How History Speaks on Instagram: "What looks like a simple marble toy is actually a 700-year-old mathematical idea in motion. This device demonstrates the Tusi Couple, where two perfectly synchronized circular motions combine to create a straight-line path. Invented by 13th-century Persian scholar Nasir al-Din al-Tusi, this principle was once used to explain planetary motion in astronomy. The steel balls only appear to move straight, but their path is the result of pure geometry cancelling curve

David Heavens on Instagram: "Glasses or glasses?樂"