Halley's comet was identified in 1682, when Edmond Halley used the calculations of his friend Sir Isaac Newton to prove that the same comet seen in the sky that year had also appeared in 1607 and 1531. After calculating the effects of planetary gravity on the comet, he predicted its return in 1758. Halley did not live to see the comet's return, but astronomers did observe it in the sky as predicted, proving Halley right.
Halley's comet visits the solar system every 74 to 79 years, with most orbits taking around 75 to 76 years. The period changes slightly due to the effects of planetary and solar gravity on the comet, which can slightly alter its trajectory and speed. Using knowledge about its period and movements, astronomers have been able to identify its presence in historical records dating back as far as 240 B.C. The comet's last visit in the 20th century was in 1986, and its first appearance in the 21st century will occur in 2061.
Halley's comet passed close to the sun in 1835, and two weeks later, noted writer and humorist Mark Twain was born. Toward the end of his life in 1909, he suggested that the comet's impending return might signal his own demise. He died on April 21, 1910, just one day after the comet's closest approach to the sun.