A shearing interferometer block consisting of a beamsplitter cube, a right angle prism and a corner cube is used in conjunction with a dual detector to measure when an input beam is normal to the cube to better than 0.001 arc seconds. The two arms of the interferometer have optical path differences of X/4. The wavefronts of the exit beams, one from the corner cube and the other from the right angle prism are tilted in opposite directions from a moving input beam. Zero-crossings of the detector pair output occur when they each receive the same optical power. The output maximum and minimum occur just before and after the zero-crossing associated with normal in-cidence of the input beam. Analysis shows the zero-crossing to be detectable to an accuracy of better than 0.0001 arc seconds for 2.5 cm optics, 1 mW optical power and 1 MHz electronic bandwith with a dual p-i-n photodetector. Laboratory tests confirm this result. The interferometer block has many precision measurement applications. Those discussed herein include: a precision angular reference point; a building block to define a precision reference beam; a sensing element for high precision active control to maintain alignment between two surfaces; a star tracker sensing element. All of the above instruments should have a precision better than 0.001 arc seconds.