# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: A rate of change rule
From: Lu Abel
Date: 2016 Oct 31, 17:04 +0000
You are absolutely right on accuracy, Frank.

Where I find the Rule of 12ths useful is in estimating tidal height over the course of a tidal cycle.  An accuracy of 6" (or even 1 ft) is enough -- I'm not going to sail anywhere where I don't have at least 7' of water for my 6' 3" draft boat to sail through.  And even though I carry one aboard, I don't want to have to whip out my scientific calculator to do the math.

Lu

To: luabel@ymail.com
Sent: Monday, October 31, 2016 9:13 AM
Subject: [NavList] Re: A rate of change rule

Lu Abel, you wrote:
"Just remember 1,2,3,3,2,1  (which sums to 12). My understanding is that the Rule of Twelfths is accurate to 2 or 3% of the true sine wave"
This is a very old rule from an era when integers were essential to teaching and comprehension. Today, wouldn't it be just as easy today to remember the values that are accurate to three significant digits: 0.80, 2.20, 3, 3, 2.20, 0.80? That's the change in the value of the sine for every 30° change in the angle, expressed as sixths. Better yet, why not just memorize the sine values for the angles in question, especially since there are only four independent values: sin(0°)=0, sin(30°)=1/2=0.5, sin(60°)=sqrt(3)/2=0.8660..., sin(90°)=1. We can throw in one more that's easy to remember: sin(45°)=sqrt(2)/2=0.7071... This short list completely covers the "Rule of Twelfths" as a special case, and it's 100% accurate.
Note: the little rule I gave in my first post here is not contained in the above; it's independent. But you can get close using the "Rule of Twelfths" (or its exact, mocern equivalent as detailed above). The sine values alone tell you that, in one half period, from maximum to minimum, the value of the quantity changes by 2 units (from +1 to -1). They also tell you that in the middle third of that range, the sine value change by 1 unit (from +0.5 to -0.5) which means that the average rate of change in the middle third is 1.5 times the average rate of change in the whole range from max to min. As I noted yesterday, the actual maximum rate (in the pure sine curve case) is pi/2 or 1.57 times the average rate. So it's just a question of the level of detail you seek... what sorts of numbers you're comfortable memorizing... and how much time you want to spend working it out.
Frank Reed

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