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.

….Easter, a Christian holiday commemorating Jesus Christ’s resurrection from the dead, is defined not only by Church liturgy but also by astronomical circumstance. As specified by the First Council of Nicaea, Easter is not a straightforward anniversary, always recurring on the same date from now until eternity. Instead, it is to be celebrated on the first Sunday after the first full moon after the spring equinox—an occasion already laden with astronomical significance. The equinox, after all, is a day when time is split equally into 12 hours each of light and darkness, of illumination and obscurity.

The stakes of getting the date right were unusually high, Heilbron writes. If the faithful were to worship Easter on the wrong Sunday, out of sync with the rest of Christendom, then their very souls could be at risk.

Why Catholics Built Secret Astronomical Features Into Churches to Help Save Souls | Atlas Obscura

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