page_title: Gnomon of Saint-Sulpice
section_title: Computation of the Paschal equinox
hierarchical_section_title: Gnomon of Saint-Sulpice / Usage of the gnomon / Computation of the Paschal equinox
caption_attribution_description: English: Gnomon_obelix inscription at Saint Sulpice
context_page_description: The Gnomon of Saint-Sulpice is an astronomical measurement device located in the Church of Saint-Sulpice in Paris, France. It is a gnomon, a device designed to cast a shadow on the ground in order to determine the position of the sun in the sky. In early modern times, other gnomons were also built in several Italian and French churches in order to better calculate astronomical events. Those churches are Santa Maria del Fiore in Florence, San Petronio in Bologna, and the Church of the Certosa in Rome. These gnomons ultimately fell into disuse with the advent of powerful telescopes.
context_section_description: After this first attempt, Languet de Gergy resumed the project in 1742, this time with the objective of properly defining the Easter Equinox. The task was given to Pierre-Charles Le Monnier, member of the French Academy of Sciences.
  The inscription at the base of the obelisk mentions Charles Claude Le Monnier, as well as the mission of the gnomon in Latin: "Ad Certam Paschalis Æquinoctii Explorationem" ("To determine precisely the Paschal Equinox").
  The dating of Easter was modeled on that of the Jewish Passover, which marks the liberation of the Jews from Egypt and which traditionally falls in the Jewish lunar calendar on the 14th of the month of Nisan, the day of the first full moon after the vernal (spring) equinox. Roman Christians however had a twelve-month calendar, first the Julian calendar until 1582, and then the Gregorian calendar. Since the Council of Nicaea in 325, the Western Church had required that Easter be celebrated on the Sunday on or after the full moon following March 21, which at that time corresponded indeed to the vernal equinox. The Julian calendar being imprecise however, by the 16th century March 21 fell about 10 days after the vernal equinox, a problem that was solved by the introduction of the Gregorian calendar. (The Eastern Christian Church continues to date Easter by the Julian calendar.) Languet de Gergy, however, wished to verify independently the exact date of the vernal equinox through the gnomon in order to ascertain the date of Easter.
caption: Latin and French inscriptions at the base of the obelisk. The mentions of the King and his Ministers were deleted during the French Revolution.
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