2026-09-24 · history
Sangaku (1683) — Votive Tablets Hung at Shrines Not to Thank the Gods for a Solved Problem, but to Challenge the Next Visitor to Solve One
Sangaku are geometry problems painted on wooden votive tablets and dedicated at shrines and temples in Edo-period Japan. The earliest dated record is from 1683, and the oldest surviving tablet dates to 1686. Some were dedicated without an answer, as a challenge to the next visitor; one from 1822 recorded what is now called Soddy's hexlet, 115 years before its Western discovery. The spirit of building figures with only a compass and straightedge still lives on in puzzle games like Euclidea and Ecocoru.
