Grady W. Amy retired from the National Aeronautics and Space Administration after 42 years at the Johnson Space Center located in Houston, Texas. During his last ten years, he worked in a branch that did mathematical modeling of Space Shuttle dynamics. It occurred to him that an economic system is an oscillating system, and he developed a mathematical model to describe such a system, which resulted in a book, "Macroeconomics: Developed into an Exact Science."