What causes the sky to remain illuminated after the Sun dips below the geometric horizon?

Answer

Scattering of light by the higher layers of the atmosphere

The persistent illumination after sunset is due to the Sun's light continuing to reach and be scattered down toward the ground by the upper layers of the atmosphere, even though the direct solar disk is blocked by the Earth's curve.

What causes the sky to remain illuminated after the Sun dips below the geometric horizon?
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