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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » How close can I get with LAN?

Author Topic: How close can I get with LAN?

 - posted February 03, 2021 12:53 PM      Profile for Jon           Edit/Delete Post 
Although I believe my inputs to be good, I'm only at 4.5 nmi accuracy.

Sextant: Astra IIIB (calibrated with IC=0)
Fixed known location (south of West Point lighthouse, Discovery Park)
Natural horizon at 180T (i.e., no dip short)
HE = 11' (included in SR)

I took several sights near LAN and plotted the sun's rise and fall. My sights felt good; clear sun outline, unrushed and repeatable. All lower limb. Was hopeful on having greater accuracy but unsure on how to improve.

From: Parallel 47
David Burch

 - posted February 03, 2021 01:31 PM      Profile for David Burch           Edit/Delete Post 
We can help track down the issue. You are right that you should be able to do much better in those circumstances.

Pleased tell us the exact times of the sights, the Hs of each, and the GPS location, and what you got for the index error. We can figure HE and tide etc.

we will need at least 4 sights taken in a row.

This is the power of cel nav that we can find these errors given that info.

From: Starpath, Seattle, WA

 - posted February 04, 2021 09:50 PM      Profile for Jon           Edit/Delete Post 
I appreciate the offer. For this most recent attempt, I unfortunately discarded my source documents so I'll attempt again when the sun peeks out again.
From: Parallel 47
David Burch

 - posted February 04, 2021 09:54 PM      Profile for David Burch           Edit/Delete Post 
OK. The offer stands. If you are not getting results below a mile or so, then save all that data and we can try to figure out why. Need at least 4 sights in a row, 5 or 6 would be better, plus the several sights taken to measure the index correction using the toward and away methods we describe in our textbook.

Note this does not have to be at noon. we can still learn if the LOP goes through true position. Or even better, so a set before LAN and then an hour or so after LAN and we can get a fix. Check table 11.15-1 in our book to find the optimum or minimum time before and after noon for such a fix.

From: Starpath, Seattle, WA

 - posted February 08, 2021 01:44 PM      Profile for Jon           Edit/Delete Post 
Will do, but, to use the table provided in the book, isn't the Hs too low this time of year (e.g., get a good cut)? (Although, I'm guessing your suggestion of an hour will provide the requisite 30 deg....)
From: Parallel 47
David Burch

 - posted February 09, 2021 08:12 AM      Profile for David Burch           Edit/Delete Post 
A guideline is this: if the peak height of the sun is less than 45º then Zn is moving at roughly 15º per hour along the horizon...

peak height = 90 - lat - dec

in feb in seattle this is roughly 90-47-14 = 29 so an hour is about 15º.

But with all that said, we do not need to get a fix to test the sights. we can just check that the average LOP goes through your true position and then most important look at the spread in the values.

From: Starpath, Seattle, WA

 - posted February 27, 2021 10:38 AM      Profile for Jon           Edit/Delete Post 
Happy to report that, with yesterday's sun, I was able to achieve LAN accuracy of .3 NMI. Quite pleased.

One resulting question, however. I have an Astra IIIB. Since I walk a fair distance down to the beach for my sightings--and don't want to carry the wooden box--I've taken to removing the telescope and wrapping both it and the main component in towels and placing them in a backpack. Upon reaching my location, I reassemble. At home, I had achieved zero IC; at the beach, I was Off 1.6'. I was curious how much IC people regularly see from attaching/removing telescope after reassembly. (PS If anyone has a portability solution for transporting a sextant outside of the wooden box, I'm all ears.)

From: Parallel 47
David Burch

 - posted February 27, 2021 10:50 AM      Profile for David Burch           Edit/Delete Post 
Congratulations. Very nice work.

Unfortunately, taking the scope on and off should not affect the IC at all. There is something else going on. On the other hand, everything is very sensitive, so something else most likely got bumped in the process.

When you say you "achieved" at home a 0' ic that sounds like you are adjusting it. does that mean you also were adjusting it at the beach?

You might find that it is best to just adjust it once to be close, and then let that go. No more adjustments. Just measure what the error is before each set of sights.

