Finding deviation from a sun line

A good way to check compass deviation on a heading is to use the azimuth value from our sight reduction every time we do a sun site. As we know, a sight reduction will yield not only an intercept, but also an azimuth — or, the true direction of the sun. If you compare a reading from your compass to the azimuth, factoring in the known variation, you can calculate the deviation of the compass, if there is any, for the heading that you are on. As we recall, deviation is a function of a vessel’s heading, while variation is a function of the vessel’s geographical position. I usually find the variation from a North Atlantic Pilot chart.

For the sake of this example we will say that the variation for the DR position of 35° 13’ N by 67° 19’ W is 15° westerly. The day is July 15, 2013, and the time of the observation of the lower limb of the sun is 12:25:37 GMT. The Hs is 34° 12.2’. Our height of eye is 10 feet. We want to find the Ho and then enter into HO249 Vol. 2 to find Hc and Z.

Hs    34° 12.2
-dip    3.1’
App alt 34° 9.1’
3rd cor +    14.6’
Ho    34° 23.7’

GHA @ 12 hrs 358° 30.1’
+ 25:37     6° 24.3’
GHA     364° 54.4’
-ass long    67° 54.4’
LHA    297°
Dec at time of sight N 21° 26.9’ (d 0.4’)
        – d corr .2’
Dec         N 21° 26.7’

Vol II HO249

Hc 33° 33’     d+ 29’    Z 87°
+    13’
Hc 33° 46’
-Ho 34° 23.7’
Intercept    37.7 nm toward

From the Pilot chart we can say that the variation is 15° west. Now we can calculate the deviation on this heading. The compass bearing of the sun is 104° therefore we do the following:

When going from True direction to Compass direction westerly increments are added thus:

True 087° (the azimuth of the sun)
Variation 15° westerly (from the chart)
Magnetic 102° (the sum of the True and Variation)
Deviation ________
Compass 104°

The only number that yields 104° for compass is 002° west. Thus, the deviation is 002° west.

The deviation is found by the difference between the compass and magnetic headings. Remember the old saw: True Virgins Make Dull Company (“At Weddings” for “add west” when going from True to Compass).

By Ocean Navigator