What makes the night sky dark?

The simplest answer to this question requires that you use your imagination to picture our local star – the sun – and its family of planets. The sun’s light pours outward to illuminate every portion of our solar system so that the space around the sun is almost entirely flooded with light.

But there are dark places. These are in the shadows of planets, moons and other objects in orbit around the sun. And it’s these shadows that create night. Earth’s shadow extends over a million kilometers into space. Our planet’s shadow is so long it can brush the face of the moon during a lunar eclipse. Lunar eclipses are relatively rare. But, every day, as Earth turns on its axis, the part of the planet you’re standing on turns for a time so that you face into Earth’s shadow. When you face into the shadow, it’s night. When Earth turns so that you again face the direction of the sun, it’s day.

On any clear evening, you can see the curved line of Earth’s shadow ascending in the eastern sky. It gets higher in the east as the sun sinks below the western horizon.

October 9, 2009

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