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Sample Nautical Almanac problem

Apr 30, 2019

Find the GHA and declination for a sun sight taken at 11:26:15 GMT on July 19.

First turn to the daily page for three pieces of information: GHA (hours), declination (hours) and “d.” (This section of the Nautical Almanac is reprinted on page 44.) Follow down to the “11” hour row in the section for the 19th. The first column to the right is GHA (343° 26.1’) and the next is for declination (20° 48.0’ N). Notice that the “N20” for declination is not printed for every hour. It is extremely important to find this each time.

The d is found at the bottom of the declination column. Here it is 0.5. The navigator must determine if the sign for d is positive or negative. If declination increases during the course of the day, then d is positive; if it decreases, d is negative. Here the declination of the sun is decreasing, so d = –0.5. The d is nothing more than an interpolation factor for the change of declination over the course of an hour.

Now go to the increments and corrections pages at the back of the Nautical Almanac. Turn to the page for the minutes of the time of the sight, 26 in this case. (The table is reprinted at left.) First, enter the left column with the seconds of the sight to find the increment to add to the GHA. Go down to 15 seconds and then to the first column labeled “SUN PLANETS.” The increment is 6° 33 .8’. This is added to GHA:

GHA (1100)
increment (26:15)
GHA (11:26:15)
  343° 26.1’
+     6° 33.8’
349° 59.9’

Now, enter at the top of the table in the first column labeled “v or d” and go down that column until the d value of 0.5. Stop at 0.5 and move one column to the right to get the correction. The correction is 0.2’. It is –0.2’ because d is negative. Therefore:

declination (1100) 
d correction (26 mins)
declination (1126)
  20° 48.0’ N
–           0.2’
20° 47.8’ N

And that is what we need: the GHA and declination.

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