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Astronomers ready as comet returns

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One of the most studied comets in history will be favorably passing by the Earth in the next few days. Aside from Halley’s Comet, Comet Encke is the most famous and richest in history of all of those mysterious icy wanderers that wend their way among the planets. While Encke is not expected to be visible to the unaided eye, it will be an interesting target through binoculars and small telescopes, for those experienced enough to find it.

ENCKE IS THE COMET with the shortest orbital period known — taking about 3.3 years to complete one revolution around the sun. It does not approach giant Jupiter as closely as do some other periodic comets. So unlike other comets, whose orbits get gravitationally adjusted by Jupiter, Encke’s orbit has remained more or less stable for hundreds of years.

This year, Encke’s Comet will reach perihelion — its closest point to the Sun — on Dec. 29. It will be closer to Earth, however, on Monday Nov. 17, providing the best viewing opportunity in more than six decades.

Whenever perihelion falls in November, December or January, the comet becomes very favorably placed for skywatchers in the Northern Hemisphere. Conversely, when perihelion is in May, June or July, the comet is difficult or impossible to see north of the equator, but can be well seen from the Southern Hemisphere.

LOST AND FOUND The history of Encke’s comet stretches back more than two centuries. This will be the 59th observed return of this object since it was first seen as a fuzzy object on the edge of naked-eye visibility by the Parisian comet hunter Pierre Méchain on Jan. 17, 1786.

Because three revolutions of this comet so nearly equal ten years, it retraces almost the same path across the sky at such intervals. True to this 10-year interval, the comet was not seen again until Caroline Herschel accidentally ran across it on Nov. 7, 1795. Comet Encke was then about 24 million miles from Earth, and her brother William reported that he could even glimpse it without any optical aid.

Another observer compared the comet in brightness to the Andromeda Galaxy. The comet was visible for three weeks before it disappeared into the evening twilight, but unfortunately, astronomers were unable to calculate an adequate orbit for it.

Yet another 10 years passed. The comet was discovered independently by not one, but three observers: Pons (Marseilles), Huth (Frankfurt-on-Oder), and Bouvard (Paris) within several hours of each other on the morning of Oct. 20, 1805.

The comet would pass unseen through the inner solar system three more times before it was again recovered in 1818, when its unusually short period was finally recognized.

GAINING A NAME Jean Louis Pons at Marseilles discovered a comet on Nov. 26, 1818, but had no way of knowing it was the same object that he had previously seen in 1805. Only when Johann Franz Encke, then just twenty-seven years old, worked out the orbit, did it become clear to him that the comets observed in 1786, 1795, 1805, and 1818 were, in fact, one of the same.

Bringing his calculations forward, Encke predicted that the comet would next come to perihelion on May 24, 1822, which it did.

So accurate was his forecast that astronomers universally attached the name of Encke to the comet. And yet, to his dying day, Encke always refused to accept credit for the comet that now bears his name. He always maintained that he merely calculated its orbit and referred to it as “Pons’ comet.”

Since then it has been seen on every one of its returns with the sole exception of August 1944, when its unfavorable position in the sky made observations difficult at a time when most major observatories were hampered by wartime conditions.

Encke’s Comet is also the first comet that has been observed throughout its orbit, for it has even been photographed at the far end of its orbit (aphelion), first in September 1913 and again in August 1972.

CLOSE CALL Comet Encke was at aphelion in May 2002, at a distance of 381 million miles from the Sun, and is now hurtling into our neighborhood where, on Dec. 29, it will swing within the orbit of Mercury, 31.4 million miles from the Sun.

What makes the upcoming visit of Encke so favorable is the comet’s comparatively close pass by the Earth. According to Brian Marsden at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, Encke will come to within 24.3 million miles of the Earth on Nov. 17.

For the Northern Hemisphere, this will likely result in the chance to view the comet since 1937.

In fact, over the past 200 years, Encke has come closer to Earth only twice before: June 1832 (23.9 million miles) and July 1997 (17.7 million miles). The 1997 pass was a good one for Southern Hemisphere skywatchers.

Despite its close approach, skywatchers should not expect an impressive showing from Comet Encke. You will need a telescope or at the very least, a good pair of binoculars to locate it, as well as a chart depicting its projected path against background stars.

Rarely does Comet Encke develop much of a noticeable tail. With such a small orbital period, and countless hundreds, if not thousands of visits to the Sun’s vicinity, this comet is probably worn out. By now, most of its ices have been vaporized by the Sun, and it probably consists of a fairly compact silicate residue, perhaps thinly mixed with the remnant of its original ices.

WHERE IS IT? Currently, the comet is in the constellation of Cygnus, the Swan, located in the Swan’s outspread right wing and shining at around ninth magnitude, too dim to see with the naked eye.

On Friday evening, Nov. 14, it will be passing close to the second-magnitude star Gienah. For the next couple of weeks, the comet will move swiftly south and west against the background stars.

Interestingly, Encke will be passing through two unusual star patterns that we’ve highlighted here on Night Sky over the past year. On Nov. 18 it will passing across the upper part of the “Cowboy Boot” of Vulpecula, while on Nov. 22 it will be very close to the “Coat Hanger” star cluster.

Although the comet will be moving away from the Earth after Nov. 17, its continued approach to the Sun should offset its fading. In fact, Encke will noticeably brighten, probably reaching magnitude 6.5 — the threshold of naked-eye visibility — by Dec. 5.

“It might even be two magnitudes brighter, since it has often in the past become unusually active two or three weeks before perihelion,” notes veteran comet observer Alan Hale in the Astronomical Calendar 2003. By this time, however, Encke will have dropped low into the western sky as darkness falls and will pretty much be gone from our evening sky

If you spot Encke’s Comet, you will have seen it more times than Encke himself.

As Robert S. Richardson (1902-1981), former Associate Director of the Los Angeles’ Griffith Observatory and Planetarium once noted: “Although he devoted about 40 years of his life to keeping track of this comet, Encke apparently never took the trouble to look at it through a telescope. A desk man to the end!”

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