szczys writes: You'd think tide prediction would be quite easy: it comes in, it goes out. But of course it's driven by gravity between the moon and earth and there's a lot more to it. Today, computer models make this easy, but before computers we used incredible analog machines to predict the tides. The best of these machines were the deciding factor in setting a date for the Allies landing in Europe leading to the end of the second world war. From the Hackaday story: "In England, tide prediction was handled by Arthur Thomas Doodson from the Liverpool Tidal Institute. It was Doodson who made the tidal predictions for the Allied invasion at Normandy. Doodson needed access to local tide data, but the British only had information for the nearby ports. Factors like the shallow water effect and local weather impact on tidal behavior made it impossible to interpolate for the landing sites based on the port data. The shallow water effect could really throw off the schedule for demolishing the obstacles if the tide rose too quickly. Secret British reconnaissance teams covertly collected shallow water data at the enemy beaches and sent it to Doodson for analysis. To further complicate things, the operatives couldn't just tell Doodson that the invasion was planned for the beaches of Normandy. So he had to figure it out from the harmonic constants sent to him by William Ian Farquharson, superintendent of tides at the Hydrographic Office of the Royal Navy. He did so using the third iteration of Kelvin's predictor along with another machine. These were kept in separate rooms lest they be taken out by the same bomb.