The present invention utilizes, in combination, an irradiator, a screen and an optical sensor. The irradiator generates a parallel light band to form a linear image region on the screen. A spherical flying object is made to pass transversely through the parallel light band to form a silhouette within the image region. The silhouette is detected by the optical sensor to determine the instantaneous spatial position of the flying object. The position thus determined may be utilized for example to calculate the initial trajectory angle of a golf ball hit from a fixed position.