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.
Sungrazing comet
One of the first comets to have its orbit computed was the sungrazing comet (and Great Comet) of 1680, now designated C/1680 V1. It was observed by Isaac Newton and he published the orbit results in 1687. Later, in 1699, Jacques Cassini proposed that comets could have relatively short orbital periods and that C/1680 V1 was the same as a comet observed by Tycho Brahe in 1577, but in 1705 Edmond Halley determined that the difference between the perihelion distances of the two comets was too great for them to be the same object. However, this marked the first time that it was hypothesized that Great Comets were related or perhaps the same comet. Later, Johann Franz Encke computed the orbit of C/1680 V1 and found a period of approximately 9000 years, leading him to conclude that Cassini's theory of short period sungrazers was flawed. C/1680 V1 had the smallest measured perihelion distance until the observation in 1826 of comet C/1826 U1.