Twilight Clouds

Twilight

twilight clouds

stacks of twilight clouds

Last night's cloudscape at sunset and this evening's cloud cover got us researching sunset specifications:

astronomical sunset:   when the upper limb of the Sun disappears below the horizon (although atmospheric refraction actually means the solar disk is already one diameter below horizon at visible sunset)

civil twilight:   begins when Sun disappears below the horizon (Sun 0° below horizon)

nautical twilight:   when the horizon is no longer visible (Sun 6°–12° below horizon)

astronomical twilight:   when all stars are visible to the naked eye (Sun is 12°–18° below horizon)

dusk:   the end of astronomical twilight until night

night:   no sunlight visible (Sun more than 18° below horizon)

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