Figure A. After sunset, the sun is below the horizon and its rays illuminate the sky overhead. The sky between the ground and the sun's rays is in the Earth's shadow. A twilight photometer measures the reduction in sunlight at the top of the Earth's shadow as the sun continues to sink below the horizon. (The reverse occurs before sunrise.) Aerosols alter the amount of sunlight atop the Earth's shadow, which enables us to estimate their altitude.
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