NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: When even a Davis Mark 3 is too much
From: Brad Morris
Date: 2017 Jan 15, 23:38 -0500
From: Brad Morris
Date: 2017 Jan 15, 23:38 -0500
John
You wrote
My first try, years ago, was good to 1/10 degree without a vernier. I just wanted a hands-on model to hold what I was reading about. With a little skill and time I am sure a handy person could build a sextant to one minute arc.
My response was that the centering error would preclude a wooden sextant being built accurate to one minute. Franky, I have no idea how you know that your first try was accurate to 1/10th degree (6 minutes), but I will accept that very bold assertion! David provided a very believable 1/4° or 15 minutes as his accuracy. Perhaps your device really is 2.5 times better!!
I provided you with a standard method of determining rotational error as a function of centering error and it's inverse. That is, given the maximum error, how good must the centering be. It's simple trigonometry and fairly straight forward.
Your response stated
Wooden octants were made and used for a very long time, even after the metal sextant - sometimes at the same time.
Which I understood to mean that wooden octants were accurate to one arc minute. Journal wear at the axis of rotation and centering error, in wood, would likely preclude such accuracy. Not only would the wooden joint have to be centered beyond the capabilities of most wood shops today, let alone 200 years ago, just rotating the index arm would produce wear and slop. I challenged you to provide any data which supported your assertion. That is, against any known standard.
You replied that the "standard is getting to your port."
Er, no. That isn't the accuracy of the device at all. That's seamanship and navigation in general. Heck, nearly anyone can plod down a latitude line and hit port. Add a bit of northern error at sea and when you hit land, turn south. Viola, there's the port. I can do that with string and a plumb bob, to come in 1 or 2 degrees north of my port before turning south to find my port. Not very efficient, but i did find my port! Your supporting argument does not determine an "accurate to one arc minute octant", not at all.
While I do understand that making a mockup can be useful, and that a modern metal working machine shop can provide the required centering (Ala Örjan), I still think you are wildly underestimating the requirements.
You wrote that a "handy person could build a sextant to one minute". I simply disagree. No disrespect intended or implied. The relationship between the arc and the index arm must be held very tightly. Chauvenet states so explicitly and gives further argument as to a double index arm to minimize the centering error effect. Chauvenet is a noted authority you are certainly familiar with.
I get to be wrong lots of times. It happens. Just recently my 36 year old memory of time zones aboard a Navy vessel was wrong....gasp!! I think in this case, that it happens to be your turn to be wrong sir! Hopefully, you treat this as a technical discussion of the merits, and certainly not a personal attack on yourself.
Brad
On Jan 15, 2017 8:10 PM, "John D. Howard" <NoReply_Howard@fer3.com> wrote:
Brad, you said:
"With all due respect, John, after 30 years if professional leadership in the motion control field, I believe I understand centering error, accuracy and repeatability.You mentioned some older octants and etc. Please provide proof, wherein the ACCURACY of the device is measured against a standard. The resolution has nothing to do with it. It's the accuracy that is the claim and the accuracy that counts."The standard is getting to your port. The proof is that for hundred of years sailors used wooden octants to do that. You seem to be saying that a plastic sextant cannot give useful results. I am saying that a wooden octant can be as good as a plastic sextant and can be used for navigation.Frank had asked the question about a home shop making a pivot and my answer was a ball bearing.John H.
