Let It Shine: Solar Energy in Ancient Rome

Reader Contribution by John Perlin
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The following post summarizes the author’s Chapter 2 of Let It Shine: The 6,000-Year Story of Solar Energy.

Rome’s greatest architect Vitruvius saw solar houses while on duty as a military engineer in recently conquered Greece. When writing his great work On Architecture, he emphasized proper solar orientation for buildings and bath houses. From literature of the time it appears many followed Vitruvius’ instructions. As an example, Varro, an agricultural expert writing around the time that Octavian Caesar came to the throne, observed, “Men of our day aim at having their winter rooms facing the falling sun [southwest], because the setting sun renders the area warmer in the evening.”

Baths were especially popular among the Romans but demanded a great amount of heat. From the times of the early empire onward, most faced the afternoon sun in wintertime when they had maximum use. They also had their large windows covered with either transparent stone like mica or clear glass, a Roman invention of the 1st century ACE, one of the great breakthroughs in building and solar technology. Transparent glass, the Romans discovered, acts as a solar heat trap, admitting sunlight into the desired space and holding in the heat so it accumulates inside. Recent experiments show that on a clear winter day in Rome the sun entering a properly oriented, glass-covered bath would keep the temperatures inside above 100 degrees Fahrenheit throughout the day, the desired temperature in the caldarium, the hot bath.

The Romans also used glass-covered receptacles for growing plants from warmer climes in temperate Italy, as well as raising native plants out of season. As wood, their principal fuel source, became scarcer during the later days of the Roman Empire, architectural writers in their building manuals stressed self-sufficiency in which solar building strategies played a major role. Facing structures to the winter sun became so popular in Roman times that sun-right laws were passed, making it a civil offense to block one’s access to the south.

Read John’s previous post: “Let It Shine: Solar Design in Ancient Greece.”