The number of sidereal days that have elapsed at Greenwich since a given starting date. It is analogous to Julian Date, and the initial epoch is chosen as the beginning of the sidereal day that was in progress at JD 0.0. The whole-number part of the Greenwich sidereal date is the number of sidereal days that have elapsed since that time (the Greenwich Sidereal Day Number), and the decimal part is the Greenwich Sidereal Time expressed as a fraction of a sidereal day.