Sky & Telescope - May 2021 - 63

was perfect for the task because of his strong work ethic and
afﬁnity for collaboration.
These same traits were to bear remarkable fruit during
the favorable Mars opposition of 1672. Cassini organized a
group of astronomers to observe the event from several different locations. He remained in Paris while his colleague
Jean Richer traveled to Cayenne in French Guiana, near the
equator, and John Flamsteed, the ﬁrst Astronomer Royal,
observed from Derby, England. Flamsteed had predicted that
Mars would occult the 4.4-magnitude star Psi2 Aquarii, providing an ideal opportunity to use parallax to calculate the
distances between Earth and Mars and between Earth and
the Sun, thereby reﬁning the value of the astronomical unit.
The successful observations were a pivotal moment in the
understanding of our place in the universe.

The Birth of Big Science
For all his many discoveries and impressive range of observations, perhaps Cassini's greatest contribution to science
was introducing three elements of modernity: a distinction
between astronomers and telescope-builders; a carefully
organized observing method; and conceiving science as the
collaboration among groups of scientists in order to achieve
goals inaccessible to a single individual.
From our present perspective in the era of " big science, "
these developments might seem obvious and quite normal.
However, that sense of normalcy arises only because scientists
such as Cassini made it possible.

GABRIELLA BERNARDI is a freelance science journalist

STAT UE DE TAIL: G A BRIELL A BER N A RDI; OLE R Ø MER:
SCIENCE HISTORY IM AG ES / A L A M Y STOCK

based in Turin, Italy. She is the author of six astronomy books,
including Giovanni Domenico Cassini: A Modern Astronomer
in the 17th Century.

S CARVED IN STONE Housed at the Paris Observatory is a statue
of Cassini. Next to the left foot are scrolls listing some of the great astronomer's most notable discoveries. Cassini was the observatory's first
director - a position he held until his death in 1712.

Cassini and the Speed
of Light
As notable as his list of discoveries is, Cassini also had a
few near misses.
Most readers know that Danish astronomer Ole Rømer
made the first estimation of the speed of light. He has
earned a rightful place in history because of it, though he
wasn't as devoted and prolific an astronomer as Cassini.
However, in the 1660s and '70s as he systematically observed the moons of Jupiter, Cassini noticed
discrepancies between the timings of his observations
and his predictions. This was obviously a problem since
determining longitude required accurate tables. He set
about calculating the corrections that would have to be
applied to his tables in order to improve their accuracy.
Interestingly, in a communiqué to the French Academy of
Science of 1672, Cassini mentioned the possibility that
the observed discrepancies could be ascribed to a finite
speed of light. However, Cassini never quite carried the
idea to its full conclusion as Rømer did.
S MYSTERY MOONS Puzzled by discrepancies between predictions and observations, Danish astronomer Ole Rømer discovered
that the apparent errors were due to the finite speed of light - an
idea that Cassini had also had but failed to pursue.

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

http://www.skyandtelescope.org

Sky & Telescope - May 2021

Table of Contents for the Digital Edition of Sky & Telescope - May 2021