page_title: Gnomon of Saint-Sulpice
section_title: Obliquity of the ecliptic
hierarchical_section_title: Gnomon of Saint-Sulpice / Usage of the gnomon / Obliquity of the ecliptic
caption_attribution_description: English: Gnomon_southern_plaque in Saint Supilce
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: Le Monnier further used the gnomon from 1744 to establish the variations of the ecliptic, or the variations in the obliquity of the Earth's axis. The endeavor is recorded on the plaque at the southern end of the meridian, in the South transept: "Pro nutatione axios terren. obliquitate eclipticae" ("for the nutation of the earth's axis and the obliquity of the ecliptic"). 
  As mentioned on a brass plaque that covered the stone plaque, the obliquity of the ecliptic was 23°28'40".69 in 1744. From 1745 to 1791, Le Monnier visited Saint-Suplice at each summer solstice and, focusing the light with a lens fixed to the opening in the stained-glass window so as to produce a sharp image of the sun on the floor, noted the exact position of the image at noon. From these observations, he calculated a variation of the obliquity of 45" per century (the exact figure is 46".85 per century).
caption: The plaque at the southern end of the meridian, mentioning the work on the obliquity of the ecliptic
exif: {"Image ImageDescription": "", "Image Make": "SONY", "Image Model": "DSC-T10", "Image Orientation": "Horizontal (normal)", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Paint Shop Pro Photo 12.00", "Image DateTime": "2009:11:17 22:08:44", "Image YCbCrPositioning": "Co-sited", "Image ExifOffset": "272", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "860", "Thumbnail JPEGInterchangeFormatLength": "5239", "Thumbnail YCbCrPositioning": "Co-sited", "EXIF ExposureTime": "1/13", "EXIF FNumber": "7/2", "EXIF ExposureProgram": "Program Normal", "EXIF ISOSpeedRatings": "800", "EXIF ExifVersion": "0221", "EXIF DateTimeOriginal": "2007:10:18 17:04:35", "EXIF DateTimeDigitized": "2007:10:18 17:04:35", "EXIF ComponentsConfiguration": "YCbCr", "EXIF CompressedBitsPerPixel": "8", "EXIF ExposureBiasValue": "0", "EXIF MaxApertureValue": "29/8", "EXIF MeteringMode": "Spot", "EXIF LightSource": "Unknown", "EXIF Flash": "Flash did not fire, compulsory flash mode", "EXIF FocalLength": "633/100", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "sRGB", "EXIF ExifImageWidth": "1424", "EXIF ExifImageLength": "1082", "Interoperability InteroperabilityIndex": "R98", "Interoperability InteroperabilityVersion": "[48, 49, 48, 48]", "EXIF InteroperabilityOffset": "718", "EXIF FileSource": "Digital Camera", "EXIF SceneType": "Directly Photographed", "EXIF CustomRendered": "Normal", "EXIF ExposureMode": "Auto Exposure", "EXIF WhiteBalance": "Auto", "EXIF SceneCaptureType": "Standard", "EXIF Contrast": "Normal", "EXIF Saturation": "Normal", "EXIF Sharpness": "Hard"}