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The term “civil twilight” is defined as...

  • A

    The period before sunrise or after sunset where the midpoint of the sun disk is 6 degrees or less below the true horizon.

  • B

    The period before sunrise or after sunset where the midpoint of the sun disk is 6 degrees or less below the apparent horizon.

  • C

    The period before sunrise or after sunset where the midpoint of the sun disk is 12 degrees or less below the true horizon.

  • D

    The period before sunrise or after sunset where the midpoint of the sun disk is 12 degrees or less below the apparent horizon.

Each twilight phase is defined by the solar elevation angle, which is the position of the Sun in relation to the horizon. During civil twilight, the geometric center of the Sun's disk is at most 6 degrees below the horizon:

  • The morning civil twilight starts when the centre of the Sun is 6° below the sensible (or true) horizon and ends at sunrise.
  • The evening civil twilight starts at sunset and ends when the centre of the Sun is 6° below the sensible (or true) horizon.

Night is defined as the hours between the end of evening civil twilight and the beginning of morning civil twilight or such other period between sunset and sunrise, as may be prescribed by the appropriate authority.

The duration of each twilight phase depends on:

The latitude - At higher latitudes, in both hemispheres, the Sun's path makes a lower angle with the horizon, so the twilight phases last longer.

The declination of the sun (and, consequently, the time of the year) - when the sun's position coincides with the observer's hemisphere, the twilight phases last longer (declination increases).

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