crepuscular rays, shafts of light which are seen just after the sun has set and which extend over the western sky radiating from the position of the sun below the horizon. They form only when the sun has set behind an irregularly shaped cloud or mountain which lets the rays of the sun pass through a cloud in bands. The radiating appearance of the bands is caused by perspective, as demonstrated by the fact that when on rare occasions the rays extend across the entire sky, they appear to converge again on the eastern horizon. The name is sometimes applied to shafts of sunlight seen when the sun shines through cumulus clouds, but the derivation of the word (from the Latin crepusculum, “twilight”) indicates that it most accurately applied only to the after-sunset occurrence.