Ephemeris Longitude is the terrestrial dynamical longitude assuming a uniformly rotating Earth. True Longitude is calculated by correcting the Ephemeris Longitude for the non-uniform rotation of Earth.
Longitude is measured positive to the East.
Since T is not known in advance, the value used in the predictions is an extrapolation based on pre-1992 measurements. Nevertheless, the actual value is expected to fall within 0.3 seconds of the estimated T used here.