clouds at dusk

In a place with a very dark night sky, clouds appear black against a brightly-lit backdrop of stars on a moonless night.

Most people live very far away from a place dark enough to see the night sky this way. Light from civilization on the ground illuminates the clouds from below—just a little light is enough to make them brighter than the stars.

In the day, light from the sun scatters to illuminate the clouds from above. During dusk and dawn, you can watch the transition point where light scattered in the sky becomes brighter than the clouds, and for a short while, they are about equal in luminance, contrasting in hue.

This marks the margin between day and night—a margin emergent from human activity, where artificial lighting and the sun take turns illuminating the earth, the clouds offering a visualization to the participants below.