Geodesy, which is the science of measuring the size and shape of the Earth, explores the theory, instrumentation and results from modern geodetic systems. The beginning sections of the volume cover the theory of the Earth's gravity field, the instrumentation for measuring the field, and its temporal variations. The measurements and results obtained from variations in the rotation of the Earth are covered in the sections on short and long period rotation hanges. Space based geodetic methods, including the global positioning system (GPS) and Interferometric synthetic aperture radar (SAR), are also examined in detail.