A lithographic apparatus including a projection system configured to project a patterned radiation beam onto a target portion of a substrate includes a position measuring system configured to determine the position of a movable object in at least one direction. The measuring system includes at least a first sensor arranged spaced from one side of the object and at least a second sensor arranged spaced from an opposite side of the movable object, and a calculation device configured to calculate the position of the object on the basis of distances measured by the first sensor and the second sensor between the first and the object and the second sensor and the object, respectively. By using the (weighted) difference between measured signals for calculation of the position of the object, the errors in the sensor signals caused by disturbances, such as global and/or local changes of the refractive index and expansion can be substantially reduced.