The phase angle (φ) is the angle between light source (in this case, the Sun) and the observer (here, the HRSC camera on Mars Express) when seen from the viewed object (the Martian moon Phobos). Over the course of the animation, this schematic representation illustrates how this angle changes due to the observer’s movement. In the animation, the phase angle of the individual Phobos observations changes from 17 degrees (A) at the beginning to almost 0 degrees (B) to 15 degree (C) at the end.

Image Credit: DLR