Apparatus (4; 40; 400) for determining a position and orientation of a first rigid body (1; 10; 100) and a second rigid body (2; 20) in relation to each other, comprising: an at least essentially punctual light source (13; 130) mounted stationary in relation to the first rigid body (1; 10; 100); an image sensor (15; 150) mounted stationary in relation to either the first rigid body (1; 10; 100) or the second rigid body (2; 20) and positioned in a field of light propagated by the light source (13; 130); a shadow mask (11; 110; 1100) mounted stationary in relation to either the first rigid body (1; 10; 100) or the second rigid body (2; 20) and arranged in the field of light propagated from the light source (13; 130) between the light source (13; 130) and the image sensor (15; 150); and a computation unit configured to calculate at least one angle (α, β) defining the position and orientation of the first rigid body and the second rigid body relative to each other by evaluating a shadow image (12; 120) generated by light propagated by the light source (13; 130) which passes the shadow mask (13; 130) and which is detected by the image sensor (15; 150), characterized by further comprising a mechanical constraint (3; 30) mounted to a first connection spot of the first rigid body (1; 10; 100) and to a second connection spot of the second rigid body (2; 20), wherein the mechanical constraint defines a fixed distance between the first connection spot of the first rigid body (1; 10; 100) and the second connection spot of the second rigid body (2; 20), is stationary in relation to one of the first rigid body (1; 10; 100) or the second rigid body (2; 20), and is variably inclinable in relation to the other one of the first rigid body (1; 10; 100) or the second rigid body (2; 20). an at least essentially punctual light source (13; 130) mounted stationary in relation to the first rigid body (1; 10; 100); an image sensor (15; 150) mounted stationary in relation to either the first rigid body (1; 10; 100) or the second rigid body (2; 20) and positioned in a field of light propagated by the light source (13; 130); a shadow mask (11; 110; 1100) mounted stationary in relation to either the first rigid body (1; 10; 100) or the second rigid body (2; 20) and arranged in the field of light propagated from the light source (13; 130) between the light source (13; 130) and the image sensor (15; 150); and a computation unit configured to calculate at least one angle (α, β) defining the position and orientation of the first rigid body and the second rigid body relative to each other by evaluating a shadow image (12; 120) generated by light propagated by the light source (13; 130) which passes the shadow mask (13; 130) and which is detected by the image sensor (15; 150), characterized by further comprising a mechanical constraint (3; 30) mounted to a first connection spot of the first rigid body (1; 10; 100) and to a second connection spot of the second rigid body (2; 20), wherein the mechanical constraint defines a fixed distance between the first connection spot of the first rigid body (1; 10; 100) and the second connection spot of the second rigid body (2; 20), is stationary in relation to one of the first rigid body (1; 10; 100) or the second rigid body (2; 20), and is variably inclinable in relation to the other one of the first rigid body (1; 10; 100) or the second rigid body (2; 20). defines a fixed distance between the first connection spot of the first rigid body (1; 10; 100) and the second connection spot of the second rigid body (2; 20), is stationary in relation to one of the first rigid body (1; 10; 100) or the second rigid body (2; 20), and is variably inclinable in relation to the other one of the first rigid body (1; 10; 100) or the second rigid body (2; 20).