Fourth period on a midwinter Thursday, Christmas vacation a fading memory by now, and Lars Brownworth took his accustomed place in front of an American history class at the Stony Brook School here. He had been guiding these seniors through the Gilded Age lately, and for this session he planned to personify the era in the form of the oil tycoon John D. Rockefeller.

For 45 minutes, Mr. Brownworth deftly orchestrated lecture, discussion and archival photographs to evoke Rockefeller in both his rapacious capitalism and social conscience. When the bell rang, out shuffled the audience, a dozen teenagers who might or might not remember any of this material beyond the next exam. In its satisfactions and its limits, such was the life Mr. Brownworth, the son of teachers, had gladly chosen.

That night, though, Mr. Brownworth, 31, set to work in his own apartment, writing an essay about Alexius I Comnenus, the Byzantine emperor from 1081 to 1118. After revision and rehearsal, the text would become the script for the latest installment of Mr. Brownworth’s podcast. And if form held, something like 140,000 listeners from Afghanistan to White Plains would hear it.

In barely 18 months, Mr. Brownworth’s podcast, “12 Byzantine Rulers” (at http://www.anders.com/lectures/lars_brownworth/12_byzantine_rulers/), has become one of the phenomena of the podcasting world. A survey of 1,200 years of rather abstruse history, starting with Diocletian in 284 and finishing with Constantine XI Palaeologus in 1453, “12 Byzantine Rulers” routinely ranks in the top five educational podcasts on iTunes, and in the top 50 of all podcasts.

In the digital era, this self-taught amateur has emerged as a figure somewhat akin to Will Durant in books or Jacob Bronowski on public television, an effective and engaging popularizer. Mr. Brownworth’s podcast competes favorably with far more conventional and credentialed online fare — university courses in beginning French or Psychology 101, test-prep drills for the SAT. Even the other highly rated personal podcasts, like “Word Nerds” and “Grammar Girl,” appeal to dependably large audiences for etymology and grammar.

“It’s a slightly frightening idea to think there are so many people,” Mr. Brownworth said. “But without question it’s the most exciting part of my professional life. We’re in the middle of a revolution, and I feel incredibly blessed to be part of it.”

While listeners address him in their e-mail messages with the respectful honorific “Professor,” Mr. Brownworth, in fact, holds only a bachelor’s degree in history, from Houghton College in upstate New York. He started teaching at Stony Brook, an independent school, only in 1999, and his initial assignment was in the science department. To the extent that he had any specialty as an undergraduate, it was the Battle of Hastings, a long way from Constantinople.

What Mr. Brownworth always possessed was a sweeping intellectual curiosity about antiquity, which inspired him while he was growing up on Long Island to learn to read hieroglyphics and sound out the Greek inscriptions in the ruins of Herculaneum. He also had a talent for dramatizing himself, whether donning the set of armor owned by family friends or imitating characters from a firefighter to a gorilla in a series of home movies called “Lars’s World.”

Still, Mr. Brownworth had fallen into the passive assumption that between Rome’s fall and the Renaissance there existed nothing but barbarism. It took a casual mention of a Byzantine empress in a book about Charlemagne that he read a few years ago for Mr. Brownworth’s curiosity to be kindled. He followed it into such standard texts as “A History of the Byzantine State” by George Ostrogorsky and “The Fall of Constantinople” by Steven Runciman. On a school trip to Turkey, he walked into the very church where every Byzantine emperor had been crowned.

“There was something mysterious about the Byzantine empire to me, this sense that it was lost history,” Mr. Brownworth recalled. “America is very much a Protestant country, and we really don’t feel like we’re connected to the Eastern world, that we don’t share values. But it’s not a coincidence that the Renaissance kicks off after the fall of Constantinople. A lot of those Greek-speaking intellectuals fled to the West, bringing their knowledge of the classics. That knowledge had been kept alive with the Byzantines.”