Solstice, alla Romana

December 21, 2011 / Events
Rome, Lazio
An amazing moment in the holiday season occurs today in Santa Maria degli Angeli e dei Martiri , every year. It occurs thanks to the confluence of the Ancient Roman, Papal Baroque, astronomical and mathematical realms.

This unique church, built in the 16th century inside the ruins of the huge and cavernous baths of Diocletian, was a design of Michelangelo’s. About 100 years later Pope Clement decided that an ultra-sophisticated time-piece was needed to determine the religious calendar and dates precisely, Easter first and foremost. This time-piece was also to ensure the accuracy of the then 100 year old Gregorian calendar reforms such as leap years, etc.

By pure coincidence, it just so happened that Pope Clement had performed his first mass ever inside this very church, and for that reason he decided to locate the sundial here. Fortunately for us this southern facing church was characterized by a large dark interior space within which a ray of light would be very visible and whose Roman foundations were unlikely to shift (having so far stayed put for the better part of 15 centuries). All these four conditions are requirements for any precision sundial worthy of the name.

How precise? Well, this 120 foot long ray of light, and its beautiful marble and brass markings along the church’s floor, is precise to within 1/7200ths of one degree of arc… that’s 3/100ths of a second! No surprise then that all of Rome’s bell tower clocks were set according to this sundial until 1848.

This chap is the Departmental Chair Professor of Astronomy of the Universita’ della Sapienza of Rome. He and your editor spent the better part of the morning lying on the church floor calibrating measuring instruments, plotting the winter solstice as dots on graph paper, and running calculations to make sure that the sun went where (actually, earth rotated as) it was supposed to (good news, it/they did). On winter solstice, the noon sun (“highest” point during the day) is as “low” as it’ll get compared to any other of the 365 noons. This is the darkest day of the year, yet it also means that days start getting longer, that new light returns.

In the final photo, notice the shield just off the lower right-hand corner of the large window and just above the right column of the arch. It is the gnomon, or “eye”, of the meridian. (Look carefully and you can actually see the pin-hole fleck of light in its center.) From there, the spear of light runs down and left for 120 feet at about a 35° angle to the lower left hand corner of the photo and strikes the floor (beautifully inlaid with brass astronomical tick marks) near the photo’s leftmost visible pinkish column, in front of the pews.

While perhaps not so portable, the Clementine Sundial is all in all a most satisfying and accurate time-piece… which has officially marked the high point of the (astronomical, at least) holiday season. Auguri! (Best wishes!)


by GB Bernardini

Editor, Italian Notebook

17 Responses to “Solstice, alla Romana”

  1. Evanne

    It’s close to noon here, about one hour north of Rome, and by the brightness of the sky on this day, seems unimaginable that it is the darkest day of the year. What joy we have to look forward to! What fun you must have had doing the research! Auguri to you, dear Editor!

  2. Thanks GB for such an interesting read! Italiannotebook always inspires and educates. Yet another church to be added to my list of “must-see” when I return to Rome next November.

    Merry Christmas and best wishes for 2012!


  3. giuseppe spano (jojo)
    giuseppe spano (jojo)

    Some say we are increasing in knowledge and ‘know how’ yet the the past scholars achieved things incredible, with so little with which to work.Amazing!

  4. Another good story. I remember stumbling by chance into this church and discovering the sundial without appreciating its significance. And none of the guidebooks talked about the accuracy of the construction. What intrigues me is how they managed to lay the brass and marble so accurately. I wonder if it took several years to fine tune it.

  5. Accumulated thanks for the previous year’s articles. I read each with interest and enjoy knowing there is a community of readers doing the same! Happy Solstice from Rhode Island.

  6. What a wonderful story. I to, will look in on this church when I am next in Rome. I have google earthed it to see exactly where it is. I learn so much about my adopted home from this notebook. Such a pleasure to read all the tit bits.
    Auguri to you GB and everyone else on the list.

  7. Adding my thanks to you,GB, and all who contribute to the Notebook!
    Travel opportunities are fewer as one ages but memories of Bella Italia remain forever aided by the charming, informative and picturesque views presented here. Buon Natale from Maryland, USA!

  8. Grazie, this is a beautiful and amazing note. The church is stunning. I also love the sundial in the Basilica di San Petronio in Bologna. And I love how it was OK to combine science and faith back in those days. Buon Natale Italian Notebook, Buon Natale tutto Italia!!

  9. I’ve been to Rome several times and have never found this noted in any guide book. GB: you truly find amazing things. Thanks for sharing. Merry Christmas from Pennsylvania, USA.

  10. Grazie GB for a fascinating story.
    A lifetime of travel to Italy isn’t enough to comprehend all that this fabulous country offers.

  11. Penny Ewles-Bergeron
    Penny Ewles-Bergeron

    Thoroughly enjoyed this note, as with all your articles, GB. Such an impressive achievement for the 17th century men of science and craftsmen. Thank you for sharing it all with us.

  12. Thanks a squillion times for the detailed and accurate note on the Meridiana! The faculty where I studied in Piazza Esedra (later Piazza della Repubblica) was at that time in the building next door the Basilica of Santa Maria degli Angeli, so faculty and students were pretty familiar with this church and its meridian line. Thanks again and happy holidays from Washington DC!
    BTW, the Basilica has its own website (in different languages), with a section for the Meridian line. Here is the website:

  13. I had only about 24 hours in Rome (I know, a travesty) and thus sightseeing was constrained. Just by chance this is the one church I managed to visit. The exterior pulled me in, but the interior took my breath away. There was also a great exhibit on the history of the church, in a side room. Against the timeline of Rome we are but specks of dust.

  14. Giancarlos:
    Solstice, alla Romana
    You always manage to tell such fascinating stories about one of the most precious places on Earth … Rome.
    Mille Grazie, Auguri e Buon Natale. Please post a larger picture of the shield and pin hole of light.
    ( It is the gnomon, or “eye”, of the meridian. (Look carefully and you can actually see the pin-hole fleck of light in its center.) From there, the spear of light runs down and left for 120 feet at about a 35° angle to the lower left hand corner of the photo and strikes the floor (beautifully inlaid with brass astronomical tick marks) near the photo’s leftmost column, in front of the pews.)
    Suzanne and Ron

  15. And a very very auguri at this wonderous Solstice time. Everything feels well into place knowing that our dearest editor has been on the scene!!


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