PS. I know very well that walk down to the beach, and that by the shorter woods trail involves a lot of bumps. so This is not too much a surprise that it might have changed. You can look through the sextant once adjusted, and just push a bit on the mirror as you watch the horizon to see how sensitive this is.

From: Starpath, Seattle, WA
Chris Faris

 - posted February 27, 2021 10:54 AM      Profile for Chris Faris           Edit/Delete Post 
Hi Jon...new to the forum and just finishing the classroom book. In Chicago this morning, I was able to get an LOP 3nm away from my actual position (got to LOPs from 2 sights). The horizon on Lake Michigan was foggy this morning, so I know I have work to do, plus my HE=16 feet (one needs to be cautious with the ice this time of year, so I might be off there). I was pleased with the first time using my Astra IIIB Pro sextant, but know I need way more practice under more optimal conditions...fingers get cold quickly.

I carry my sextant in a Pelican case from Celestaire (Pelican-like) with the foam already cut out. I'm pleased with this setup and don't have to take the telescope off.

More practice is needed on my part, looking forward to it!


 - posted February 27, 2021 03:38 PM      Profile for Jon           Edit/Delete Post 
Thank you, Chris. I have a Celestaire brochure around here that I have not looked through. I will find it and check out the Pelican case (they make nice products). I appreciate the tip. BTW, how do you like the Pro model?

PS I learned the hard way about how sensitive these tools are. I had wrapped the entire sextant--telescope attached--in a towel and discovered days later that the stud on which the "nut (cap)" screws to affix the telescope had sheared off. (It appears to be made of a relatively thin casted material.) Celestaire kindly sent me a new part (the piece holding that threaded stud with two screws which attaches to the frame) for no charge; not even shipping. I have no recollection of bumping or dropping my backpack, but, be forewarned, they can break. I have hopefully learned my lesson.

From: Parallel 47

 - posted February 27, 2021 03:42 PM      Profile for Jon           Edit/Delete Post 
David, I only adjusted the mirrors at home, never at the beach. I simply used the horizon for IC. And, given the "horizon"--landfall behind it > 4 miles away, it was not as crisp as I'd like.

My IC adjustment at home was both using rooftops and the 32.4' sun check. Side error checks done using stars.

From: Parallel 47
Chris Faris

 - posted February 27, 2021 05:30 PM      Profile for Chris Faris           Edit/Delete Post 
Jon, I like my sextant a lot, but I have to confess, I don't have a lot of experience with them, so not a lot of basis to compare. It was a gift. I do have a Davis plastic one as well (split mirror) and this Astra IIIB Pro (full mirror)...still in learning mode and loving every minute of it! A great skill to pick up during a pandemic! I did put a long list of questions out there, but it hasn't been posted yet (I attached a picture, so I assume it is getting screened, not sure).
David Burch

 - posted March 01, 2021 09:51 AM      Profile for David Burch           Edit/Delete Post 
Pictures must be less than 800 pixels wide and less than 300 kb. Only pictures can be posted. no other file type.

As for the long list, please raise only one question per post, so it can be addressed that way. Can be as many posts as you like, but one topic per post please. thanks.

From: Starpath, Seattle, WA
David Burch

 - posted March 01, 2021 10:36 AM      Profile for David Burch           Edit/Delete Post 

We have just realized that you have actually paid for the full online Starpath Cel Nav course, but it looks like you have not taken the course. We have no quiz records nor questions posted within the course. Can you please call the school at your convenience and we can look into this and get you going on that part of the training. 206-783-1414. I am sure that will address your long list of questions. The online course has its own discussion forums, with much more content and activity.

From: Starpath, Seattle, WA
Chris Faris

 - posted March 04, 2021 11:27 AM      Profile for Chris Faris           Edit/Delete Post 
Hi David, I'll give you guys a ring...thanks. Yes, I purchased the "home study" course (some time back), got all the piece parts, and now, thanks to the pandemic have completed the home study (just wrapping up moon shots topic)...loved every minute of it! I'll ring you guys, I guess I lost track of the 'online part of it'. Toby has been great too in answering my questions.

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