GBL angle calculation

Schematic diagram for the calculation of the GBL angle, due to the Sun, for a light beam coming from planet Mercury (Ma) and reaching the Earth (Ea). The deflected beam is illustrated as the solid red curve, where r0 stands for the closest approach to the Sun. Due to this relativistic phenomenon, a virtual position of each planet occurs (represented by MV and EV, respectively) which are shifted with respect to their actual positions. The GBL angle is computed as the difference between the actual angle αa and the virtual angle αV.