compass deviation by the sun, revisited

[duckmanton]

Hi Team, l would have thought this nav problem would have got more people involved, or am l the only thick one in the group.
Dave, could you answer this one,(or anyone else please) regarding the example PBO391 or my previos post below on the index.
What was the reason behind subtracting the 13deg, the example was W13 deg S,is there a reason to add or is it always subtract.
Just high lighting, that this is a calculator project not tables.
I would also like any one elses opinion regarding is it degrees or radicon?(degrees l believe)
I Have e-mailed John the Nav man but as yet have not received any responce.
Thanks in advance

<hr width=100% size=1>

 

[duckmanton]

Just to add, the adage error west compass best may be the reason behind the subtraction, but if this is the case what is the significance behind the South

<hr width=100% size=1>

 

[Evadne]

I don't claim to be a serious mathematician and I don't have my copy of PBO to hand, but is (say) W13 subtracted and E13 added? This would be something to do with longitude taken as -180 to 0 to +180, so negative angles are required depending on your longitude relative to the Greenwich meridian. Mathematically it is arbitrary whether East is positive or negative and I'm not enough of a navigator to state with certainty which is the convention, although I'd have thought West would be negative for a right-handed observer. Hopefully someone may come up with the answer before Monday, I'll try and read the article in more detail before then.

<hr width=100% size=1>

 

[bedouin]

I don't have the article you refer to so excuse me if this is nonsense.

If I understand the situation correctly - The the decision whether to add or subtract the 13 degrees depends on whether the declination is the same 'sign' (e.g. N or S) as the latitude

<hr width=100% size=1>

 

[snowleopard]

to avoid having separate tables for sun rising and sun setting, they simply give the azimuth in terms of degrees above or below the east/west line. so W 13 deg S means 13 degrees South of due West which is 270 - 13 = 257. That would be for a setting sun in winter, i.e. with a southerly declination.

E x deg N: subtract from 90
E x deg S: add to 90
W x deg S: subtract from 270
W x deg N: add to 270

much easier with a diagram...

<hr width=100% size=1>

 

[duckmanton]

l am thankfull that there is some ( l was going to use a big word but l have just got home from HAY STREET and the words are to big to spell) discussion arising from this but who is correct. I know l have left this very late for me personaly
but l do have an articule in Cruising Helmsman (AUST) that delves into this, but alas l have been on the piss for the last 11 hr and l haven't double focussed it yet.
As l said early A i think JOHN could of explained this better for idiots like me ( and l would think later that with no responce from his articule, he may of thought that everyone was either up to his level or where not willing to show their ignorance)
l have now one focussied the articule.
The amplitude as found is applied to either East(90deg) or West(270) thus anyamplitude of S10 W is infact 260 deg true.
but is this error west compass best ? how come the minus if not this reason
looking forward to a lively discussion.

<hr width=100% size=1>

 

[snowleopard]

no error

the amplitude calculation has nothing to do with correcting errors so the old rhyme doesn't apply.

<hr width=100% size=1>

 

[duckmanton]

Thanks Snowleopard, you are correct as far as the articule l have now read ( with one eye closed and the othere shut) thanks for answering this, the sun is now popping it's head up(05.15, 21.15GMT) the wife has locked the bedroom door, and l'm of to Esperance and Sunday off across the bight.
IF someone has given false info and l end up in the North you people will be unlucky. Regards The DRUNCK l ment the "DUCK"

<hr width=100% size=1>

 

[trev]

Just compare the declination to your latitude - if the dec is to the north of you then add the azimuth for setting sun (subtract for rising) and vice versa. Declination is 'latitude' on the celestial sphere, so if the dec is higher than your lat then the sun is to the north of you.

<hr width=100% size=1>Trev

 

[snowleopard]

declination...

if the declination is greater than your latitude, the sun will be north of you...AT NOON.

at sunrise or sunset the sun will be north of you if the declination is north, regardless of your latitude. at the equinoxes the sun rises due east and sets due west.

(above assumes you are in northern hemisphere)

<hr width=100% size=1>