Casting a Long Shadow: Celebrating the Spring Equinox with the Roman Technology of Sundials

Nathalie Roy

The spring equinox is upon us – twice a year (once in spring and once in fall) the number of daylight hours equals the number of night time hours. Many ancient cultures celebrated this time with special festivals related to agriculture and fertility, and the Romans were no exception. The Hilaria, held on March 25, celebrated the goddess Cybele, Phrygian mother of the gods, also associated with the Greek goddess Rhea.

Have you ever wondered how the Romans knew when certain holidays were actually going to be celebrated? How did they keep time to mark such celebrations? As modern people with access to official clocks and calendars, we rarely consider how difficult ancient timekeeping must have been. “Meet me in the forum at the 4th hour,” Caecilius might have said to Barbillus, but how did they know what time it really was or what day it was for that matter?

One timekeeping technology used by the Romans was the sundial. In basic terms, a sundial works by using the sun’s light to cast the shadow of a gnomon (or indicator) on a dial oriented to a specific location on earth. Some types of sundial tell the time of day; others tell the month or approximate date.

Probably the most famous Roman sundial, the horologium Augusti, stood in Rome next to the first emperor’s mausoleum (his family tomb) and the Ara Pacis (Altar of Peace) in a complex of buildings along the Tiber River. Erected in the late 1st century BCE, the horologium (sometimes called the solarium), was described in great detail by 2nd century CE Roman writer Pliny the Elder in his Historia Naturalis (or Natural History, XXXVI.72–73). It consisted of an Egyptian obelisk topped with a gilt bronze sphere. On top of the sphere was a needle-shaped bronze tip which served as the gnomon.

The dial of the horologium Augusti consisted of a marble surface, the entire shape and size of which is unknown. Near the obelisk, incised in the marble surface, was a line filled with bronze inlay. Labeled in ancient Greek were the names of the zodiac signs, dividing up the line into the months associated with the respective astrological signs and seasons. Some parts of this marble meridian line still exist – they are in the cellar of a nearby building (which regularly floods).

According to experts who have studied the structure, the gnomon cast a shadow on the meridian line to show the date. Thus, the monument indicated not the hour of the day, but the day of the year. At least that’s what the current research points to (excuse the pun).

The process of taking the obelisk from its original location in the Egyptian town of Heliopolis and transporting it to Rome was quite an engineering feat. Re-erecting and repurposing it as a sundial was no doubt a sign of Augustus’ political prowess – imagine watching the scene of the obelisk floating down the Tiber on a giant barge. Unfortunately, due to flooding in the Campus Martius area during the 1st C BCE, the elevation of the ground fluctuated, the obelisk sunk, and not long afterward, the horologium was useless for indicating the date.

If you’re teaching the Cambridge Latin Course, you may want to connect this intriguing monument to the Egyptian stages (17–20) in Book 2 in which obelisks, the Pharos (the fabled lighthouse of Alexandria), and other famous monuments are discussed. The Romans pillaged and reused obelisks all around Rome, and the one used as the gnomon for Augustus’ sundial, destroyed and buried for many years and then re-erected in the 1700s, still stands in Rome’s Piazza di Montecitorio today. It still casts a long shadow, reminding its viewer of the marvelous Roman technology behind it.

Resources and curricular connections

1. This article explains the current archeological understanding of how the horologium Augusti worked, and the references list the most relevant scholarship about it.

2. Pliny’s description of the horologium Augusti is not difficult Latin, though somewhat technical. Your students may enjoy trying it out.

3. Ancient Roman architect, Vitruvius, also discusses sundials in Book IX.7 of his De Architectura, particularly, the analemma – an analemmatic sundial – is a fun and doable project for students.

4. The process of acquiring obelisks from Egypt, transporting them to Rome via the Tiber, and then erecting them was a marvel of Roman engineering and a statement of Rome’s political prowess. Read about it in more detail.

5. Ever wonder how obelisks were raised without cracking them? Watch Nova’s Secrets of Lost Empires: Pharaoh’s Obelisk where experimental archaeologists try to figure it out.

6. Not all sundials were for the public. Read here about ancient Roman portable sundials.


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About the author:

Nathalie Roy teaches Roman technology, classical mythology, and Latin at Glasgow Middle School in Baton Rouge, Louisiana, USA, where she enjoys researching and introducing the classical world to students through experimental archaeology. She earned her B.A. and M.A. in Classics at Louisiana State University where she also focused on pedagogy. She is a National Board Certified Teacher and a STEM Fellow for the Foundation for East Baton Rouge Parish Schools. In January, Nathalie was named Teacher of the Year for the East Baton Rouge Parish School System. You can follow her on Twitter @MagistraRoy.