Is Seeing a Comet Like Halley’s a Once-in-a-Lifetime Event?

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You don’t have to be an astronomer to be familiar with Halley’s Comet. The glowing ball of ice and dust, shown here, passes Earth about every 76 years. English astronomer Edmond Halley first predicted the comet’s recurring orbit in 1705. Halley identified a pattern of comets that had approached Earth in 1531, 1607 and 1682, and predicted it would return again in 1758. Although he died before he could witness what would become his namesake comet’s return, from its periodicity he concluded that it would orbit the sun on a regular basis. Halley’s has become famous because “it’s the only short-period comet that a human can count on seeing in his lifetime,” says Bill Cooke, lead at NASA’s Meteoroid Environment Office. His comet last passed Earth in 1986, and it was about as bright as Polaris, aka the North Star. Astronomers measure the brightness of objects in the sky on a scale of apparent magnitude—the brighter the object, the greater the negative number; the dimmer, the greater the positive number. The sun is very bright and registers –26 magnitude, compared with the full moon, which is about –13 magnitude to the naked eye. The 1986 visit by Halley’s was 2 magnitude. Halley’s next pass should be in July 2061, at an expected brightness of about –0.3. “It’s not going to be a fairly spectacular apparition,” Cooke says. More via Scientific American.

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