Geoscience Reference
In-Depth Information
The telescope was moved from one feature to the other, and as viewed from the
telescope, the angle between two features was the angle entered into the trigono-
metric calculations.
Places
The
mire
at Montmartre
Picard put the original trigonometrical station on the hill of Montmartre in 1673. The
present stone obelisk dates from 1736 and was placed here by Jacques Cassini. Originally
it was inscribed:
L'an M DCC XXX VI cet obélique a été élevé par ordre du Roi pour servir d'alignement à
la méridienne de Paris du côté du nord. Son axe est à 2931 toises 2 pieds de la face
mériodionale de l'Observatoire.
“In the year 1736 this pyramid was erected by order of the King to serve to align the merid-
ian of Paris on the northern side. Its axis is 2931 toises 2 feet from the southern face of the
Observatory.”
As a result of weathering, only the date can now be read. Presumably in the eighteenth
century the houses and trees less obscured the view of the obelisk from the Observatory.
Now, the obelisk stands overshadowed by a clutter of urban outbuildings placed without
any sympathy for its importance and reducing it from its former glory.
The survey starts off with a measured baseline that can then be extended by
measuring the angles of triangles to the most distant point. As a check on the proc-
ess and to determine the accumulated error, it is best if the survey includes at check
points the extremities of the triangulation line or another baseline, whose length is
determined not only from the trigonometry but also from direct measurements.
According to the check, Picard's survey was accurate to 5 meters. Nearly 100 years
later, though, it was concluded that this was a misleading impression of the accu-
racy and there must have been canceling errors.
IN 1669 while Picard was working on his first survey in the field, Colbert, in the
name of the King, invited the Italian astronomer Gian Domenico Cassini to visit
Paris and work at the Observatory. This was the beginning of a long collaboration
between the two men.
People
Gian Domenico Cassini (1625-1712)
Gian Domenico was known as Cassini I (
Fig. 12
) to distinguish him from his son, grandson,
and great-grandson (all served as senior figures in the Paris Observatory). He was born in
Perinaldo, near Naples. Attracted to astrology in his youth, he became Professor at Bologna
studying the rotation periods of the planets and the satellites of Jupiter. In the 1650's he
reworked and corrected the construction of the accurate sundial, or
meridiana
, in the Church
of San Petronio in Bologna to measure the motion of the Sun, timing and dating the position
of the Sun's transit and image across an accurate north-south line (see Chapter 4).
When he moved to France in 1669 and was appointed to the Paris Observatory, Cassini had
intended it as a limited visit, but his stay became permanent, his intentions changed by a
woman. He married this woman, Geneviève de Laistre, daughter of the lieutenant general
of the Comte de Clermont north of Paris. Her dowry included the Château de Thury, where
the Cassini family lived for years thereafter. He took up French citizenship and altered his
name to Jean Dominique, becoming fully adopted into the French establishment.
When Cassini arrived in Paris he erected and used a telescope of 17 feet (5.2 m)
focal length given to him by the instrument maker Giuseppe Campani and with this
made more accurate measurements of the Sun's rotation. The excellence of the
