Info Theme: Medieval astronomy in Europe

Entity: 33

Subentity: 1

Version: 10

Status: PUB

Date: 2012-08-16 22:09:10

Author(s): Stephen McCluskey with contributions by Clive Ruggles

With the construction of the west front, the cathedral obtained its earliest known display of astronomical time—one that the cathedral shares with many other large churches—the carved reliefs depicting the signs of the zodiac and the labours of the months in the bases of the statues flanking the right portal (Fig. 1).

Fig. 1: Occupations of the months and signs of the zodiac, west façade, right portal—May, Gemini; June, Cancer; July, Leo; August, Virgo.

Photograph © Ad Meskens, Creative Commons Licence

The most widely known of these astronomical displays of are the cathedral’s elaborate astronomical clocks. The 14th-century clock included a calendar, a mechanically driven stereographic projection showing the movement of the stars, and pointers showing the positions of the Sun and Moon. Atop the clock was an automaton of a cockerel, which crowed at noon, flapping its wings.

The 16th-century clock added to these elements a rotating celestial sphere on which were depicted all 1020 stars of Ptolemy’s star catalogue together with figures of 48 constellations, a disc showing the ecclesiastical calendar for 100 years, and depictions of all eclipses over an interval of 32 years. A stereographic projection of the stars, Sun and Moon, like the one in the original clock, was enhanced with additional pointers showing the positions of all the visible planets and the Dragon, or lunar node, which served to explain eclipses (Fig. 2). Elements of the case and display were incorporated into the current clock. Although the clock reflected the geocentric model of astronomy, its decoration included a portrait of Nicolas Copernicus.

Fig. 2: Astrolabe planetary dial of the second astronomical clock. Detail from Woodcut by Tobias Stimmer (1574)

The 19th-century clock reflected Copernican astronomical concepts. The geocentric stereographic projection of the Sun, Moon, and planets was replaced by a heliocentric model of the visible planets, plus the Earth and Moon, in the solar system (Fig. 2b). It displayed both uniform civil time and the apparent time indicated by the daily motions of the Sun. The stellar globe now portrayed more than 5000 stars, extending down to faint sixth magnitude ones. In addition, the clock incorporated a perpetual calendar, computing the solar cycle of 28 years, the lunar cycle of 19 years, the date of Easter, and other calendrical parameters traditionally found in ecclesiastical computus.

Fig. 3: Heliocentric planetary dial of the third astronomical clock. Photograph © Didier B (Sam67fr), Creative Commons Licence

The concern with time that we see in the cathedrals clocks also appears in its fourteen sundials, which date from the 13th to the 18th centuries. The oldest sundial, dated between 1225 and 1235, marks seven times of prayer in the course of the day, beginning at dawn and continuing until sunset. The 15th century saw the addition of three more sundials, dividing the day into twelve hours from sunrise to sunset (Fig. 4). In the 16th century, five dials were installed at the platform level of the tower and three mathematical dials, designed by the builder of the second clock, were installed on the gable of the south transept (Fig. 5). The builder of the 19th-century clock, Jean-Baptiste Schwilgué, installed a vertical meridian line inside the entrance to the south transept (Fig. 6), marking local apparent noon to regulate the clock.

Fig. 4: Astrologer with a sundial, south portal. Photograph © Coyau, Creative Commons Licence

Fig. 5: Three sundials on south gable: altitude/azimuth dial (left), vertical sundial (centre), and dial reading hours from sunrise and sunset (right).

Photograph © Jean-Marie Poncelet, Creative Commons Licence

Fig. 6: Schwilligué’s meridian line (detail), inside entrance, south transept. Photograph © Jean-Marie Poncelet, Creative Commons Licence