Celestial navigation requires complicated computations. Performing these calculations in cramped open cockpits with low temperatures and wind speeds of over 160 kilometers (100 miles) per hour was part of what made navigation difficult in the early years of aviation. Thankfully, Capt. L. C. Bygrave developed this handy slide rule shortly after World War I. It provided the best shortcut method of speeding up celestial computations at the time. Piece of cake, right?