The difference between the photographic and photovisual magnitudes of a star or, more generally, the difference between the magnitudes of a single star in two separate and specified regions of the spectrum. By convention, the color index (C. I. ) is then defined as: magnitude in short wavelength region minus magnitude in long wavelength region. The C. I. for an A0 star is defined in the Johnson Morgan UBV system of stellar classification to be zero. The C. I. of the Sun is 0. 62. The color index appears in geodesy when the coordinates of stars are to be determined very accurately from stellar photographs. It is one of the quantities appearing in the equations used in reducing the measurements of the photographs.