The Twilight Phenomenon

The Twilight Phenomenon refers to a series of optical events that occur in the atmosphere when the sun is close to the horizon, marking the transition between daytime and nighttime.

The primary factor influencing these phenomena is the scattering of sunlight in the Earth's atmosphere and the reduction in the intensity of direct solar rays. As the sun descends towards the horizon, its rays travel through a longer path in the atmosphere, resulting in decreased brightness and reduced illumination of the Earth's surface. This decline occurs not only due to the weakening direct sunlight but also because of light scattered throughout the atmosphere. When the sun is 5-10 degrees below the horizon, the diminishing illumination accelerates significantly, indicating the onset of twilight.

As the direct solar rays illuminating the Earth's surface diminish, the atmosphere becomes the primary source of illumination, particularly after sunset. However, the Earth's shadow gradually rises, occupying a larger portion of the atmosphere. The lower atmospheric layers, engulfed in shadow, no longer contribute to the brightness of the sky, and the scattered light increasingly originates from the higher layers, which are still illuminated by direct sunlight. Since air density decreases with altitude, the scattering coefficient also decreases, resulting in weaker scattering of sunlight, reduced sky brightness, and lower surface illuminance. When the sun dips to a depression of 10-15 degrees below the horizon, the upper atmospheric layers start emitting a faint glow alongside the starlight, and the lighting conditions gradually resemble those of nighttime. Typically, the transition to night is considered complete when the sun is around 17-19 degrees below the horizon. However, in exceptionally hazy conditions, twilight may persist until the sun reaches a depression of 22-23 degrees. Consequently, the boundary between the twilight period and night is not well-defined and can vary depending on atmospheric conditions. The duration of twilight is influenced by the speed at which the sun descends below the horizon, which in turn depends on the geographical latitude and time of the year.