A prototype sun sensor has been developed. It consists of a thin slice of silicon coated with a layer of chrome and a layer of gold with hundreds of small pinholes, placed on top of an active pixel sensors (APS) image detector at a distance of 500 microns. Images of the sun are formed on the APS image detector when the sun illuminates the mask. Image processing of the sun image is performed externally to the chip. Sun angles are derived by determining the precise location of the sun images on the detector - just like a sundial. The envelope of the sun sensor is 10/spl times/15/spl times/1.5 mm, it has a mass of 0.5 gram and the power consumption is 30 mW. The accuracy of the MEMS based sun sensor on a chip is estimated to be 1 arcminute.