Astronomy in Athens began downtown, at the National Observatory in Thiseio, on a site chosen in the nineteenth century by the architect Eduard Schaubert back when the night sky over Athens was still genuinely dark. By the 1930s it no longer was. Stavros Plakidis, then director of the observatory, set out to find darker skies and settled on the Koufos hill in Penteli, 500 metres up the slope of Mount Pentelicus and about 18 kilometres from the city centre - far enough to escape the glow, close enough to remain useful. Plakidis began observing there in 1936, equipping the new station with telescopes and meteorological instruments. The chase after darkness is the recurring story of every great observatory, and Penteli's founding is a textbook case of it.

The instrument that gives Penteli its fame is the Newall Refractor. Built in 1869 by the renowned instrument-maker Thomas Cooke for the engineer Robert Stirling Newall, its twenty-five-inch lens made it the largest refracting telescope in the world at the moment of its completion. It served at Newall's private observatory, then moved to the Cambridge Observatory, before being donated to the National Observatory of Athens and installed at Penteli in 1959. A building to house the great instrument had been started two years earlier. For a time it was the largest telescope in Greece, and through the 1960s and 1970s the observatory was a serious centre of research - a standing built on Plakidis's decades of international collaboration, including his earlier partnership with the eminent British astrophysicist Arthur Eddington on variable-star work at Cambridge.

The very enemy the observatory had fled to Penteli to escape eventually caught up with it. As Athens sprawled outward through the second half of the twentieth century, light pollution crept up the mountain, and by 1980 the glow had begun to ruin serious observation through the historic refractor. Rather than abandon the site, the observatory changed its purpose. In 1995 a Visitor Center opened, turning the grand old telescope toward education instead of research - offering seminars, documentaries, and a chance for schoolchildren to put an eye to the same lens that once probed the frontiers of nineteenth-century astronomy. Meanwhile, real research continued in a different key: since 2000 the site has hosted the Athens Digisonde, an instrument that sounds the upper atmosphere and feeds open data to ionospheric networks across Europe and the world.

In August 2024, wildfires swept across Attica and climbed Mount Pentelicus, burning part of the same slopes that had once promised dark and peaceful skies. The flames advanced on the observatory, and the building was evacuated as the fire bore down. It reached the very yard of the observatory before firefighters halted it - extinguishing the blaze just a few metres from the main building that shelters the Newall Refractor. A telescope that had survived a private collector, a transatlantic journey, a world war, and the slow creep of city light came within a few steps of being lost to fire. The marble mountain held; the dome still stands.