Tuesday, February 07, 2012
By Jim Kaler, University of Illinois
Nightfall. When day turns softly to evening. But night does not fall. It rises.
Wait for a brilliant clear day and find a place of contemplation, one with a clean horizon that allows you to see the changing colors of sunset and twilight. Like everything else in sunlight, the Earth casts a shadow into space. When our Moon passes through the shadow at full phase (opposite the Sun), we see a lunar eclipse. But the shadow starts at the surface of the Earth itself and must first cut through our own atmosphere. So as the Sun dips below the western horizon, a gray band rises in the air to the east, the earthshadow. The Sun is far and the air near, which produces a lever action that causes the shadow to climb faster and faster until it sweeps across the darkening sky, allowing the stars to shine. Night thus rises. It is in morning twilight, just before sunrise, when the gray band descends to the western horizon, that night falls, the Earth's shadow finally disappearing as the Sun pops up in the east and a new day begins. Welcome to the sky -- half your world. Look upward and you won't want to look back.