System. 



