OPOD - Siding Spring

Dusk at Siding Spring

James Watson Photography) captured this series of optical effects at Siding Spring Observatory. Here, the rising Earth's shadow fringed by the pink Belt of Venus backdrops the 3.9m Anglo-Australian Telescope.

Opposition effect or shadow hiding:
The dome shadow marks the antisolar point. There are no tree shadows nearby because each shadow is hidden by the shadow casting tree. Further and further from the dome, the shadows become more visible. The lack of shadows near the antisolar point render it brighter than the surrounding landscape. It is also slightly yellow because the blue of shadows is absent. Shadow hiding also contributes - with other effects - to the brightness of the Moon at full and of Mars at opposition.

Aerial Perspective:
Landscape objects become bluish with distance. The effect, long known by painters to indicate distance or depth, was first labelled aerial perspective by daVinci in his Treatise on Painting.

Haze and airlight contribute to the blue. Blue haze results from sunlight scattered by aerosol, smoke or dust particles smaller than wavelengths of blue light. They are Rayleigh scatterers and preferentially scatter blue light.

Airlight is also a Rayleigh scattering effect but the scatterers are air molecules themselves. It is strongest when the sun is high.