Plutarch's Platonic questions (8.3)

It is therefore most proper to understand the earth to be here an instrument of time; not that the earth is moved, as the stars are; but that, they being carried about it, it standing still makes sunset and sunrising, by which the first measures of time, nights and days, are circumscribed. Wherefore he called it the infallible guard and artificer of night and day. For the gnomons of dials are instruments and measures of time, not in being moved with the shadows, but in standing still; they being like the earth in intercepting the light of the sun when it is down,—as Empedocles says that the earth makes night by intercepting light. This therefore may be Plato’s meaning.