Susan Houde-Walter, Greg Pierce
The thin blanket of air that envelopes the Earth is rarefied at high altitudes and densifies with proximity to the surface. Thus the refractive index of the atmosphere increases in a continuous manner from unity in the vacuum of space to approximately n=1.000293 at sea level (λ=0.5893 μm). Everywhere the gradient of the refractive index points toward the Earth's center, and so light passing from the sun to an observer on Earth will take straight or curved paths depending on whether the light travels parallel or perpendicular to the index gradient. At high noon, the direct sunlight observed from the ground travels through the atmosphere along a straight trajectory. Direct sunlight reaching us at sunrise or sunset has taken a curved path, bending toward the high refractive index layers near the ground.
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