capnsensible
Well-Known Member
Because it was neither north or south. Try it.
Its a strange feeling that no matter where you point the sextant, the sun seems to drift around the horizon. Weird.
Question for the classically trained Astro navigators; when Dec approaches Lat
- Thread starter Skylark
- Start date
Its a strange feeling that no matter where you point the sextant, the sun seems to drift around the horizon. Weird.
capnsensible
Well-Known Member
Well obviously not.
Refueler
Well-Known Member
I was ..... c'mon ... I suggested looking at sunrise and sunset so he would understand altitude is constant change .... playing the teacher way of making student look at it ...
Time to do it to you ??
Refueler
Well-Known Member
Because it was neither north or south. Try it.
Agree .... but because you can still get within a reasonable accuracy - its valid ...
A MP / PL noon position is only good to an average 10nm anyway ... so time to look at it in real terms.
Refueler
Well-Known Member
capnsensible
Well-Known Member
Well Somethin We Can Agree On........ second part.
Refueler
Well-Known Member
Maybe you misunderstood..... YOU are wrong because the accuracy of a MP / PL noon fix is only to about 10nm anyway - means your post saying its not possible is wrong.
I am sure all those guys who have stood on bridge wings taking MP would love to have been excused the job by your error !!
Alan S
Well-Known Member
Just point the sextant south by compass, take a series of sights and take the maximum as the sun passes the meridian. If the ALT is more than 90 then your lat is south of Dec so subtract alt from 180.
Refueler
Well-Known Member
Conversation between 2nd Mate and Master :
2/0 : Sorry Captain .. MP is too near 90 deg today for us to do noon fix ...
Master : What ? So no Noon run data for me to report to company / charterers ? Are you sure ???
2/O : Yes Captain .... "Capnsensible" om YBW said its impossible to take sights for MP / noon fix ...
Master : May i suggest 2/O - you seek another employment (words to that effect).
2/0 : Sorry Captain .. MP is too near 90 deg today for us to do noon fix ...
Master : What ? So no Noon run data for me to report to company / charterers ? Are you sure ???
2/O : Yes Captain .... "Capnsensible" om YBW said its impossible to take sights for MP / noon fix ...
Master : May i suggest 2/O - you seek another employment (words to that effect).
capnsensible
Well-Known Member
Thats a useful tip I would like to try. I was indeed getting sextant angles over 90 degrees.
capnsensible
Well-Known Member
As well as my own experiences I quoted from a Yachtmaster Ocean exam paper. So you had as I said, best tell the RYA training and the authors of several training books instead of posturing. And thats being nice.
Skylark
Well-Known Member
Yep, fair enough, I didn’t engage brain and think about what I was trying to say, typed it too quickly in between cleaning my pushbike and oiling the gear change ? Guess you’ve never made a silly mistake.
I’ll try again. Normal MP procedure is to take sights from, say, 15 minutes before, based upon Almanac and DR Long. If Dec and Lat are similar, how do you establish the correct horizon? The sun will be more or less directly overhead viewed from 360 azimuth so the Hs will be unreliable. The only way I can think of is to use a compass to sight either north or south, depending upon diff between Dec and Lat, but is that accurate enough? That’s why I asked if there was a lore to use a different celestial body in these circumstances (but didn’t expect sarcasm as a response).
Shame that my genuine interest to learn something has deteriorated into a politics and Brexit spat.
I’ll consult Bowditch to see if he makes a non-judgmental, explanation.
capnsensible
Well-Known Member
For those reading who this is an Odd Thing, the voice on this is a bit yuk but the principle is explained:
Refueler
Well-Known Member
I wasn't meaning to be rude - just trying to get you to look at what you posted. Which you have done. Forget the posturing crap others have posted on this bit - they are just trying to be smart.
The accuracy of course diminishes - but you are making a hard job for yourself .. you have good idea of the direction needed and you take your MP ... the sextant is not a 90 deg only instrument.
As a Professional Navigator who has on a number of occasions been in this situation -I post based on actual experience and solution ... not on some syllabus or tutorial paper as some others.
Skylark
Well-Known Member
Thanks, I know but didn’t engage brain before I wrote the comment about alt. I’m struggling with the location of the horizon when measuring Hs; azimuth will be due East then due West (that’s the all-day constant that I should have quoted, not alt). Near MP I can imagine that the sun is pretty much overhead from all directions but true alt needs to be measured from a perpendicular horizon, ie due north or south.
Alan S
Well-Known Member
Skylark, when lat and Dec are very near the same, as long as you are facing due south or North, the max angle on your sealant will after the normal corrections give you an ALT as accurate as any "normal " noon sight.
Refueler
Well-Known Member
As well as my own experiences I quoted from a Yachtmaster Ocean exam paper. So you had as I said, best tell the RYA training and the authors of several training books instead of posturing. And thats being nice.
Being nice ... as a professional navigator who has had this on a number of occasions - I will say it once more and final : MP and producing a noon fix by sextant sun sights when Dec is near or at Lat is practically possible and not impossible as you claim.
Fundamental errors by YOU and why I dismiss them as such :
N / S position lines cannot be used for running fix ... CRAP ! Only when your course is N / S ... any other course - the position line can still be run based on course / speed.
Second -falling over backwards taking noon sight .....the MP is only to provide latitude and given that overall you are expecting to be in a 10nm probability - its good enough to take your sight for MP in the N or S direction expected. If you get it 180 deg out - the sextant has range exceeding 90 deg ... you will still have the results.
I say this plain and blunt : I was navigating ships before GPS ... before Transit ... the Sextant was our third hand. I do not take lightly stupid comments or implied crap after many years of successfully navigating ships ... where sextant use was a daily routine.
Some years ago - I stopped giving talks to clubs about navigation matters because of the crap some yotties came out with ...
Sorry to bring in the Professional vs Yottie ... but I see this crap again ... On that note - I shall love and leave you to your world ...
Alan S
Well-Known Member
Refueler, I agree with what you say about taking noon sights with the sun overhead (even if your choice of words is sometimes a bit abrasive).
I don't agree that a fix is possible with only 2 north/south lops (ie longitudes) no matter what your course and speed is. Sorry Skylark, this last bit is not really relevant to your original question.
I don't agree that a fix is possible with only 2 north/south lops (ie longitudes) no matter what your course and speed is. Sorry Skylark, this last bit is not really relevant to your original question.
capnsensible
Well-Known Member
N / S position lines cannot be used for running fix ... CRAP ! Only when your course is N / S ... any other course - the position line can still be run based on course / speed.
Two straight lines running N/S have no angle of cut. Therefore no fix. Perhaps you have forgotten?
Two straight lines running N/S have no angle of cut. Therefore no fix. Perhaps you have forgotten?
Refueler
Well-Known Member
N / S position lines cannot be used for running fix ... CRAP ! Only when your course is N / S ... any other course - the position line can still be run based on course / speed.
Two straight lines running N/S have no angle of cut. Therefore no fix. Perhaps you have forgotten?
Sorry ... yes - with all the other - forgot that one !!