IF DR LONITUDE IS TO THE LEFT ( EAST ) OF THE STANDARD MERIDIAN THEN ADD THE TIME.
morning and evening twilight are usually the most important periods of the day. Ordinarily,these are the only times during which you can fix your positions by
Remember:
IF DR longitude is ...
to the right (EAST) of the STD. Meridian
THEN ...
subtract the time.
to the left (WEST) of the STD. meridian
add the time.
MORNING or EVENING TWILIGHT
Twilight is that period before sunrise when darkness is giving way to daylight, and that period after sunset when daylight is giving way to darkness. In celestial navigation, morning and evening twilight are usually the most important periods of the day. Ordinarily, these are the only times during which you can Fix your position by obtaining nearly simultaneous lines of position from celestial observations. At nautical twilight, the sun is 12° below the celestial horizon. At the darker period of civil twilight, the center of the sun is 6° below the celestial horizon, and during good weather, bright stars are easily distinguished, and the horizon is sharp and clear. This is approximately the mid-time of the period during which the experienced navigator makes twilight observations. In order for the navigator's eyes to adjust to the darkness, the navigator usually wants to get on the bridge 20 or 30 minutes prior to civil twilight. The Nautical Almanac lists data for obtaining nautical twilight as well as civil twilight for three day intervals.
With the given information in the following problem, determine the zone time of morning civil twilight.
At 0400 zone time, on the 24 June 1981, your DR position is LAT 23° 10.0'N, LONG
85° 33.0'W. You are steering a course of 295 T at a speed of 10.0 knots. What is the zone time of civil twilight?
A.0433 B.0450 C.0517 D.0458
Procedure Follow these steps to calculate the zone time of civil twilight.
Step Action
1. Enter ''The Nautical Almanac" and locate
the page that correspond to the date you are using.
2. Using a universal plotting sheet determine
the DR position (latitude & longitude) of your ship at the approximate time of sunrise [ 04 58 ].
3. Select the proper section.
IF you need AM twilight then use the upper section
IF you need PM twilight then use the lower section
4. Go down the latitude column and locate the tabulated latitudes that bracket your DR latitude and extract the latitude and sunrise data.
JUNE 24 1981
LAT. CIVIL
N 30 0433
N 20 0458
5. Determine the difference between tabulated latitudes and the time values.
N 30 0433
N 20 0458
10 25 min
6. Determine the ratio of change in time for each degree of change of latitude by dividing the latitude difference into the time difference.
25 min / 10 degree's = 2.5 min
7. Determine the difference between your DR latitude and the tabulated latitude nearest yours. Convert the difference into degrees to the nearest whole degree.
DR LAT N 23
TAB LAT N 20
DIFF. 3 degrees
8. Determine the latitude time correction by multiplying the difference by the ratio in step 6 and rounding it to the nearest minute.
3 degrees x 2.5 min = 7.5 min or 8 min
9. Apply the latitude correction to the tabulated latitude time value. This is the time of twilight at the standard meridian at that latitude.
N 20 0458
- 8
N 23 0450
10. DR ahead using this time to get a more accurate DR postion.
Twilight at STD MER 0450
DR time 0400
50 min
50 min x 10 kts = 8.3 mi
DR postion at 0450 is LAT. 23-13.2N , LONG. 85-40.9W
11. Determine what standard meridian is closest to your DR position.
STD MER 90-00.0W
12. Determine the difference between your DR longitude and the standard meridian.
STD MER 90-00.0W
DR LONG. 85-40.9W
DIFF. 4-19.1
13. Convert the arc ( degrees and minutes ) into time ( hours and minutes ) . use either the arc to time table in the Nautical Almanac or the arc to time formulas.
4 degrees = 16m 00s
19.1 = 1m 16s
17m 16s =17 min
14. Apply the difference of time to obtain the exact time sunrise.
SR at STD MER 0450
- 17
ZT of twilight 0433
REMEMBER - IF DR LONGITUDE IS TO THE RIGHT ( EAST ) OF STD MERIDIAN THEN SUBTRACT THE TIME.
IF IT IS TO THE LEFT ( WEST ) OF THE STD MERIDIAN THEN ADD THE TIME
