Paris Observatory

A royal institution

Louis XIV's minister Colbert founded the Académie des sciences in 1666 and the observatory the next year; the building, designed by the physician and architect Claude Perrault, was completed in 1672. Its first director was Giovanni Domenico Cassini, brought from Bologna, who stayed for the rest of his life; four generations of his family directed the observatory.

Cassini discovered four moons of Saturn, Iapetus in 1671 and three others later, and described in 1675 the division in Saturn's rings that bears his name. The observatory's meridian line, the Paris meridian, served French cartography until the Greenwich meridian replaced it in 1884.

The speed of light

In 1676 the Danish astronomer Ole Rømer, working at Paris, noticed that the eclipses of Jupiter's moon Io occurred later than predicted when Earth was farther from Jupiter, and concluded that light takes time to travel. His estimate of the speed of light was about 220,000 km/s, some 25 per cent below the true value, but it settled that the speed was finite. The observatory marked the 340th anniversary of the finding in 2016.

From 1679 it published the Connaissance des temps, an almanac of astronomical ephemerides that is still published.

Weather, time and a wider site

Under Urbain Le Verrier the observatory turned to meteorology; in 1863 it published the first modern weather maps, and a telegraph network of stations fed a daily bulletin. In 1913 it used the Eiffel Tower as a radio antenna to measure longitude with the US Naval Observatory, a pioneering use of time signals.

As Paris brightened, work spread: the observatory has a site at Meudon, with the Great Refractor, and a radio-astronomy station at Nançay, in the Sologne. It is now part of PSL University.

How it compares

Paris and Greenwich were founded within eight years of each other, with similar aims and different methods. Greenwich concentrated on positions of stars and the Moon for navigation; Paris, under the Cassini family, explored the planets and geodesy, and measured the arc of the meridian across France. The result was a rivalry in the standards of time and of longitude: until 1884 French charts used the Paris meridian. After the Greenwich meridian was adopted, Paris remained an observatory with a wide range, from the planets to radio astronomy and to the measure of time, with the Paris laboratory that keeps the French time scale today. The comparison shows how national observatories shape the standards a country uses.

Instruments and technologies

• Meridian circle A telescope fixed to rotate in the plane of the meridian; the time a star crosses and its altitude give right ascension and declination.
• Long-focus refractors Telescopes with focal lengths of tens of metres, used by Cassini to separate Saturn's moons and ring features before achromatic lenses existed.
• Nançay radio telescope A large decimetric telescope with a fixed spherical mirror and a tilting flat mirror, set up in the 1960s.

The people

• Giovanni Domenico Cassini First director; discoverer of four moons of Saturn and the ring division.
• Ole Rømer Danish astronomer who demonstrated the finite speed of light from Io's eclipses.
• Claude Perrault Architect of the observatory building.

Timeline

• 1667 Observatory founded.
• 1671 Cassini discovers Saturn's moon Iapetus.
• 1675 Cassini describes the division in Saturn's rings.
• 1676 Finite speed of light demonstrated from the eclipses of Io.
• 1679 First issue of the Connaissance des temps almanac.
• 1863 Publishes the first modern weather maps.
• 1913 Uses the Eiffel Tower as an antenna to measure longitude by radio with the US Naval Observatory.

Key lessons for observation today

• A public service outlasts its first purpose The Connaissance des temps and the weather bulletin gave the observatory a use that survived changes of science. Services that people rely on keep programmes funded.
• Move the work when the sky gets worse Meudon and Nançay kept the science going as city light grew. Siting is a design variable, reviewed over the life of a network.
• Compare the same event from two places Rømer's result depended on repeated, dated observations. Time-stamped data from many sites is what lets anomalies be spotted.

Sources

• Wikipedia, Paris Observatory
• Wikipédia, Observatoire de Paris
• Observatoire de Paris – PSL, 340 years ago Rømer showed that light has a finite speed
• Wikipedia, Giovanni Domenico Cassini
• Observatoire de Paris, history