The illumination of twilight at horizontal surface has been quite difficult to search for, but I did find it. At:
Sun overhead (angle +90 degrees above horizon) - 129 klux
This is total - direct Sun disc plus blue sky combined.
As the Sun falls, the illumination decreases - for example with Sun at altitude 11 degrees, the illumination is just 12,7 klux. Obviously the illumination on horizontal surfaces would fall on airless bodies like Moon as well.
But in absence of air, the illumination on surfaces perpendicular to sunlight would be constant until the disc edge reaches horizon.
Can anyone link at data which would specify separately
1) the illumination on horizontal surfaces and the illumination on surfaces perpendicular to sunlight at the same solar position, and
2) provide the breakdown between the light from Sun?s disc and that from the sky around (separately for horizontal and perpendicular direction)?
General physics and astronomy discussions not directly related to Celestia
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