However, the disappearance of the Sun's upper limb below the horizon does not mean night falls instantly, like someone shutting off a light switch. Instead, it is the beginning of a period called twilight. Twilight is the period between sunset and true nightfall, when the last bit of sunlight finally vanishes, and the sky becomes completely dark. Twilight, as it turns out, is further divided into three phases: Civil Twilight, Nautical Twilight, and Astronomical Twilight, and it is the phases of twilight, plus sunset, which are indicated by the four dots clustered at sundown. As each type of twilight darkens, it reaches its Dusk phase, which marks the transition into the next Twilight period. (The word twilight, incidentally, is Old English in origin ... appropriately enough for a term describing visual obscurity, it is unclear what the prefix "twi" actually means; the word may mean, "half light.")

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In-Depth: The Eerie Beauty Of The Apple Watch Solar Face, And The Anatomy Of Nightfall - HODINKEE