On Earth, daytime is roughly the period on any given point of the planet's surface during which it experiences natural illumination from indirect or (especially) direct sunlight.

Other planets that rotate in relation to a luminous primary, such as a local star, also experience daytime of a sort, but this article primarily discusses daytime on Earth.

https://en.wikipedia.org/wiki/Daytime

"If we crave some cosmic purpose, then let us find ourselves a worthy goal."
Carl Sagan
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