Why is the Night Sky Dark?

October 03, 2013


Photographer: David K. Lynch; Dave's Web site
Summary Author: David K. Lynch

Why is the night sky dark? Well, actually it isn’t. When a tree or your hand is silhouetted against it, the night sky is seen to be surprisingly bright. Above we can clearly see trees and mountains silhouetted against the night sky. Faint light comes our way from myriad stars and galaxies, most of which are too dim to be seen individually but together form a luminous veil. The Earth’s airglow also makes the night sky shine.

Of course, it's a lot darker at night than during the day. Have you ever wondered why? Your first thought might be: “Sunlight brightens the day, and without sunlight, it’s dark.” This seemingly obvious explanation misses some profound information about the Universe. Starting as early as 1596 with Thomas Digges, scientists such as Kepler, De Cheseaux and Olbers and even Edgar Allen Poe assembled some powerful arguments about the Universe based on the night sky’s relative darkness. For example, according to Olbers:

“If the Universe is infinite and contains an infinite number of stars, then any direction we look should point to a star. Therefore the sky should be as bright as the surface of the Sun both day and night. Since it’s not, the Universe must not contain an infinite number of stars and therefore, the Universe must be finite.”

While we know now that the universe is expanding, and as it does so it further dims (reddens) distant starlight, the reasoning by Digges and others is nonetheless correct. Not a bad lesson from simply gazing at the night sky! Photo taken in the Sierra Nevada of California in 2009.

Photo details: Camera Model: PENTAX K200D; Focal Length: 18.0mm (35mm equivalent: 27mm); Aperture: f/3.5; Exposure Time: 43.890 s; ISO equiv: 800.