At
sunrise and
sunset, rays emanating from the
sun to the
perpendicular arc that meets the viewer's
zenith are called
crepuscular rays, or
solar rays. Their distinctive bands all point towards the sun. When these same rays are visible at sunset from the viewer's zenith arc to the
antisolar point, even though they are emanating from the sun,
perspective makes them appear to emanate from an
empty spot on the opposite horizon. (It's a truly cool,
invisible sun effect.) Thus these rays merit their own name, called
anti-crepuscular rays or
anti-solar rays. You can also find both terms as single words without the
hyphen.
Linguistic note: Given that "crepuscular" is from the Latin crepusculum for twilight, anti-crepuscular is a fitting—if a little ham-handed—name for such a beautiful atmospheric effect.
Sources:
- http://www.meteoros.de/indexe.htm
- http://www.sundog.clara.co.uk/atoptics/phenom.htm
- At the time of writing, a beautiful example is available at
http://antwrp.gsfc.nasa.gov/apod/ap030226.html