“Pitch black” describes conditions with a complete absence of light from any source. True pitch-black conditions are uncommon in most inhabited areas. The rarity of such darkness stems from various natural and human-made light sources that often prevent the sky from becoming entirely devoid of light.
Phases of Nighttime Darkness
The progression from daylight to the deepest night involves distinct phases of twilight, each defined by the sun’s position below the horizon. Civil twilight occurs when the sun is between 0 and 6 degrees below the horizon. Enough natural light remains for outdoor activities, and only the brightest stars are visible.
Nautical twilight follows, with the sun positioned between 6 and 12 degrees below the horizon. At this point, the horizon often remains discernible, but artificial light becomes necessary for most outdoor tasks. Astronomical twilight takes place when the sun is between 12 and 18 degrees below the horizon. A faint scattering of sunlight still occurs, which can affect the observation of very dim celestial objects. True astronomical night, or “pitch black” conditions, begins only after the sun drops more than 18 degrees below the horizon, when indirect sunlight no longer contributes to sky illumination.
Environmental Influences on Darkness
Even after astronomical twilight, environmental factors can diminish true darkness. Light pollution, from urban and artificial sources, brightens the night sky. This artificial light scatters, creating “skyglow” that reduces star visibility and prevents full dark adaptation. Air pollution, clouds, fog, and haze further scatter artificial light, making the night sky brighter.
The moon’s phase and position also influence night sky brightness. A full moon illuminates the night sky, acting as a light pollution source that washes out fainter celestial objects. Conversely, a new moon phase offers the darkest conditions. The moon’s altitude above the horizon also affects its brightening impact, with higher moon positions leading to brighter skies.
Achieving and Perceiving Ultimate Darkness
Experiencing true pitch-black conditions requires seeking remote locations away from human settlements and light pollution. International Dark Sky Places, certified by the International Dark-Sky Association, are prime examples of such areas. These locations, which include national parks, preserves, and sanctuaries across the globe, implement responsible lighting policies to protect their naturally dark night skies. Examples include Central Idaho in the United States, NamibRand Nature Reserve in Namibia, and Aoraki Mackenzie in New Zealand.
The human eye’s ability to perceive darkness is also a factor, as our vision system adapts to low-light conditions through a process called dark adaptation. This adjustment involves the pupil dilating to allow more light to enter the eye and a biochemical regeneration of photopigments in the retina’s rod cells. While the quickest gains in sensitivity occur within the first few minutes, full dark adaptation, particularly for rod cells responsible for night vision, can take 20 to 30 minutes, or even several hours for maximum sensitivity. This physiological process means that even in a truly dark environment, a period of adjustment is necessary before the eye can fully appreciate the absence of light.