In what could be one of the coolest examples of astronomical timekeeping Atlas Obscura has detailed the clocks built secretly into old Catholic Churches. These clocks – usually sundials or small pinholes dotting the ceiling – could tell time as accurately as any mechanical clock and were a fascinating marriage of science and religion.

For example, in the Basilica di San Petronio in Bologna, Italy a small hole surrounded by a painted sun shoots a single beam of light on the floor. Where it hits the priests and scientists meticulously placed astronomical signs and solstice markers and the calendars of San Petronio were considered the best of their day.

Stranger still, on either side of this brass line, words and celestial images have been carved directly into the rock. There are the 12 signs of the zodiac interspersed amongst Roman numerals and references to solstices. There is Aquarius, the water bearer; Capricorn, with its confusing mix of shaggy horns and the coiled tail of a sea creature; Sagittarius, preparing to fire a magnificent bow and arrow; and the pouting fish of Pisces. At first glance, these symbols seem pagan, even sacrilegious, as if the astral remnants of an older belief system have somehow survived beneath the feet—and beyond the gaze—of daily worshippers.

Yet these symbols are not there to cast horoscopes, let alone spells. They are there for purposes of church administration and astronomical science. This cathedral, the Basilica di San Petronio in Bologna, Italy, also doubles as a solar observatory—at one point, one of the most accurate in the world—and these signs of the zodiac are part of an instrument for measuring solstices.

Why were these clocks important? The Catholic Church wanted to know when to celebrate various holidays including Easter. As they moved towards the Gregorian calendar they found themselves in need of true methods for assessing the day and date. Using these astronomical tools they were able to build true calendars.

You can read more about these astronomical calendars here. It’s a great ride through history.

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Last Update: November 17, 2016

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