No, night is not completely dark.
While we often perceive night as being entirely dark, especially in urban settings with light pollution, this isn't the case when considering the absence of direct and refracted sunlight. The term "night," particularly in an astronomical context, has specific criteria. According to astronomical definitions:
- Astronomical twilight continues until the Sun is 18 degrees below the horizon. This means that even after sunset, a faint amount of sunlight, refracted through the atmosphere, can still be visible.
- Astronomical night begins only when the sun is more than 18 degrees below the horizon. At this point, refracted sunlight is no longer visible. This period could be considered as when true darkness occurs, absent other light sources like the moon, stars, or artificial lights.
Understanding Degrees of Darkness
To better illustrate the concept, consider these points:
- Civil twilight is the time immediately after sunset or before sunrise, when the sun is just below the horizon, and artificial light is usually needed.
- Nautical twilight follows civil twilight, when the sun is further below the horizon, but some light is still present for navigation purposes.
- Astronomical twilight then takes over, where the remaining sunlight is extremely faint, and celestial observation becomes more accessible.
| Twilight Phase | Sun's Position Below Horizon | Light Visibility |
|---|---|---|
| Civil Twilight | Up to 6 degrees | Noticeable light |
| Nautical Twilight | 6 to 12 degrees | Faint light |
| Astronomical Twilight | 12 to 18 degrees | Very faint light |
| Astronomical Night | Over 18 degrees | No sunlight |
Factors Affecting Perceived Darkness
- Lunar Phase: The moon's phases significantly impact the perceived darkness of the night. A full moon can brighten the night sky considerably, even without any sunlight.
- Light Pollution: Artificial lights from cities and towns significantly reduce the darkness of the night. This light pollution can obscure fainter stars and celestial objects.
- Atmospheric Conditions: Factors such as cloud cover, humidity, and air quality can affect the perceived darkness of the night sky.
Therefore, while astronomical night represents the time when direct and refracted sunlight is no longer visible, various natural and artificial factors mean that complete darkness is not always achievable.
