Measurement · a Light

A marble column in Cairo set the year's tax rate

On Rawda Island in the Nile stands a stone pit with a marble column at its centre, marked off in cubits. It was rebuilt in 861 under the caliph al-Mutawakkil, and the astronomer Ahmad ibn Muhammad al-Farghani is named as its designer. Each summer the river rose, and the height it reached on that column decided everything: too low meant famine, too high meant destroyed villages, and a good flood meant the year's taxes could be levied. Officials read the mark and criers announced the number in the streets. The nilometer still stands, its Kufic inscriptions intact, one of the oldest Islamic structures in Egypt and a piece of public infrastructure that doubled as a public forecast.

Rawda Island, Cairo · 861 CE