It’s well known that the sky gets darker as you get farther away from human sources of light. But what’s the limit? How dark can the night sky get? Can the surface of the Earth ever get so dark on a moonless night that you literally cannot see a thing?
The answer is no. In fact there’s a fundamental limit to how dark the night can get. That’s because there are three completely natural sources of light that prevent the night sky from ever being completely black.
First, there’s light from stars themselves, which gets scattered in the atmosphere and contributes a glow to the sky.
Second, there’s what astronomers call airglow. It’s a permanent, faint aurora painted by solar activity. Airglow varies on a time scale ranging from minutes to hours, as well as over the 11-year cycle that governs the sun’s output of energy. In periods of high solar activity airglow at truly dark sites can make the night sky look noticeably lighter, sometimes producing rippled structures that resemble thin bands of high cloud.
Finally, the largest natural source of skyglow is zodiacal light. That’s caused by space dust reflecting sunlight back to us.
Combined, these sources of light can make the sky appear bright even a long way from artificial lights. Still, the best views of the stars come well away from towns and cities. Visit the KNAU website to see an animated view of what those look like from space at night.
