Sky & Telescope - November 2020 - 60

Cometary History
t SPLITSVILLE Comet Biela in February
1846, not long after it broke into two separate comets

Panic Among the People
Comet Biela was next observed during its 1832 return, when
it created quite a stir amongst the public. This wasn't because
it became visible to the unaided eye - it remained strictly a
telescopic sight - but because predictions showed that Comet
Biela would intersect Earth's orbit, albeit a month after our
planet had vacated the vicinity. Nonetheless, the popular
press was captivated by the notion of a close comet-Earth
encounter, and the fact that the two were predicted to miss
each other by 32,000 kilometers (20,000 miles) got lost in
the resulting excitement. Newspaper accounts of the day
reveal that a bit of a mass hysteria ensued. The frenzy was not
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One Comet Becomes Two
The comet was recovered on November 26, 1845, by the Italian astronomer and Jesuit priest Francesco de Vico, himself
a famous comet hunter. A month later, on December 29th,
two observers using a 5-inch refractor at Yale College in New
Haven, Connecticut, sighted a faint secondary comet one
arcminute northwest of the primary comet; the American
astronomer and naval ofﬁcer Matthew Fontaine Maury found
the same on January 13th. Astronomers subsequently observed
the slow separation of what was clearly a pair of comets. No
one had observed a comet splitting up before. Reports indicated that the two alternated in brightness, with one brighter
on one night, then the other brighter the next night.
There were even drawings and reports of a bridge of material linking the two comets for a short time. Much later, during the mid-1900s, astronomers thought this bridge had likely

In Comet
Biela's Orbit

Many people over the past
several hundred years figure,
even if indirectly, into the
story of Comet Biela. See the
main text for details on how
these individuals - amateur
astronomers, mathematicians,
and others - factor into the
questions surrounding this icy
visitor from afar, which may or
may not still exist in any form
beyond the particles that make
up the almost-extinct Andromedid meteor shower.

Edmond Halley
(1656-1742)

CO ME T BIEL A: E. WEISS / BILDER ATL AS DER STER NENWELT / WIKIMEDIA COMMONS / PUBLIC DOMAIN;
HALLE Y: RICHARD PHILLIPS / NATIONAL PORTR AIT G A LLERY / WIKIMEDIA COMMONS / PUBLIC DOMAIN

vations. If you wanted it done, you did it yourself.
In 1806 Carl Friedrich Gauss, the greatest mathematician
of his day and one of the greatest of all time, went further in
calculations of what would become known as Biela's Comet.
He tentatively linked the 1805 comet to one that French
astronomers Jacques Leibax Montaigne and Charles Messier
had seen in 1772. After further mathematical analysis, the
three comets were tied together. Despite the previous sightings, Wilhelm von Biela's name became attached to the comet
because his calculations ﬁrst conﬁrmed it to be periodic.
Comet Biela was unexceptional, however. It usually brightened to no more than 8th magnitude during its apparitions.
Its orbital period of 6.6 years (speciﬁcally the 0.6) ensured
that at least half the time it would be on the opposite side of
the Sun from our planet when it arrived in the inner solar
system, making it almost impossible for telescopes of the
time to see it. Except when it approached near to Earth, one
needed a good-quality telescope in order to detect it.
As far as anyone knew, Comet Biela had probably traveled
this same path for hundreds of years. It was later determined
to be part of the Jupiter family of comets - those short-period
comets with orbital periods of less than 20 years whose orbits
are primarily determined by Jupiter's gravitational inﬂuence.

about whether the comet's gas would
lethally poison everyone, but whether
or not the comet would collide with
our planet.
Of course, nothing untoward
happened. Comet Biela passed
safely across the skies, witnessed
only by those who sought it out
with telescopes.
In 1839 the comet was not
favorably positioned for observation, and no one saw it. This was
due to that 0.6 portion of a year in
Comet Biela's orbital period, which
resulted in the icy body getting lost
in the Sun's glare some years. So, astronomers waited for the
comet's 1845 approach.

Sky & Telescope - November 2020

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