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
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.
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!"
Night software brings the universe to your desktop. Map the sky from
your location, or just sit back and let the cosmos come to you.
-- a veteran of eleven total lunar
eclipses -- 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.
the standard by which astronomers measure the apparent brightness of objects
that appear in the sky. The lower the number, the brighter the object.
The brightest stars in the sky are categorized as zero or first magnitude.
Negative magnitudes are reserved for the most brilliant objects: the brightest
star is Sirius (-1.4); the full Moon is -12.7; the Sun is -26.7. The faintest
stars visible under dark skies are around +6.