Already the Greeks, some two centuries BC, had invented the first meridian lines. Later, there were many of them, which obviously created confusion when it came to navigation. The Paris Meridian exists officially since 1634. The Paris Observatory was built 1667-71 – a few years before the Royal Greenwich Observatory - on a hill where since then the centre of the building is crossed by the Paris Meridian in a perfect North-South direction. The Paris Observatory was then outside the city borders in a calm area, today it’s in the middle of the city.
The architect was Claude Perrault (brother of Charles Perrault, who wrote “Little Red Riding Hood”, “Puss in Boots”, “Cinderella”…). For that time it was a very simple building with hardly any decorative elements, but the quality of the stones and how perfectly well they are fitted together, is amazing – hardly any concrete needed.
The basic activity of the Observatory was of course to study the sky. Telescopes and other optical instruments were, to start with, installed in the central hall of the building, which had very high windows. In this hall was also installed the way to check exact hours and dates (summer and winter solstice, autumn and spring equinox) by help of the sun, a little hole in the wall and a line (the meridian) on the floor. (The same can be seen elsewhere, in Paris e.g. in the Sainte Sulpice Church (see previous post).)
Later, with larger telescopes, special buildings were added to house the instruments – there are a number of cupolas in the surrounding garden.
One of the cupolas is to be found on the top of the building, added during the 19th century. Today all these installations are just historical. Visual sky observation is not done here anymore, but elsewhere, and the Paris Observatory is today only a leading scientific and educational centre.
You have the impression that the Meridian runs through the middle of the Luxembourg Palace (see previous post) and I, also, mistakenly believed so. However, not quite. The Palace was of course built some 50 years before the Observatory.
It seems to be admitted that cartography in its modern sense originated at the Paris Observatory. This led also later to the invention of the meter – defined as one ten millionth of the distance between the North Pole and the Equator. The metric system became the official system of measurement in France in 1795 (also involving litres, kilogrammes…).
In 1884 an International Meridian Conference decided that the Greenwich Meridian should be the standard for the zero degree longitude. France (of course) voted against and got as a “compensation” that the British would adopt the metric system - finally launched a bit later, in 1965, with an official metric program. Today, the only major country which has not adopted the metric system is the USA (see the map of countries which have not adopted the metric system.).
Standing on the roof top, you have of course some nice views of Paris.