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This map shows the sky looking directly overhead this time of year with North toward the bottom.


Hercules: See the Celestial Strongman
By Joe Rao
SPACE.com's Night Sky Columnist
posted: 05:45 am ET
16 July 2004

With the bright Moon out of late evening sky, look high overhead to see the "celestial strongman," the constellation of Hercules.

Itís a star pattern in which the traditional mythological figure is difficult for modern skywatchers to visualize. Astronomer Robert H. Baker (1880-1962) described its six brightest stars as a "butterfly with outspread wings." Others sometimes describe those same stars as outlining the initial "H" for Hercules.

But primitive people seemed to have no difficulty in picturing these stars as forming the figure of a kneeling man, and the old Arabic name of its brightest star, Ras Algethi, means "The Head of the Kneeler."

About 260 B.C., the Greek poet Aratus noted that "no one knows how to read that sign clearly, nor on what task he is bent." Yet, it was also the Greeks who apparently declared that "The Kneeler" was their great hero, Hercules.

Also within Hercules is quite possibly the most celebrated object in the summertime skies: The Great Cluster in Hercules, known also as M13. The M, of course, is the initial of the famed comet observer, Charles Messier (1730-1817).

Messier was deeply interested in discovering comets but he was plagued by the same trouble that besets all comet hunters. He kept finding "comets" that were not comets at all but only star clusters and nebulae. His hopes were dashed so often that for his own convenience he kept a list of these deceiving objects, which he published in a catalogue.

To locate Messier 13, look toward the four stars, known as the "Keystone" that supposedly forms the body of Hercules.

A keystone is the stone atop an arch, and has this shape, narrower at one end. Itís between the two western stars of the keystone that we can find the Great Globular Cluster of Hercules. Itís about a third of the way along a line drawn from the stars Eta to Zeta. Actually, it was not Messier, but Sir Edmund Halley (of comet fame), who first mentioned it in 1715, having discovered it the previous year: "This is but a little Patch," he wrote, "but it shows itself to the naked Eye, when the Sky is serene and the Moon absent."

Located at a distance of about 25,000 light-years, the Hercules Cluster has been estimated to be a ball of tens of thousands of stars roughly 160 light-years in diameter.

Messier first saw the cluster in June 1764 and described it as a "round and brilliant nebula with a brighter center, which I am sure contains no stars."

Today, if you use good binoculars and look toward that spot in the sky where M13 is you likely will see a similar view: a roundish glow or patch of light. Moving up to a telescope, the view dramatically improves. With a 4 to 6-inch telescope, the "patch" starts to become resolved into hundreds of tiny pinpoints of light. In larger instruments, Messier 13 is transformed into a spectacular celestial chrysanthemum.

In his Celestial Handbook, Robert Burnham (1931-1993) describes the view of the cluster in a 12-inch or larger telescope as "an incredibly wonderful sight; the vast swarm of thousands of glittering stars, when seen for the first time or the hundredth, is an absolutely amazing spectacle."

Basic Sky Guides


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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.


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