A nilometer was a structure for measuring the Nile River's clarity and the water level during the annual flood season. There are two nilometers at Elephantine Island. The more famous is a corridor nilometer associated with the Temple of Satis, with a stone staircase that descends the corridor. It is one of the oldest nilometers in Egypt, last reconstructed in Roman times and still in use as late as the nineteenth century AD. Ninety steps that lead down to the river are marked with Arabic, Roman, and hieroglyphic numerals. Visible at the water's edge are inscriptions carved deeply into the rock during the Seventeenth Dynasty.

The other Nilometer is a rectangular basin located at the island's southern tip, near the Temple of Khnum and opposite the Old Cataract Hotel. It is probably the older of the two. One of the nilometers, though it is not certain which, is mentioned by the Greek historian Strabo.

Many sources claim that the fabled "Well of Eratosthenes", famous in connection with Eratosthenes' presumed calculation of the Earth's circumference, was located on the island. Strabo mentions a well that was used to observe that Aswan lies on the Tropic of Cancer, but the reference is to a well at Aswan, not at Elephantine. Neither nilometer at Elephantine is suitable for the purpose, while the well at Aswan is apparently lost.

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Nilometers
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