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12/26/06 - Antisolar Crepuscular Light

After our Christmas dinner, but shortly before sunset we took a walk along the Strait of San Juan de Fuca, starting at Morse Creek. As we headed east back to the parking lot we were extremely aware of the failing light and the cloudy sky. We also noticed dark streaks in the sky towards the east, directly opposite the direction of the setting sun. These were not pillars of smoke, but something more interesting. They were the shadows of clouds cast by the setting sun. (Move your mouse over the picture to see a contrast exagerated view).

Basically, when the sun is low, cloud shadows can be hundreds of miles long. They are most visible when viewed parallel to the direction of the casting light and area least visible when viewed perpendicularly. Because of perspective effects, the rays appear to diverge from the setting sun, which was obscured by the coastal bluffs we were at the base of, and they appear to converge at the antisolar point on the horizon opposite the sun.


Antisolar crepuscular light at Morse Creek

Antisolar crepuscular light as seen from the coastal trail along the Srait of San Juan de Fuca, not far from the mouth of Morse Creek

Keywords: christmas, science, atmosphere, morse creek