The history, definition and pecularities of the Earth centered inertial (ECI) coordinate frame and the scales that measure time

The process of star catalog reduction is described in the sequence and terminology familiar to engineers working with space systems. This process converts a star catalog from a stationary coordinate frame located near the center of the Sun to an Earth centered coordinate frame at another time. Most of the material has been extracted from the Astronomical Almanac. The equations for proper motion, parallax, gravitational bending, and aberration corrections and the transformations for precession and nutation are presented. A discussion of the scales that measure time and why leap seconds are required, as well as the process of transforming from Earth centered inertial coordinates to Earth centered Earth fixed coordinates is included