Was Comet McNaught the best or brightest?

By Joe Rao

Space.com skywatching columnist

updated 3:20 p.m. ET Jan. 19, 2007

Last week favorably placed observers viewed a comet so brilliant that it could be seen with the naked eye in broad daylight, if the Sun was hidden behind the side of a house or even an outstretched hand. 

Comet McNaught, which was discovered last August by astronomer Robert McNaught at Australia’s Siding Spring Observatory, was one of the greatest comets in recent times. It evolved into a brilliant object as it swept past the Sun on Jan. 12, at a distance of just 15.9 million miles.

The comet's show is mostly over for those North of the equator.

Yet even as the comet puts on a fantastic show now in the evening sky for viewer's in the Southern Hemisphere (McNaught himself produced a fantastic photograph), an incredible sight is still visible to seasoned observers in the Northern Hemisphere. From Colorado, Mary Laszlo of used a 20-second exposure to capture the outer extremities of Comet McNaught's tail on Jan. 17. Paul Robinson of Boulder was credited as having realized such an image might be possible.

Brighter than Venus
According to reports received from a worldwide audience at the International Comet Quarterly, it appears that the comet reached peak brightness on Sunday, Jan. 14 at around 12 hours UT (7:00 a.m. EST).  At that time, the comet was shining at magnitude -5.1.  On this scale, larger numbers represent dimmer objects; the brightest stars are generally zero to first magnitude, while superbright objects such as Venus—and Comet McNaught—achieve negative magnitudes. 

I determined the comet’s peak magnitude by averaging out more than a dozen observations that were reported to the ICQ on Jan. 14.  Some observers, such as Steve O’Meara, located at Volcano, Hawaii, observed McNaught in daylight and estimated a magnitude as high as -6, noting.  “The comet appeared much brighter than Venus!” 

From Jan. 12-16, Comet McNaught ranked as the third brightest object in the sky behind only the sun and the moon!

Was Comet McNaught the best or brightest comet ever seen?  While it’s true that comets that are visible with the naked eye during the daytime are rare, the case of McNaught is not unique. In the last 263 years, it has happened seven other times:

Creat comet of 1744: First sighted on Nov. 29, 1743 as a dim fourth magnitude object, this comet brightened rapidly as it approached the Sun.  Many textbooks often cite Philippe Loys de Cheseaux, of Lausanne, Switzerland as the discoverer, although his first sighting did not come until two weeks later.  By mid-January 1744, the comet was described as 1st-magnitude with a 7-degree tail. By Feb. 1 it rivaled Sirius and displayed a curved tail, 15-degrees in length.  By Feb. 18 the comet was equal to Venus and now displayed two tails.  On Feb. 27, it peaked at magnitude -7 and was reported visible in the daytime, 12-degrees from the Sun.  Perihelion came on March 1st, at a distance of 20.5 million miles from the Sun.  On March 6, the comet appeared in the morning sky, accompanied by six brilliant tails which resembled a Japanese hand fan.

Great comet of 1843: This comet was a member of the Kruetz Sungrazing Comet Group, which has produced some of the most brilliant comets in recorded history. It passed only 126,000 miles from the Sun’s photosphere on Feb. 27, 1843.  Although a few observations suggest that it was seen for a few weeks prior to this date, on the day when it made it closest approach to the Sun it was widely observed in full daylight.  Positioned only 1-degree from the Sun, this comet appeared as “an elongated white cloud” possessing a brilliant nucleus and a tail about 1-degree in length.  Passengers on board the ship Owen Glendower, off the Cape of Good Hope described it as a “short, dagger-like object” that closely followed the Sun toward the western horizon. In the days that followed, as the comet moved away from the Sun, it diminished in brightness but the tail grew enormously, eventually attaining a length of 200 million miles. If you were able to place the head of this comet at the Sun’s position, the tail would have extended beyond the orbit of the planet Mars!

Great September comet of 1882: This comet is perhaps the brightest comet that has ever been seen; a gigantic member of the Kreutz Sungrazing Group.  First spotted as a bright zero-magnitude object by a group of Italian sailors in the Southern Hemisphere on Sept. 1, this comet brightened dramatically as it approached its rendezvous with the Sun.  By the 14th, it became visible in broad daylight and when it arrived at perihelion on the 17th it passed at a distance of only 264,000-miles from the Sun’s surface.  On that day, some observers described the comet’s silvery radiance as scarcely fainter than the limb of the Sun, suggesting a magnitude somewhere between -15 and -20!  The following day, observers in Cordoba described the comet as a “blazing star” near the Sun.  The nucleus also broke into at least four separate parts. In the days and weeks that followed, the comet became visible in the morning sky as an immense object sporting a brilliant tail.  Today, some comet historians consider it as a “Super Comet,” far above the run of even Great Comets.

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