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Ibn al-Haytham writes the Book of Optics
In Cairo, under house arrest for failing to dam the Nile, the Basra-born scientist completed a seven-volume treatise arguing that vision happened when light reflected from objects struck the eye. His experimental method, controlled and geometrical, was a foundation stone of the modern scientific revolution. The work overturned a thousand years of Greek theory and would profoundly influence Bacon, Kepler, and Descartes centuries later.
West Baray reservoir begun at Angkor
Suryavarman I ordered the construction of an immense artificial reservoir eight kilometers long and two kilometers wide at the Khmer capital, a feat of hydraulic engineering that dwarfed anything in contemporary Europe. The West Baray stored monsoon floodwaters for dry-season irrigation, sustaining the rice agriculture that fed Angkor's vast population and its armies of temple builders.
Druze faith emerges from al-Hakim's disappearance
After the Fatimid caliph vanished into the Muqattam hills and never returned, a group of his followers declared him divine and in occultation rather than dead. Hamza ibn Ali codified their beliefs into a distinct theology that rejected mainstream Islam's rituals. The Druze community, closed to converts, would endure in the mountains of Lebanon and Syria for a millennium.
Disappearance of Caliph al-Hakim
The erratic Fatimid ruler, who had persecuted Christians and Jews, banned chess, and fasted until he hallucinated, rode out of Cairo one night into the Muqattam hills and never returned. His followers insisted he was in hiding; his cult would become the Druze faith of the Lebanese mountains. Only his donkey and bloodstained garments were found, and the mystery of his fate has never been solved.