This year marks the 150th anniversary of a great comet that took astronomers by surprise, creating a fearsome spectacle in the night sky.
A comet is a ball of ice and rock that originates in the distant reaches of the solar system, beyond Neptune and Pluto. The Great Comet of 1861 approached Earth from the south, so an observer in Australia was the first to see it. He discovered the comet on May 13th.
The comet headed north and a month later, on June 12th, came closest to the Sun. In an era before rapid communications, though, word of the comet didn't reach the northern hemisphere until the comet itself did. On June 30th, people in Europe and North America were stunned to see what an observer in Greece called "a comet of truly fearful appearance." The comet's huge head looked as large as the full Moon, and its tail stretched across the heavens.
The comet looked so huge in part because it came quite close to Earth -- about 12 million miles. It was so close that Earth probably passed through the comet's tail.
The Great Comet of 1861 is on such a long orbit around the Sun that it won't return any time soon. But a similar comet swung past in 1996. Named Hyakutake, this comet also passed close to Earth and sported a long tail. Both comets are reminders that you never know when the next cometary spectacle will grace the sky.
Script by Ken Croswell, Copyright 2011