Today is Hamilton Day. (William Rowan, not Alexander.) 173 years ago today, William Rowan Hamilton figured out how to multiply quaternions. The story goes that he was trying to figure out how to multiply triples of numbers, but eventually realized he was going to have to multiply quadruples. He figured it out while walking along Broom Bridge in Dublin, where he carved the equation . There’s a plaque (you should always read the plaque), and a few years back John Baez posted some pictures. Maynooth University does an annual Hamilton Walk retracing his steps on this day, which you missed, but there’s an audio guide.