Syene was the southernmost important Greek-Egyptian city of the Ptolemaic kingdom. It sat at the First Cataract of the Nile, where the river narrows between substantial granite outcrops; it had been a frontier military post since the late New Kingdom and a major granite-quarrying centre since the Old Kingdom (the obelisks of Karnak and Heliopolis were cut from the Syenite granite quarries on the city’s southern edge). By the 3rd century BC it was also a major Ptolemaic trade hub for Nubian gold, ivory, and ebony moving northward toward Alexandria.
It also happens to sit at almost exactly 24° 5′ N latitude — within a few minutes of the position of the Tropic of Cancer in the Hellenistic period (the latitude has shifted slightly over the past two millennia due to long-term axial precession). The Tropic of Cancer is the northernmost latitude at which the Sun reaches the zenith at any point in the year. At Syene, at noon on the summer solstice — the day the Sun reaches its highest annual point — the Sun stood almost exactly directly overhead. Vertical sticks cast no measurable shadow. Reflected sunlight reached the bottom of a deep well known to local astronomical observers.
The geographical fact had been part of local Egyptian astronomical knowledge for centuries before the Greeks arrived. Eratosthenes — the chief librarian of the Library of Alexandria under Ptolemy III Euergetes, around 240 BC — used the Syene noon-solstice observation as the second of the two empirical measurements he needed to calculate the circumference of the Earth. The other was the shadow length cast by a vertical gnomon at Alexandria at the same solar moment (about 7.2°), and the assumed terrestrial distance between Alexandria and Syene (about 5,000 stadia, given by the standard Ptolemaic land-surveying records).
The arithmetic was simple. If Syene had no shadow while Alexandria had a 7.2° shadow, the arc between them along Earth’s curved surface was 7.2°. 7.2° is one-fiftieth of a full 360° circle. The Earth’s circumference therefore was approximately 50 × 5,000 = 250,000 stadia — somewhere between 39,000 and 46,500 km depending on which version of the Greek stadion Eratosthenes was using. The modern figure is approximately 40,075 km.
The Syene observation also figured in Strabo’s Geographika a century later as the standard demonstration of the latitudinal-coordinate system. The well at Syene was still being maintained as a working solstice-observation instrument under the Romans. Modern Aswan retains substantial archaeological remains of the Hellenistic and Roman astronomical infrastructure on Elephantine Island, immediately opposite the old city.
What had begun as a piece of local Egyptian astronomical lore became one of the foundational measurements of European scientific cosmology — the first known successful measurement of the size of a planet.