• from Old English twi- ‘two’ (used in an obscure sense) + light (English) = twilight (late middle English)

-OxfordLanguages

• literally 'second light, half-light'

--wiktionary

Twilight occurs when Earth's upper atmosphere scatters and refracts sunlight which illuminates the lower atmosphere.

The morning twilight is often called dawn , while the evening twilight is also known as dusk .

There are 3 Types of Twilight:

1. Civil Twilight
2. Nautical Twilight
3. Astronomical Twilight

Astronomers define the 3 stages of twilight on the basis of how far the Sun is below the horizon.

“The length of twilight is an element useful to be known: by prolonging the day, it permits the continuance of labor.”

-French physicist Auguste Bravais

Civil twilight occurs when the sun is less than 6 degrees below the horizon

CIVIL DAWN

In the morning it begins when the center of the sun is 6 degrees below the horizon; ends at sunrise

CIVIL DUSK

In the evening, it begins at sunset; ends when the center of the sun is 6 degrees below the horizon

Civil twilight is the brightest form of twilight. There is enough natural sunlight that artificial light may not be required to carry out outdoor activities