The "lunar terminator paradox" is an optical illusion that occurs when observing the moon, particularly when it is full, at sunset. The paradox arises because, although the sun is below the horizon, the moon appears to be pointing upwards, with the bright side at the top and a dark slice at the bottom. This is due to the fact that, as the moon rises in the sky, the observer's viewpoint shifts from looking at the moon from a direct side view to looking at it from below, causing the illuminated portion to appear at the top. The effect is most pronounced with a full moon and becomes less noticeable with less full moons, such as a half-moon, which does not exhibit this paradoxical behavior.