October 13, 2000
This photograph was taken last month just after sunset in Washington D.C., and it shows one very common phenomenon, congested streets at rush-hour, and another uncommon phenomenon, anticrepuscular rays. The traffic is heading east, away from the setting Sun, and the faint anticrepuscular rays can be seen converging at the antisolar point, just below the horizon. Of course, the light rays from the Sun are actually parallel to each other. However, after sunset, if clouds or dust beyond our horizon block some of the rays while other rays find gaps between the obscuring material, it's possible to see the beams of light spread out across the sky. When looking west, near or after sunset, the beams are called crepuscular rays. However, if conditions are right, these same rays can be seen on the opposite side of the sky as well (anticrepuscular rays). It's analogous to looking at railroad tracks in the distance and noticing that they appear to converge to a point. Now, if you turn around and look in the opposite direction, the tracks will again appear to converge. That's what's happening in this photo. This is one of many interesting twilight phenomena that you may be able to see if you look up every once in a while - it'll give you something to do when your stuck in rush-hour traffic.