Atmospheric
Optics
Antisolar Rays & Belt of Venus ~ Zhuo-xiao Wang captured this dramatic scene from the summit of Mauna Kea, Hawaii. Antisolar (anticrepuscular) rays and shadows stream downwards to the point directly opposite the sun. The sky is suffused by the warm pink glow of the Belt of Venus. Beneath all that is the dark shadow of the earth itself cast through the atmosphere.
Image ©Zhuo-xiao Wang, shown with permission
Belt of Venus
At the top of the Earth’s shadow the sun is just setting. Its rays travel a long path through the air and in particular the lower atmosphere. They are scattered by air molecules, dust and aerosol.
Blue light is scattered much more. The resulting direct rays are highly reddened. The red light mixes with the blues of the sunlit atmosphere above. Stratospheric ozone adds its own effects.
The combined light scatters backwards to the eye and forms the rosy glow of the Belt of Venus.
Antisolar Rays
Shadows of clouds on the western horizon cross the sky. They are parallel. But to the eye they appear to converge - like rail-road tracks - to the point directly opposite the sun.
The antisolar point is below the horizon by day. After sunset on the ground it rises into the sky. We may still see antisolar rays because they are then passing through the still sunlit upper atmosphere.
