Halley's Comet

Halley's Comet on its 1910 appearance. This Yerkes Observatory photograph is highly enlarged; the comet would not have appeared this large or bright to the naked eye.

Halley's Comet (also known as Comet Halley and pronounced to rhyme with "valley") is a famous periodic comet which becomes visible to the naked eye when it approaches the Sun, about every 76 years. The last appearance was in 1986 and the next one will be in 2061.

Halley's Comet as depicted in the Bayeux Tapestry, c. 1070 AD. (The observers are point to a star with a tail in the frieze at the top). A caption says "Isti mirant stella," ("they stare, amazed at the star")

It is named for astronomer Edmond Halley (1656-1742). Halley was certainly not the "discoverer," in the sense of being the first person to see it. In fact it was recorded on Babylonian tablet in 164 BC, and its appearance in 1066 was depicted on the Bayeux Tapestry. Halley, however, was the first person to tie together accounts of older comet appearances and realize that these were periodic appearances of the same comet. This was an important step in astronomers' understanding of comets and the Solar System. (Halley, incidentally, was also a believer in the "hollow earth" theory: that the Earth is a spherical shell and humans live inside, on the inner surface.)

Illustration of how Halley's Comet might have looked to a naked-eye observer standing on the grounds of Yerkes Observatory in 1910. Moon at left, Halley's at right. Although appearance suggests motion, comets do not swoop through the sky; position changes slowly over many hours.

The period of 76 years gives Halley's comet an intriguing "once-in-a-lifetime" quality. Mark Twain was born in 1835, when Halley's Comet was visible, and said "I came in with Halley's Comet and I expect to go out with it." He in fact died in 1910, on Halley's next appearance.