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Refer to figure.
At what UTC time is the end of the Evening Civil Twilight on the 19th of October 2017 at Bodo airport in Norway (N67°16', E014°22')?

  • A

    15:15 UTC

  • B

    16:13 UTC

  • C

    17:15 UTC

  • D

    18:20 UTC

Refer to figure.

The Air Almanac includes tables that give the Local Mean Time (LMT) of sunrise and sunset and the LMT of the start of morning civil twilight and the end of evening twilight. The times given in the Air Almanac are calculated for the Greenwich meridian, but can be taken as LMT at all longitudes.

The time of sunrise and sunset varies with latitude and time of the year. Higher latitudes experience longer days in the summer and shorter days in the winter, because of the inclination of the Earth's axis of rotation to the ecliptic.

The morning civil twilight starts when the centre of the Sun is 6o below the sensible horizon and ends at sunrise. And the evening civil twilight starts at sunset and ends when the centre of the Sun is 6o below the sensible horizon.

For this question:

  • Find the LMT end of the Evening Civil Twilight on 19th October 2017 interpolating for N67° 16' (between N66° - N68° and between 17th - 20th Oct): 17:10.
  • Convert Arc to Time: Bodo airport is 14o 22' East off Greenwich, which corresponds to: 14º x 4 min + 22' x 4 sec = 57 minutes of time.
  • Convert LMT sunset into UTC: The Earth rotates from West to East, thus the UTC end of Evening Civil Twilight at Bodo airport on 19th October 2017 is: 17:10 - 57 min = 16:13 UTC.

NOTE: Arc to time conversions: 360º = 24 hours, 15º = 1 hour, 1º = 4 minutes, 1' = 4 seconds.

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