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

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Re: Bowditch 2002
From: Mitch B Burrill
Date: 2002 Oct 4, 10:19 -0400

```Jared's point on accuracy is well taken, but one must use five-place trig
values to get full sextant ( 0.1-0.2 min ) accuracy. I've had the privilege
to help train many young engineers, and I've found that one of the toughest
things to inculcate is "enough with the 6 decimal places already !".
I told my HP48 to leave default mode, gave it 7 decimal places, asked it for
5, and it disagreed with Bowditch. How many  places should I give it ? I
think this is interesting because when Briggs published his 14 place log
tables in 1624, it contained over 1000 errors, almost all of them in the last
decimal place (still an astonishingly low error rate).

Anyway, to digress : anyone out there have any practical experience with
longitude by Jovian moons? I found a website giving all the transit/eclipse
data and am ready to start. Hurricane Lili is not cooperating.....

-----Original Message-----
From:   Herbert Prinz [mailto:hprinz@ATTGLOBAL.NET]
Sent:   Thursday, October 03, 2002 5:20 PM
To:     NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject:             Re: Bowditch 2002

Mitch,

You are misunderstanding the idea about rounding to the next even or odd
number. This applies only to the case where you have exactly the sequence
5000... following the digit to
which you are rounding. The goal is to avoid systematic rounding error while
still minimizing individual error. What you have suggested would produce
individual rounding errors
greater than 0.5 times the place value of the place to be roundet to and would
thus be unacceptable as a rounding method.

You are probably thinking of the special case where you are given n decimal
places and must round to n-1 places. Then you would correctly round to the
nearest even (or odd) decimal
number if the n-th decimal is a 5. *) But when you produce tables such as in
Bowditch, you always compute your values to SEVERAL places more than you
tabulate, and you avoid
systematic rounding error altogether by simply picking the nearest value to the full computed result.

*) Interesting enough, the US Power squadron does not think so. For them you
must round up, or else you flunk their tests. Could it be that they want to
be able to check your work
with a pocket calculator?

Herbert

Interesting enough

Mitch B Burrill wrote:

> The only algorithms I know are :
>
> 1. Always round up, like a modern electronic calculator (which I think leads
to serious error accumulation in hand calculation)
> 2. Round up if the digit to the right of the 5 is odd (or even)
> and 3. Round up to make the digit to the left of the 5 odd (or even).
>
> To get the values below (which are published in Bowditch 2000 AND the 23rd
edition of the CRC math tables),
> Bowditch is using Round if Right is Odd.
> But then that algorithm fails with the published values for 32-22 and 32-33.
>
> I'm stymied (for now)  !!
>  -----Original Message-----
> From:   Herbert Prinz [mailto:hprinz@ATTGLOBAL.NET]
> Sent:   Thursday, October 03, 2002 3:01 PM
> To:     NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
> Subject:             Re: Bowditch 2002
>
> Well, Mitch, which rounding algorithms are you aware of?
>
> Sin(32d 07') = 0.53164497... and Sin(32d 43') = 0.54048508...
>
> So, rounded to 5 places after the decimal point, we would expect to see
0.53164 and 0.54049, using ANY rounding algorithm that I can possibly think
of.
>
> What does Bowditch have?
>
> Herbert Prinz
>
> Mitch B Burrill wrote:
>
> > Yes, examine the sine of  32-07, 32-22, 32-31, 32-33, 32-43, and 32-59.
The values given for 32-07 and 32-43 seem mutually exclusive using the
rounding algorithms I'm aware of.

```
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