Before the advent of the modern pocket calculator, slide rules were used to do complex calculations. These devices require no electricity, resemble a ruler, and were made of plastic. After the advent of cheap pocket calculators (around 1970’s), the slide rule became remnants of a past era. As an analog computer, the slide rule could do surprisingly much. It could perform trig functions, logarithms, multiplication, and other calculations. These simple mechanical devices were all that was used to perform the calculations that took us to the moon. The engineers and scientists, lacking the modern technology of today, relied on the trusty slide rule. The astronauts on Apollo 11 even took one as a backup in case their computer failed. The model that they used (Pickett Model 600-ES) is shown bellow. Besides being a collector’s item (they have largely gone out of production, although you can still buy old ones on ebay), the slide rule is still used by sailors and pilots as backup computers. The slide rule, though obsolete, is still a reliable tool.