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but today these towers, suitably strengthened, carry permanent observing domes for the
telescopes now installed there. A terrace surrounds the building and certain instruments
were also taken on to the terrace for observations. The building was built without wood (to
avoid fire) and without iron (to avoid disturbance to any magnetic measurements); some
criticised it, however, for its unsuitability for astronomical observations because there is
nowhere inside from which it is possible to view close to the zenith.
What has become known as the Cassini Room is situated centrally on the floor above the
main northern entrance of the Observatory. It is a huge room, now reconstructed to contain
pillars to support instruments and structures placed on the roof. Here the first leader of the
Observatory, Gian Domenico Cassini, constructed the first Paris Meridian on a north-south
line on the floor. In May 1682, the King visited this meridian to regulate his watch to
accurate Paris time.
The meridian line in the Cassini room is now marked by a memorial erected by Gian
Domenico's son, Jacques, to his father: “Having extended the meridian line that passes
through the Paris Observatory in both directions, north and south, to the confines of the
kingdom, it appeared necessary to the full perfection of the work to trace within the
Observatory itself a meridian line that would be part of the one crossing the kingdom and,
at the same time, would serve for the astronomical observations pursued vigorously there
since its inception.” The line is flanked by worn and rather crudely chiselled engravings of
the zodiacal figures covered by transparent plastic overlays for their protection. The figures
mark where the image of the Sun falls as it passes into the corresponding zodiacal sign.
The Observatory is decorated with other unique works of art. For example, on the ceiling
of the Council Room, watched in perpetuity by portraits of the directors of the Observatory,
is an allegorical picture (1886) of the Transit of Venus in front of the Sun, painted by
Edmond Louis Dupain (1847-1933).
AS PART OF THE Observatory's program, even before its buildings were finished,
the Academy took up Colbert's challenge to create better maps of France (Konvitz
1987). It charged one of its members, the Jesuit astronomer Abbé Jean Picard, with
the measurements.
People
Jean Picard (1620-1682)
Little is known of the early life of the Abbé Jean Picard although he started his working life
as a gardener (or perhaps this is a misunderstanding of reports intending to say that he was
a horticulturalist). Via training in the Church, he became a professor of astronomy in Paris
and made several improvements in scientific instrumentation including measurements of the
diameters of the Sun and the Moon, which since the angular diameters change with distance
from the Earth, were used to help determine their orbits. He was considered in his time a
scientist of the first rank, and in criticising the performance of the first leader of the Paris
Observatory some historians have said that Picard ought to have been appointed instead.
Picard approached scientifically the tasks he had been given. The best available
maps of France up to then were those by Sanson, yet as detailed, clear and elegant
as these maps were, they were the product of office research and collation, not of
field work. Colbert contrasted the method of their execution with the pioneering
work by Dutch cartographers; they had based their methods on principles devel-
oped by Willebrord Snell.
People
Willebrord Snell van Roijen (1580-1626)
Snell studied law in Leiden but became interested in mathematics and astronomy. He
traveled throughout Europe and studied in Paris, and in 1613 he succeeded his father as
professor of mathematics at the University of Leiden. He published
Eratosthenes Batavus
,
