Image: McNaught comet
David Lillo  /  AFP - Getty Images
The McNaught comet as seen early January 19, 2007 from Pucon, Calafquen Lake sector in Chile.
By Space.com skywatching columnist
updated 1/19/2007 3:20:14 PM ET 2007-01-19T20:20:14

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.

Great January comet of 1910: The first people to see this comet—then already of first magnitude—were workmen at the Transvaal Premier Diamond Mine in South Africa on Jan. 13.  Two days later, three men at a railway station in nearby Kopjes casually watched the object for 20-minutes before sunrise, assuming that it was Halley’s Comet.  Later that morning, the editor of the local (Johannesburg) newspaper telephoned the Transvaal Observatory for a comment.  The observatory’s Director, Robert Innes, must have initially thought this sighting was a mistake, since Halley’s Comet was not in that part of the sky and nowhere near as conspicuous. Innes looked for the comet the following morning, but clouds thwarted his view.  But on the morning of the 17th, he and an assistant saw the comet, shining sedately on the horizon just above where the Sun was about to rise. 

Later, at midday, Innes viewed it as a snowy-white object, brighter than Venus, several degrees from the Sun.  He sent out a telegram alerting the world to expect “Drake’s Comet”—for so “Great Comet” sounded to the telegraph operator.  It was visible during the daytime for a couple of more days, then moved northward and away from the Sun, becoming a stupendous object in the evening sky for the rest of January for the Northern Hemisphere. Ironically, many people in 1910 who thought they had seen Halley’s Comet, instead likely saw the Great January Comet that appeared about three months before Halley.

Comet Skjellerup-Maristany, 1927: Another brilliant comet, first seen as a third magnitude object in early December, had the unfortunate distinction of being situated under the poorest observing circumstances possible.  The orbital geometry was such, that the approaching comet could not be seen in a dark sky at any time from either the Northern or the Southern Hemisphere.  But it reached tremendous magnitude at perihelion on Dec. 18.  Located at a distance of 16.7 million miles from the Sun, it was visible in daylight about 5-degrees from the Sun at a magnitude of -6. As the comet moved out of the twilight and headed south into darker skies, it faded rapidly, but still threw off an impressively long tail that reached up to 40-degrees in length by the end of the month. 

Comet Ikeya-Seki, 1965: This was the brightest comet of the 20th century, and was found just over a month before perihelion passage in the morning sky moving rapidly toward the Sun.  Like the Great Comets of 1843 and 1882, Ikeya-Seki was a Kreutz Sungrazer and on Oct. 21 swept to within 744,000 miles of the center of the Sun.  The comet was then visible as a brilliant object within a degree or two of the Sun, and wherever the sky was clear, the comet could be seen by observers merely by blocking out the Sun with their hands.   From Japan, the homeland of the observers who discovered it, Ikeya-Seki was described as appearing “ten times brighter than the Full Moon” corresponding to a magnitude of -15. Also at that time, the nucleus was observed to break into two or three pieces.  Thereafter, the comet moved away in full retreat from the Sun, the head fading very rapidly but its slender, twisted tail, reaching out into space for up to 75 million miles, and dominating the eastern morning sky right on through the month of November.

Comet West, 1976: This comet developed into a beautiful object in the morning sky of early March 1976 for Northern Hemisphere observers.  It was discovered in November 1975 by Danish astronomer, Richard West on photographs taken at the European Southern Observatory in Chile. Seventeen hours after passing within 18.3 million miles of the Sun on Feb. 25, it was glimpsed with the naked eye 10 minutes before sunset by John Bortle—the last daylight comet sighting until McNaught in 2007. In the days that followed, Comet West displayed a brilliant head and a long, strongly structured tail that resembled “a fantastic fountain of light.” Sadly, having been “burned” by the poor performance of Comet Kohoutek two years earlier, the mainstream media all but ignored Comet West, so most people unfortunately failed to see its dazzling performance!

What’s next for Comet McNaught?
Viewers in the Southern Hemisphere will now have Comet McNaught pretty much all to themselves in the days ahead.  It should continue to be a striking object in the west-southwest sky as darkness falls. 

If a parallel can drawn between Comet McNaught and any of the above-mentioned comets, it’s that it should gradually fade as it moves away from both the Earth and Sun.  As we have previously noted, new comets can be notoriously unpredictable to forecast, but it appears now that McNaught should be shining somewhere between magnitude 0 and 2 on Jan. 21, then fade to perhaps magnitude 5 by the end of January or early February.

Although the comet is fading as it moves higher into the sky and sets progressively later, its tail should appear to impressively lengthen. 

Amazingly, the end of the tail (called the "terminus") has been glimpsed as far north as Colorado; the multiple streamers protruding above the southwest horizon resemble faint auroral rays in binoculars.

It should, in fact, appear at its longest this upcoming week—before the increasing brightness of the waxing Moon begins to compromise the view.  Skywatchers should look for two tails.  The one appearing slender and straight and pointing nearly directly upward from the horizon, will be due to gas, while the other, appearing as broad and gentle curving fan is composed of dust expelled from the comet’s head and made visible by reflected sunlight. 

Without doubt, however, Comet McNaught’s performance will stand as one of the most spectacular in recent years. For those fortunate enough to have seen it, it will always be a comet to remember.

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