“Night” refers to the portion of Earth facing away from the Sun, into the planet’s shadow. Because Earth has an atmosphere, night doesn’t begin immediately at sunset, nor end exactly at sunrise.

Instead, observers first experience twilight, the period after sunset and before sunrise when the atmosphere scatters sunlight into the shadowed side, gently illuminating the sky. Only once Earth has rotated far enough into the shadow does true night begin.

We often describe night as “dark,” but what does that really mean? Most imagine a black sky dotted with stars, planets, and the Moon. Yet the night sky is more complex. Light from countless stars in the Milky Way merges into a bright band across the sky. Dust in our Solar System scatters sunlight toward Earth’s night side, and in polar regions, the aurora can light up the sky during periods of high solar activity. Through telescopes, even more light from distant galaxies and other cosmic sources becomes visible. Even under ideal conditions on Earth, the night sky is never as completely dark as it is from space.

This image shows some different paths taken by light rays from outdoor light fixtures. The streetlight emits light in many different directions. Some of the light rays (“1″) are directed up into the sky and travel completely through Earth’s atmosphere. Of these rays, a few (“2″) will be detected by satellites as they pass over the nighttime side of our planet. In still other cases (“3”), rays are scattered back to the ground by dust particles or molecules in the atmosphere, forming skyglow. Some downward-scattered rays (“4”) make it into astronomers’ telescopes, effectively blocking their view of the universe. Occasionally, rays directed downward (“5”) reflect off the ground into the sky, where they might escape the atmosphere and be detected by satellites. Credit: International Dark-Sky Association