Halley's Comet
Halley's periodic returns have been subject to scientific investigation since the 16th century. The three apparitions from 1531 to 1682 were noted by Edmond Halley, enabling him to predict its 1759 return. Streams of vapour observed during the comet's 1835 apparition prompted astronomer Friedrich Wilhelm Bessel to propose that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit.
The 1910 approach, which came into naked-eye view around 10 April and came to perihelion on 20 April, was notable for several reasons: it was the first approach of which photographs exist, and the first for which spectroscopic data were obtained. Furthermore, the comet made a relatively close approach of 0.15 AU, making it a spectacular sight. Indeed, on 19 May, Earth actually passed through the tail of the comet. One of the substances discovered in the tail by spectroscopic analysis was the toxic gas cyanogen, which led astronomer Camille Flammarion to claim that, when Earth passed through the tail, the gas "would impregnate the atmosphere and possibly snuff out all life on the planet." His pronouncement led to panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public. In reality, as other astronomers were quick to point out, the gas is so diffuse that the world suffered no ill effects from the passage through the tail.
Science in the Age of Enlightenment
Building on the body of work forwarded by Copernicus, Kepler and Newton, 18th-century astronomers refined telescopes, produced star catalogues, and worked towards explaining the motions of heavenly bodies and the consequences of universal gravitation. Among the prominent astronomers of the age was Edmund Halley. In 1705, Halley correctly linked historical descriptions of particularly bright comets to the reappearance of just one, which would later be named Halley’s Comet, based on his computation of the orbits of comets. Halley also changed the theory of the Newtonian universe, which described the fixed stars. When he compared the ancient positions of stars to their contemporary positions, he found that they had shifted. James Bradley, while attempting to document stellar parallax, realized that the unexplained motion of stars he had early observed with Samuel Molyneux was caused by the aberration of light. The discovery was proof of a heliocentric model of the universe, since it is the revolution of the earth around the sun that causes an apparent motion in the observed position of a star. The discovery also led Bradley to a fairly close estimate to the speed of light.