98
constellation of Taurus. The
following night, Piazzi checked his
measurements and found that the
object had moved slightly. This
meant it was definitely not a star.
Piazzi watched the object for
24 days before informing Bode.
He thought at first that it was
a comet—a relatively common
discovery—but his observations
soon suggested otherwise. He
could see no fuzzy coma, or tail,
and while comets sped up as they
approached the sun, Piazzi’s
discovery took a more stable,
circular path. In his letter to Bode,
Piazzi made his suspicions clear:
this could be the missing planet
everyone was looking for.
On hearing the news in late
March, Bode wasted no time
in announcing the discovery of
a new planet, which he named
Juno. (He had recently chosen
the name for Uranus, and clearly
felt confident in his right toTHE DISCOVERY OF CERES
do so.) Other astronomers
preferred the name Hera, but
Piazzi, still the only person who
had actually seen the object, had
opted for Ceres, after the Roman
goddess of agriculture.
By June, Ceres’s orbit had taken
it into the glare of the sun. Piazzi
had been sick in the interim and
so had not had the chance to
map anything but the simplest
orbital arc. He calculated that his
discovery would be visible again
in the fall. But, try as they might,
neither Piazzi nor anyone else
could find Ceres.Mathematical hunch
Von Zach decided to follow a
hunch, and sent the details of
Ceres’s orbit to the mathematician
Carl Friedrich Gauss. In less than
six weeks, Gauss calculated all
the places Ceres was likely to be. It
took von Zach most of December to
search through Gauss’s predictions,
but on the evening of New Year’s
Eve 1801, almost exactly a year
to the day after it was first seen,
he found Ceres once again.
The orbital distance of Ceres
was 27.7 Bode units, a remarkably
close fit to the predicted location.
However, the orbital data showed
that this new member of the solar
system was far smaller than the
known planets. William Herschel’s
early estimate put Ceres at just
160 miles (260 km) across. A few
years later, Schröter proposed a
diameter of 1,624 miles (2,613 km).
The actual figure is 588 miles (946
km), which means it would be
a comfortable fit over the Iberian
Peninsula or Texas.Piazzi’s telescope, the Palermo
Circle, was built by Jesse Ramsden.
Its precision mounting allowed it to
measure stellar positions with an
accuracy of a few seconds of an arc.