12 AUSTRALIAN SKY & TELESCOPE May | June 2018
In the view of many, Comet Donati
was the most beautiful comet ever
seen. This was a big call. Certainly it
was so spectacular that it attracted
the attention of artists of the calibre
of William Turner and others across
Europe. The comet’s passage was also
recorded with the still-embryonic
technology of photography, barely 20
years old but already taken up with
enthusiasm by astronomers. Comet
Donati was in fact the first comet
to be photographed, initially by a
portrait photographer in England andlater through a telescope at Harvard
University Observatory, though
photographers struggled to achieve
anything like the detail the painters
could capture.
The comet was also the most
observed by 19th century astronomers,
as the skies were dark and the weather
clear across northern Europe at the
time. Its appearance also sparked
an upsurge of interest in astronomy
amongst the wider public, many of
whom might have looked up with
interest at the night sky for the first
time. An erroneous calculation byDISCOVERIES by David EllyardDonati and his discovery
Remembering the most-observed comet of the 19th century.
O
ver the centuries, numerous
celestial objects have been given
names, and through those
names we remember many people and
places, some famous and some not so
famous. We all know of Edmund Halley
mostly because of the comet named
after him (though he did not discover
it) and despite his other achievements.
Many objects are named for lesser-
known astronomers. So it is with the
19th century Italian, Giovanni Donati.
A map of the Moon will show a crater
called Donati, and further out we can
find an asteroid bearing his
name... even though, like Halley,
he did not actually discover
either of them. Yet he certainly
did find Comet Donati. In fact,
he found six comets over a
10-year period. But the one that
came into his view in 1858 is the
Comet Donati and one of the
great celestial sights of all time.
On June 2 of that year, from
his observatory in Florence,
Donati glimpsed a small patch
of light near the head of Leo. At
magnitude 7 it was just out of
naked-eye reach, but it soon brightened
and moved, proving that it was indeed
a comet.
By September it had moved into Ursa
Major and had become a stunning sight
for northern viewers. Lying across the
stars of the Great Bear, its scimitar-
shaped tail with three spikes stretched
halfway from the zenith to the horizon
at the time of its closest approach to
Earth on October 9. In linear terms
that span was more than 75 million
kilometres. Only the Great Comet
of 1811 was of equal or surpassing
spectacle. Later, Comet Donati passed
into the southern sky as it began to
fade and was finally lost to view around
March, 1859.S William Turner’s watercolour of Comet
Donati’s 1858 visit to Earth’s skies.one astronomer a year or so earlier,
indicating that a comet would strike the
Earth in 1857, had caused some hysteria
— but in the event Comet Donati
missed us by half the distance between
the Earth and the Sun.
Perhaps the most famous lay
observer of Comet Donati was Abraham
Lincoln. He reported seeing it from his
porch in Illinois on the eve of one of
his famous debates about slavery with
political rival Stephen Douglas— debates
that influenced his election as a US
senator, and ultimately as President and
all that followed from that.
For Donati and others, of
course, there was science to
be done, as the true nature
of comets was then still
unknown. Donati was a leading
exponent of the still-new
technology of spectroscopy,
which extracts information
from light by breaking it up
into its constituent colours.
He had noted that as a comet
approaches the Sun and heats
up, its spectrum of colours
changes, indicating that it has
begun to shine by its own light rather
than merely reflecting sunlight. So at
least some part of a comet must be
composed of gas that can be heated till
it glows.
It will be a long time before
Comet Donati returns to our skies.
Calculations of its orbit as it rounded
the Sun suggested it was headed out
into deep space and will not be back for
about 2000 years. It will reach its most
distant point around the year 3047,
before obeying the tug of the Sun and
starting its return journey.■ DAVID ELLYARD presented SkyWatch
on ABC TV. His StarWatch StarWheel
sold over 100,000 copies. PAUL MELLON COLLECTION