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Author
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Topic: Practice Problem Sun Sight Exercise, Sun #2
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FRV
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posted March 26, 2026 02:33 PM
Hello,
I have been working my way through Celestial Nav Home Study Course. Making good progress.
My question relates to Sun Sight Exercise #2 Pg 77 and determining the LHA to enter into HO249.
I adjusted the DR Lon of 123° 00'W (box 1) to reflect the min from the GHA (box 3) which made the A-lon 123° 37.6'W and resulted in an LHA of 22° (box 4). That's how I entered HO249.
The answer sheet for the problem on pg 223 reflects the DR Lon changing from 123° 00'W (box 1) to 122° 37.6' (box 3) and then the LHA of 23° (box 4).
I don't understand why the text book changed the DR from 123° to a-lon 122°. The min did not pass 60 and I didn't need to borrow from the degrees during subtraction. All the rest of my figures matched the answer page.
I plotted the HC, D, and Z for both LHA 22° and LHA 23° respectively. The Assumed positions were 60 NM apart but the ZNs were nearly parallel and the LOPS fell almost on top of one another.
Can you help me understand why the text book reflects changing the DR Lon of 123° to 122° for the a-lon? I read back through chapter 5 discussion several times but could not figure this one out.
Thanks much.
From: NC
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RobertR
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posted March 26, 2026 09:21 PM
Hello! It is not possible to create a flat map of a section of a globe without some distortion. In the case of a Mercator chart (or false Mercator, in the case of our plotting sheets), this distortion mostly shows up in our measurements of distances, other than along a longitude line.
The greater the distance we need to measure, the greater the error in distance will be. So whenever possible, we try to minimize the distances measured.
In this particular case, an aLon of 122°37.6'W is 15.2' closer to our DR Lon of 123°00'W than 123°37.6'W is. In the real world, in the middle of the ocean, this won't make much difference. In this course, where we're trying to teach best-practices, we've chosen the smaller distance, and hence a (slightly) smaller distance-induced error.
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FRV
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posted March 27, 2026 06:53 PM
OK. Same thinking I guess as rounding to the closest degree for assumed latitude. I went back and reread chapter five, pg 63, 7th paragraph from top explained it. Pretty much says WRT assumed longitude keep the minutes the same as GHA min in west longitude but adjust degrees to keep it less than 30'.
Thanks.
From: NC
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