Paris Observatory: From Construction in 1667 to Observations across the Atlantic

Paris Observatory was founded through royal patronage under Louis XIV and the Royal Academy of Sciences. The Academy was established in 1666, and construction of the observatory began the following year. The process included acquiring land, determining the building’s orientation, and gradually beginning observations; 1667 should not be understood as the year when every facility was completed at once.

Acting for the king, Colbert purchased land beyond the Faubourg Saint-Jacques on March 7, 1667. On the summer solstice, June 21, Picard, Richer, and other Academy scholars observed the Sun at the site to establish the meridian and the building’s orientation. Construction began that year. The land-purchase deed and Academy meeting records assembled by Wolf connect administrative preparations with astronomical measurement at the same location.

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The first requirement in choosing a site was an unobstructed view for observations. Claude Perrault’s explanation noted that although Montmartre was high, it might be affected by smoke rising from the city of Paris to its south. The site ultimately chosen lay beyond the Faubourg Saint-Jacques south of the city, leaving Paris to the north and relatively open views to the east, south, and west, with access to the city along major roads.

Perrault was responsible for the architectural design, but scientific instruments imposed additional spatial requirements. The building had to provide fixed observing positions while accommodating clocks, angle-measuring instruments, and researchers’ living quarters. By the time Cassini reached Paris, construction had already reached the first story. His participation involved adjustments during construction and cannot retrospectively be treated as the starting point for the entire design.

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Jean-Dominique Cassini arrived in Paris in 1669 and moved into the observatory in 1671. The Cassini family subsequently became closely associated with it, but after comparing correspondence and appointment documents, Wolf pointed out that the first Cassini never received the title of director in its later sense. The early observatory operated through the Academy, and the organizational structure of a later independent institution cannot simply be projected onto it.

Research also depended on instrument makers. Gosselin made angle-measuring instruments, and Thuret made pendulum clocks. Observers had to check graduations, calibrate alignment, and coordinate readings of angles and time. In his study of scientists’ incomes and instrument costs, Lequeux notes that royal funding, salaries, and accommodation together provided the material conditions for research; some expensive instruments also cost far more than ordinary researchers could afford individually.

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In 1671, the Academy decided to send observers to a region near the equator so that observations could be compared with those made in Paris. Richer reached Cayenne in South America in 1672 and remained until 1673. His report, published in 1679, listed his tasks: to investigate the parallaxes of the Sun, Venus, and Mars; observe southern stars difficult to see from Europe; and measure pendulum length, atmospheric pressure, and tides.

Richer carried an octant, a quadrant, and two pendulum clocks. Before and after departure, he tested the instruments at Paris Observatory, La Rochelle, and Cayenne to distinguish genuine celestial phenomena from instrumental errors. Instruments were also retained in Paris for corresponding observations. The expedition’s value therefore lay not simply in adding another observing location, but in using comparable measurement procedures at both sites.

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Richer’s report places descriptions of the instruments before its numerical records, explaining which altitude readings required corrections to be added or subtracted and whether an observation concerned the Sun’s center or its limb. Meridian-transit records preserve the date, the name of the celestial body, and the clock reading. Departures of an instrument from the meridian and readjustments of clocks were also recorded. This method of documentation enabled later calculators to determine how the values had been obtained.

The report also records that the length required for a seconds pendulum in Cayenne was shorter than in Paris. Richer stated that he repeated the measurement over ten months and brought the locally calibrated length back to France for comparison. This was an empirical result obtained across different locations. Reading it alongside the instrument checks and repeated experiments explains how the early observatory produced knowledge more effectively than attributing the achievement solely to a scholar’s single moment of inspiration.

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According to Wolf’s research, the building was not yet entirely finished when Cassini moved in during 1671, and the main construction phase was not completed until 1672. Research equipment also faced practical difficulties: a storm damaged a large quadrant in 1671, and glazing was still being installed in the tower windows in 1672. Clocks occasionally stopped, requiring time references to be obtained from other observers.

The early history of Paris Observatory was thus a process in which buildings, instruments, personnel, and recording procedures gradually came into alignment. The year 1667 marks its foundation and the start of construction, while residence and collaborative observations in the 1670s made it a continuously functioning workplace. Cayenne was an expeditionary observing site connected with Paris, not a relocation of Paris Observatory overseas.

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