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 Edmund 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.

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." True to his word, he died in 1910, on Halley's next appearance.

Comets remain unpredictable. One factor is details of timing and geometry in each appearance, which affect where and how well a comet can be seen. A bigger factor is that the visible part of a "comet" is mostly a cloud of water vapor, consisting of ice in the comet that is vaporized by the heat of the sun. How much ice is vaporized can vary greatly from one appearance to the next. Halley's comet, visible in daytime on some appearances, was terribly disappointing in its 1986 visit.

Halley's comet made an appearance in 12 BC. It is one of many candidates to have been the historical correlate of the Star of Bethlehem, described in Matthew 2:2, 2:7, 2:9 2:10 (and not mentioned in the other Gospels). The date does not fit well, but the idea is appealing. Matthew himself could have seen Halley's comet in its 66 AD appearance.

Cultural references

Giotto depicts the star of Bethlehem as a comet in his 1305 fresco, The Nativity. The 1962 Christmas carol "Do You Hear What I Hear" describes "A star, a star/Dancing in the night/With a tail as big as a kite."