Both an expression as well as a term used to refer collectively to the city's artificial sun and moon together.

As an expression it's used to express horror, shock, disgust, or disbelief.

When referring to the heavenly bodies "city's eye" can be used interchangeably for whichever is visible at the given time. Some citizens whisper that the use of "eye" is more literal than symbolic and are quite sure that the council is spying on them through its celestial gaze.