Abstract: A lithographic apparatus includes a position quantity determination system to determine a position quantity of a movable part which is in operation at least partly surrounded by an area comprising a fluid. The position quantity determination system includes an interferometer system, a global sensor to determine a global value of a physical quantity of the fluid in the area, and a local sensor to determine a local value of the physical quantity of the fluid in the part of the area. The position quantity determination system is configured to determine the position quantity from an output of the interferometer, the global value of the physical quantity and the local value of the physical quantity. The physical quantity may include a pressure, a temperature, etc. The local physical quantity determination system may include a sensor, such as a high-speed sensor, a computational fluid dynamics model or a linear approximation model.