The Blue Shift

Perspectives from The Artist's Road

Nocturne, Railway Crossing, Chicago 1892-93 Childe Hassam

Moonlight is the light of the sun reflecting off the surface of the moon. It is about 400,000 times fainter than direct sunlight. Surprisingly, we perceive it to have a bluish cast when we are painting a nocturne outside, but it is actually a slightly warmer color than the color of direct sunlight.

The color temperature of moonlight has been measured at 4100 to 4150K versus the color temperature of sunlight which is measured at over 5000K. Why then, do we perceive a blue color from moonlight? The effect seems so natural that even filmmakers often use a blue filter over the lens to simulate a moonlit scene when they are shooting during the day. However, it isn’t really natural at all!