Observatoire de Paris

The Paris observatory was founded by Louis XIV's finance minister, Colbert, in 1667; it was designed by Claude Perrault (who also worked on the Louvre), with labs and an observation tower. The French meridian line drawn by François Arago in 1806 (which was used here before the Greenwich meridian was adopted as an international standard) runs north-south through the centre of the building. The dome on the observation tower was added in the 1840s.

You'll need to apply for an appointment at the Observatoire by letter, but it's also worth checking the website for openings linked to astronomical happenings - or visit on the Journées du Patrimoine.