Complex sentence: In order to be observed with the naked eye, a spacecraft must reflect sunlight towards the observer; thus, naked-eye observations are generally restricted to twilight hours, during which the spacecraft is in sunlight but the observer is not.
Split into simpler sentences: In order to be observed with the naked eye, a spacecraft must reflect sunlight towards the observer. Thus, naked-eye observations are generally restricted to twilight hours, during which the spacecraft is in sunlight but the observer is not.