How the Meridian Line was Drawn on the Paris Observatory’s Site on Solstice Day 21 June 1667 (2022)
Thomas Widemann (Author)
Archives Internationales d'Histoire des Sciences
Volume: 72
Issue: 189
Pages: 56-73
The meridian line drawn at the Paris Observatory became the French Prime Meridian for world cartography and navigation (a fixed origin of terrestrial longitudes) and a universal local time reference for celestial ephemeris for 244 years, from June 1667 until its demotion in favour of the Prime Meridian at the Royal Observatory of Greenwich in March 1911. This article revisits technical accounts on how the line was actually drawn on the then recently purchased site of the Observatory’s building on Solstice Day, 21 June 1667 prior to the construction of the actual edifice, in presence of members of the French Académie des Sciences Bernard Frénicle de Bessy, l’abbé Jean Picard, Jacques Buot, Jean Richer, Adrien Auzout and Claude-Antoine Couplet. We show that pendulum clocks were not used to determine the azimuth of the Sun. Instead, the technique of equivalent heights was used, as described by Adrien Auzout (1622-1691), in order to eliminate “the risk of wander resulting of any previous observations”. The drawing of the line on a stone laid by mathematician and fellow member of the Academy Claude-Antoine Couplet (1642-1722), took place on solstice day in order to minimize hour-by-hour variations of the Sun’s declination. Further descriptions on how the meridian line was drawn on the stone can be found in the Procès-Verbaux de l’Académie as well as additional accounts in Du Hamel (1624- 1706), Delisle (1688-1768) and reference to their descriptions by Fontenelle (1657-1757) and Wolf (1827-1918).
...MoreArticle Thomas Widemann; Eberhard Knobloch (2022) Introduction. Archives Internationales d'Histoire des Sciences (pp. 6-23).
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