The solar clock in the basilica's floor

A long bronze line inlaid in the marble runs across the floor of the Basilica di Santa Maria degli Angeli e dei Martiri in Rome: the celebrated meridian line, an astronomical instrument that for centuries measured solar noon and the turning of the seasons with remarkable precision. It is one of the most captivating meetings of science and art you can admire inside a church.

Who built it, and why

The meridian line was created in the early 18th century (1702) by the astronomer Francesco Bianchini, commissioned by Pope Clemente XI (Clement XI). It served several purposes:

  • Verifying the accuracy of the calendar (in particular the rule for the calculation of Easter and the leap year).
  • Giving Rome a monumental sundial to rival the one in Bologna.
  • Demonstrating the accuracy of astronomical science in the service of the Church.

The great hall of Santa Maria degli Angeli, tall and sheltered, was the ideal setting for an instrument this precise.

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How it works

The principle is simple and brilliant: high up on a wall sits a small gnomonic hole (the "pinhole"). At noon (local solar time), a ray of sunlight passes through the hole and strikes the meridian line on the floor, marking with precision the point that corresponds to the day of the year. Shifting along the line day after day, the ray signals the zodiac signs and the solstices, turning the floor into a vast solar calendar.

What to look for

  • The bronze line with its inlaid scales and zodiac signs.
  • The gnomonic hole above, where the ray of sunlight enters.
  • The images and inscriptions that decorate the meridian line.
  • The moment of solar noon, when the ray touches the line (on a sunny day).

A masterpiece of art and science

The meridian line of Santa Maria degli Angeli is far more than a curiosity: it is the record of an age in which astronomy, mathematics, and faith were in dialogue, and churches could become observatories. To admire it is to understand how, before modern clocks, time was measured with the sun, geometry, and extraordinary precision.

Frequently asked questions

What is the meridian line of Santa Maria degli Angeli? A great meridian line inlaid in the basilica's floor, used to measure solar noon and the passing of the seasons: a masterpiece of art and science from the 18th century.

Who built it? The astronomer Francesco Bianchini, in 1702, commissioned by Pope Clemente XI (Clement XI).

What was it for? To verify the accuracy of the calendar (including the calculation of Easter), to give Rome a monumental sundial, and to show the precision of astronomical science.

How does it work? A ray of sunlight enters through a hole above and, at noon, strikes the line on the floor, marking the day of the year and the zodiac signs.

Can you watch the ray touch the line? Yes, around solar noon and on a sunny day, the ray lights up the corresponding point on the meridian line.

See also

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Article #523 · Category: Monuments · Updated: May 2026