As you watch a setting sun, you see the sky go darker. Some people call that period ""dusk,"" while others may call it ""twilight."" But these words aren't arbitrary in the astronomical world. They have specific meanings.
Civil twilight occurs when the sun is between 0 degrees and 6 degrees below the horizon.
Nautical twilight occurs when the sun is between 6 degrees and 12 degrees below the horizon.
Astronomical twilight occurs when the sun is between 12 degrees and 18 degrees below the horizon.
Night is classified when the sun is 18 degrees or farther below the horizon.
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