Twilight

Twilight is the interval when the Sun is below the horizon but still illuminates the atmosphere or ground. It spans the changing light between daylight and night.

Solar depression angle: The angle of the Sun below the horizon, used to describe its position after sunset or before sunrise. Twilight stages are defined by how far the Sun has descended below the horizon.
Horizon: The apparent boundary between Earth and sky, or the corresponding geometric plane at an observer’s location. Twilight begins and ends relative to the Sun’s position below this boundary.
Sunrise and sunset: The daily events when the Sun’s upper edge appears above or disappears below the horizon. Twilight surrounds these boundaries between direct daylight and the Sun below the horizon.
Daylight: Natural illumination at Earth’s surface from the Sun when it is above the horizon. Unlike twilight, daylight includes direct sunlight at the location.
Rayleigh scattering: The scattering of light by particles much smaller than its wavelength, especially molecules in the atmosphere. It redirects sunlight through the atmosphere after the Sun has set.
Solar zenith angle: The angle between the direction to the Sun and the vertical direction at a location. It describes the Sun’s changing position as twilight progresses.
Prayer times in Islam: Daily prayer times determined by the Sun’s position and other prescribed astronomical conditions. Dawn and evening twilight help determine the times of Fajr and Isha prayers.
Night: The period when a location faces away from the Sun and receives no direct solar illumination. Twilight is the transition into or out of night, not the fully dark interval.
Atmospheric refraction: The bending of light as it passes through layers of air with different densities. It makes the Sun appear slightly higher than its geometric position near the horizon.
Earth's rotation: Earth’s spinning motion around its axis, producing the daily cycle of sunrise and sunset. It carries locations through twilight each day.

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