Extended Data Fig. 3: Illumination geometry of the skull-top crevice.

a, OSIRIS NAC image of the Abydos region on 12 June 2016 with three overlaid azimuth plots. The central point of each plot is positioned on the skull-top crevice. The two skull-top boulders are outlined by the green ellipse and the crevice by a tiny purple ellipse inside it. At this central point the viewer is at a 90° angle, that is, an overhead view; each concentric circle represents a decrease in viewing angle of 10°. The large red box encloses an azimuth plot overlaid on the image and the green line shows the horizon. The red area in the two left-hand azimuth plots shows the track of the Sun over this region during the full period of the Philae landing and to the end of the Rosetta mission, while the yellow line overlay is the crevice’s own horizon mask (created as explained in b). b, Example azimuth plot for 21 August 19:19–19:24 ut with Rosetta’s position and the Sun’s position at this time marked by the very short green and red lines. These short lines are then connected by long blue lines to the central crevice. This gives the line of sight for the OSIRIS camera and for solar illumination of the crevice during this period. We then look at the OSIRIS image (inset) to see if the crevice is illuminated. We find that it is, which is why it lies inside the red-lined horizon mask. The dot-dashed line passes right through the crevice, meaning that the right of that line is the right side of the crevice and the left, the left side.