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(For this question use Annex 061-600416).
What is the duration of morning civil twilight at 66°48’N, 095°26’W on 27th of January?
  • A
    8 hours 14 minutes
  • B
    1 hour 2 minutes
  • C
    9 hours 27 minutes
  • D
    1 hour 13 minutes

Since the question asks us to find the Twilight duration, we must use both tables provided.

(1) Determine the Sunrise Time:

Step 1: Locate the date and the latitude.

  • 27th of January is between 26th and 29th of January.

  • 66º48’N is between 66ºN and 68ºN.

Step 2: Interpolate the data.

  • Interpolate 68ºN row: △Date : △ Time
    26 – 29 is equivalent to 0947 – 0933: 3 days changes in date is equal to 14 minute changes in time. Therefore, 1 day change in date is equivalent to -14/3 = approx. -5 min. If we look at the table, we can see that time is reducing as the date is increasing.
    Sunrise at 68ºN = 0947 – 0005 = 0942 LMT

  • Interpolate 66ºN row: △Date : △ Time
    26 – 29 is equivalent to 0920 – 0909: 3 days is equal to 11 min. Therefore, 1 day is equivalent to -11/3 = approx. -4 min.
    Sunrise at 66ºN = 0920 – 0004 = 0916 LMT

  • Interpolate 37th column: △latitude : △ Time
    68º - 66º is equivalent 0942 – 0916: 2º change in latitude equals 26 min changes in sunrise time. Therefore, 1º change in latitude is equivalent to 13 min changes in sunrise time. To find the amount of sunrise times changes within 48’ changes in latitude: 48’ x 13 min = 10 min.
    Sunrise at 66º48’N equals 0916 + 0010 = 0926 LMT

Latitude/Date

26th

27th

29th

68º

0947

0942

0933

66º48’

0926

66º

0920

0916

0909


Having found the sunrise time, to find the twilight duration, we need to find the time of morning civil twilight (the same procedure applies).
(2) Determine the Morning Civil Twilight time:
Step 1: Locate Date and Latitude.
Step 2: Interpolate the data.

  • Interpolate 68ºN row. △Date : △ Time
    26 – 29 is equivalent to 0826 – 0817: 3 days changes in date is equal to 9 min changes in time. Therefore, 1 day change in date is equivalent to -9/3 = approx. -3 min.
    MCT at 68ºN (27th) = 0826 – 0003 = 0823 LMT

  • Interpolate 66ºN row. △Date : △ Time
    26 – 29 is equivalent to 0811 – 0803: 3 days changes in date is equal to 8 min changes in time. Therefore, 1 day change in date is equivalent to -8/3 = approx. 3 min.
    MCT at 66ºN (27th) = 0811 – 0003 = 0808 LMT

  • Interpolate 37th column: △latitude : △ Time
    68º - 66º is equivalent 0823 – 0808: 2º change in latitude equals 15 min changes in sunrise time. Therefore, 1º change in latitude is equivalent to 7.5 min changes in sunrise time. To find the amount of sunrise times changes within 48’ changes in latitude: 48’ x 7.5 min = 6 min.
    Sunrise at 66º48’N equals 0808 + 0006 = 0814 LMT

Latitude/Date

26th

27th

29th

68º

0826

0824

0817

66º48’

0814

66º

0811

0808

0803


Sunrise is at 0926 LMT, and it is the end of the morning civil twilight.
Morning civil twilight starts at 0814 LMT.
To find the duration, we simply subtract these two times.

  • Duration of Morning Civil Twilight = 0926 – 0814 = 1 hour and 12 minutes

Your Notes (not visible to others)



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