Nightfall was not a threat to the Earth. Anything that gets between the Earth and the Sun, which Nightfall did since it blocked sunlight, is already past us and can't turn around and come back.
In relation to the Earth the Sun is downward.
Think of the Sun as the ground, and Earth as a high flying plane. Nightfall would be between the plane and the ground. Not a problem.
Therefore somebody was lying to Clark!
It was clearly an attempt to trick Superman into killing himself trying to save Earth.
Patrick
