“Tides” refer to the changing levels of the sea, caused by the gravitational forces of the Moon and Sun and the rotation of the Earth. People have been trying to predict tides since Selecucus of Seleucia first tied them to the Moon in the 2nd century B.C. Surfers are one of the largest groups to benefit from improvements in tide chart technology. These charts show the daily times for high water and low water on a given day for a given location. For estimating the heights at intermediate times, surfers can use the rule of twelfths. Wikipedia states that:The rule assumes that the rate of flow of a tide increases smoothly to a maximum halfway between high and low tide before smoothly decreasing to zero again and that the interval between low and high tides is approximately six hours. The rule states that in the first hour after low tide the water level will rise by one twelfth of the range, in the second hour two twelfths, and so on according to the sequence – 1:2:3:3:2:1.The Internet has made it easier than ever to find accurate charts. The United States National Oceanic and Atmospheric Administration posts US charts online at http://tidesandcurrents.noaa.gov/. For Canadian tables, visit http://www.waterlevels.gc.ca/english/Canada.shtml. For UK tables, visit http://www.bbc.co.uk/weather/coast/tides/. Other countries, including Germany and the Netherlands, also post tables online, but you’ll have to be proficient in their respective languages to understand them. The best bets for other locations are local newspapers and tide prediction software. XTide is one such piece of software, a “harmonic tide clock and tide predictor” that can be accessed at http://www.flaterco.com/xtide/. Good luck, and have fun out on the waves.