Monday, October 29, 2007


We use the following phenomena with the apparent motion of the sun and the moon:

SUNRISE - The instant the upper limb of the sun appears on the visible horizon.

MOONRISE - The instant the upper limb of the moon appears on the visable horizon.

SUNSET - The instant the upper limb of the sun disappears beyond the visable horizon.

MOONSET - The instant the upper limb of the moon disappears beyond the visable horizon.

TWILIGHT - The period of semi- darkness occurring just before sunrise ( morning twilight )
or just after sunset ( evening twilight ).

The navigator uses morning and evening twilight for star observations because during twilight the darkness makes the stars visible,but permits enough light to define the horizon. Both conditions are necessary if an accurate sextant altitude (hs) is to be obtained. There are four stages of twilight,based upon the position of the sun with respect to the horizon. They are:

ASTRONOMICAL TWILIGHT - The sun is 18 degree's below the horizon. Too dark for observations.

NAUTICAL TWILIGHT - The sun is 12 degree's below the horizon. Favorable for observations.
Recorded in Nautical Almanac.

OBSERVATIONAL TWILIGHT - The sun is 10 degree's below the horizon. Best for observations.

CIVIL TWILIGHT - The sun is 6 degree's below the horizon. Too light for observations. Also recorded in the Nautical Almanac.

It is necessary to know the times of sunrise and sunset because it allows the navigator to take a amplitude ( bearing ) of the sun to help check his deviation or gyro error of the compass.
Twilight is of importance because during these periods of time the navigator will be taking sights.
You will need to know how to determine the most best time for taking sights. Basically,all three times depend on the observer's position with respect to latitude,longitude,and the location ( east or west ) of the nearest standard time meridian. Remember that the times shown for rising and setting of the sun and the moon,and twilight in ALL publications are standard times. If that zone is keeping Daylight Saving Time ( DST ) it must be accounted for after the final time is figured.
With the given information in the following problem,determine the zone time of sunrise.
At 0400 zone time on 24 June 1981, your DR position is LAT 23-10.0N, LONG 85-33.0W. You are steering a course of 295 T at a speed of 10.0 knots. What is the zone time of sunrise?
A. 0452
B. 0458
C. 0504
D. 0510
Procedure - Follow these steps to calculate the zone time of SUNRISE.
1. Enter "The Nautical Almanac" and locate the page that corresponds to the date you are using.
2. Using a universal plotting sheet determine the DR postion ( latitude & longitude ) of your ship at the approximate time of sunrise ( 0522 ).
3. Make sure you are in the proper section ( top section for sunrise ).
4. Go down the latitude column and locate the tabulated latitudes that bracket your DR latitude and extract the latitude and sunrise data.
N 30 0500
N 20 0522
5. Determine the difference between tabulated latitudes and the time values.
N 30 0500
N 20 0522
10 22 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.
22min / 10 = 2.2min
7. Determine the difference between your DR latitude and the tabulated latitude nearest yours.
Convert the difference into degree's to the nearest whole degree.
8. Determine the latitude time correction by multiplying the difference by the ratio in step 6 and
rounding it to the nearest minute.
3 degree's x 2.2 min = 6.6 min = 7 min
9. Apply the latitude correction to the tabulated latitude time value. This is the time of sunrise at the standard meridian at that latitude.
20N 0522
- 7 min
23N 0515
10. DR ahead using this time to get a more accurate DR postion.
SR at STD MER 0515
DR time 0400
DIFF 1:15
1:15 X 10 kts = 12.5 mi
DR postion at 0515 is LAT 23-14.9N, LONG. 85-44.4W
11. Determine what standard meridian is closest to your DR postion.
STD MER. 90-00.0W
12. Determine the difference between your DR longitude and the standard meridian.
Convert the arc ( degrees and minutes ) into time ( hours and minutes ). Use either the arc to time table in the Nautical Almanic or the arc to time formula
STD MER 90-00.0W
DR LONG. 85-44.4W
DIFF 4-15.6
4 Degree's = 16m 00s
15.6 = 1m 02s
17m 02s = 17min.
13. Apply the difference of time to obtain the exact time of sunrise.
SR at STD MER 0515
- 17
ZT of SUNRISE 0458