Possible Clues:

• Apparent displacement due to observer movement
• Apparent displacement due to perspective
• Seeming displacement, to astronomers
• Soldier, fifty and flabby? It's an optical illusion
• Visual displacement, in astronomy
• Optical effect as normal: a line without focus?
• Celestial phenom
• Soldier left undisciplined in apparent change of position
• Apparent change in position due to perspective
• Optical phenomenon