A Swiss named Doerfel showed that the orbit of the comet of 1680 was approximately a parabola ; Halley showed that the comet of 1682 (since called after him), which had roused terror in 1066 and at the fall of Constantinople, had an orbit which was a very elongated ellipse, with a period of about seventy-six years ; and Newton's Principia, in 1687, showed that the law of gravitation accounted as satisfactorily for the motions of comets as for those of planets. Theologians who wanted portents were compelled to fall back upon earthquakes and eruptions.
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