Sky & Telescope - May 2021 - 61

COME T ILLUSTR ATION: WIKIMEDIA COMMONS PD -US-ART;
VIE W OF BOLOG N A: G A BRIELL A BER N A RDI

around the Sun. Cassini also studied comets. In the 17th
century, little was known about the true nature of these icy
visitors, and their study lay at the forefront of astronomical
research. Cassini observed a comet that appeared in 1652-53,
and measures of its motion revealed that the object had no
significant parallax, meaning it must be very far from Earth.
This conclusion contradicted the commonly held hypothesis
that comets were " emitted " by our planet, and it constituted
an important contribution to the debate between Aristotelean
and Galilean physics.
While in Rome in 1664, Cassini viewed a second comet
from the Chigi Palace (home of the Pope's brother) and from
the Riario Palace (residence of Queen Christina of Sweden).
Cassini became the first astronomer to employ laws of planetary motion to predict a comet's path in the sky.
Cassini's instrumentation wasn't restricted to telescopes.
As mentioned earlier, he designed the meridian line in the
Basilica of San Petronio, in Bologna, Italy - one of the largest
scientific instruments of the time. By measuring the passage of the solar disk on the meridian line, Cassini precisely
determined the length of the year. Less well known is that he
also used this instrument to demonstrate experimentally the
validity of Kepler's Second Law, thus performing the first test
of the laws of planetary motion that ultimately helped give
rise to Isaac Newton's Principia.
Near San Petronio, Cassini exploited the 100-meter-high
Asinelli Tower in Bologna to view an unobstructed horizon.
This allowed him to produce a refined theory of atmospheric
refraction that was much more accurate than the previous
one proposed by Tycho Brahe.
These are just some of Cassini's scientific contributions.
From the perspective of the 21st century, many of them may
seem relatively minor; however, together they helped advance
our understanding of the universe. But perhaps Cassini's
greatest contribution isn't found in his numerous discoveries
so much as in his methodology.

S A TOWERING VIEW This perspective of Bologna and its surroundings
is from the top of the Asinelli Tower, where Cassini and other scientists
made astronomical observations and performed experiments. In the distance is the blocky profile of the Basilica of San Petronio, where Cassini's
meridian line is located.

W COMET CATCHER
The comet of 1664 is
shown near Corvus in
this engraving from a
German listing of comets from 1566 to 1664.
Cassini observed the
object and used Kepler's laws of planetary
motion to predict its
path across the sky.

s k y a n d t e l e s c o p e . o r g * M AY 2 0 2 1

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Sky & Telescope - May 2021

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