**Halley, Edmund**1656-1742- Astronomer and geophysicist; prompted Newton to write the Principia. At the age of 20 Halley went to St Helena and made the first accurate catalogue of the southern stars. His subsequent career included work on meteorology and terrestrial magnetism, as well as the astronomy for which he is famous today. In 1684 Halley travelled from London to Cambridge to ask Newton what would be the shape of planetary orbits if the force of gravity varied as the inverse square of the distance to the sun. Newton's response that it would be an ellipse, and the papers written in proof, eventually led to the publication of the Principia in 1687. Halley financed the printing himself when the Royal Society was unable, and expressed his selfless admiration for what Newton had done, in the book's Preface. Halley's modern fame is based on his identification of Halley's Comet in historical records, as it returned every 76 years, and this in turn is based on his calculation of its orbit with Newtonian techniques. During the 1696 recoinage, Halley was deputy comptroller of the Chester mint.

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