Abstract: The apparatus includes a solid state, two-dimensional, array (4) of elements or pixels fixedly attachable to the spacecraft or satellite (2) so that the array boresight (5) is aligned with the nominal Celestial Body pointing axis of the spacecraft or satellite (2). The elements or pixels are sensitive to illumination in the visible to near infra-red wavelength range, sense an image of Celestial Body (3) falling thereon and indicate the relative intensity of illumination falling on each element or pixel from the image. Optical means focus the image of Earth onto the array (4) and means for digitally sampling and processing the relative intensity of illumination information indicated by the array (4) operate to produce coordinates of the center (7) of the Celestial Body image on the array (4) whose displacement in two orthogonal axes from the center of the field of view of the array (4) provides an indication of the roll and/or pitch attitude error of the spacecraft or satellite (2).