Geoscience Reference
In-Depth Information
As soon as Cassini arrived in Paris, he was enlisted to work with Picard to extend
the meridian from its base on the plain near Fontainebleau. Between 1669 and1670,
Picard and Cassini measured the long baseline running north-south, south of Paris
from Juvisy to Villejuif, and completed the survey northwards to Amiens. They
measured the length of a degree of latitude across Paris at 57,060 toises, corre-
sponding to 111,092 meters (the modern value for the average length of a degree of
latitude is 111,220 meters). In 1671, their work was published as
Mesure de la Terre
in 1671 and resulted in a
Map of the Paris Region
published in 1678.
IN 1671-1672 PICARD took on the practical task of surveying, intending to shed
light on the most important scientific theory under development at the time - some
say the most significant scientific theory ever - the theory of gravity. He visited
Brahe's ruined observatory, Uraniborg (“Castle of Urania”
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or “City of the Heavens”),
at Hven, in order to measure its position. The reason the position of Uraniborg was
so important to Picard was that, if its accuracy was known, observations that had been
taken there a century before could be related to other measurements, particularly
those being taken by Picard and his colleagues at the Paris Observatory.
Uraniborg was the observatory of the Danish astronomer Tycho Brahe and its
conception was triggered by the appearance of a new star. In 1572 Brahe was riding
in his coach going home from a convivial dinner in Copenhagen when he noticed
some peasants marveling at the sky. It was a bright star that had appeared in the con-
stellation of Cassiopeia, where no star had been seen before. This was remarkable
because it seemed to be a change in what was believed to be the “unchangeable”
celestial heavens. Brahe was stunned that this idea, which was a key concept for
Aristotlean philosophy. The model of the Universe proposed by Ptolemy that the
Moon, the planets and the Sun revolved around the Earth within the sphere of fixed
stars was be wrong. According to this philosophy, the planets moved and changed
brightness because they were near to the Earth where everything was in flux, altering
progressively until death. By contrast, the stars were at great distances next to heaven
and were perfect and unchanging. What the peasants had seen seemed to be a star,
and it had changed from being invisible to being one of the brighter stars in the sky.
To see if the star was really in the celestial sphere, Brahe made careful measure-
ments over a year of the star's position relative to other stars. He used a version of
cartographers' surveying techniques that triangulates on heavenly bodies and deter-
mines their distance. Brahe was at his observatory on the Earth in Copenhagen but
was being carried around the Earth as it rotated in its daily motion. If he sighted on
the star at 6 p.m. as the Sun set and then again at 6 a.m. as it rose, he would have
been carried round the Earth by one half of a rotation. The sightings effectively
established the angle to the star as seen from these two positions, called the
method
of diurnal parallax
. Tycho established that the star never moved and as far as he
could measure its parallax was zero. This proved that the star was more remote
than, say, the Moon. In the topic
De Stella Nova
(“Concerning the New Star”)
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Urania was the classical muse of astronomy.
