# NavList:

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

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Re: recommendation for slide rule ?
From: Gary LaPook
Date: 2009 May 29, 03:50 -0700
```Hi Gary

Yes, the Flat Bygrave (and also the cylindrical) will provide wonderful results, making the Bygrave
my "recommendation for slide rule".

Any luck finding the Dunlap Sunrise Sunset Computer?

Best Regards
No, it turns out I don't have one. I have a Sundowners computer and I am attaching copies of it. It seems that it does about the same thing.

gl

```Hi Gary

Yes, the Flat Bygrave (and also the cylindrical) will provide wonderful results, making the Bygrave
my "recommendation for slide rule".

Any luck finding the Dunlap Sunrise Sunset Computer?

Best Regards

-----Original Message-----
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of glapook@PACBELL.NET
Sent: Thursday, May 21, 2009 6:21 PM
To: NavList
Subject: [NavList 8361] Re: recommendation for slide rule ?

And using the Bygrave you get Hc  71º17' and Az of 45º09' the same
result, see http://www.fer3.com/arc/m2.aspx?i=107414&y=200902.

gl

On May 21, 7:40 am, Brad Morris <bmor...@tactronics.com> wrote:
```
```Hi Gary

The values, using electronics are

Hc = 71.29011  (71 deg 17 minutes)

Az = 44.57333  (44 deg 34 minutes) when using 71.1
Or
Az = 45.12870  (45 deg 08 minutes) when using 72.29011

The deltas, compared to the K&E 4080, are

delta Hc = (71.29011 - 71.1) = 0.19011  (0 deg 11.4 minutes)
delta Az = (45.12870 - 44.5) = 0.62870  (0 deg 37.7 minutes)

Plotting with an azimuth with an error of less than 1 degree is unlikely to cause problems, except with very shallow LOP crossings.
The delta Hc is a larger concern, making this method probably suitable only to deep water sailings.

Workable? Yes!

Best Regards

-----Original Message-----
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of glap...@PACBELL.NET
Sent: Wednesday, May 20, 2009 7:00 PM
To: NavList@fer3.com
Subject: [NavList 8339] Re: recommendation for slide rule ?

Computing Hc and Zn on a slide rule takes 16 steps. It will be easier to explain with an example, using 34 for the latitude, 20 for the declination and 14 for LHA, the same values used in my previous example with the Flat Bygrave, seehttp://www.fer3.com/arc/m2.aspx?i=107414&y=200902

I will use a K&E 4080 for the illustrations.

Using the sine- cosine procedure we first calculate Hc using the formula, sin Hc = (sin Lat sin Dec) + (cos Lat cos Dec cos LHA).

First line up the sine (S) scale with the D scale, see IMAGP5888.

Second move the cursor to the latitude, 34, on the S scale, see -98.

Since the sine and cosine values are all less than 1 we know that when we multiply these values that the product will be smaller than the original numbers which on a slide rule means that the result is found further to the left on the scale. So, third, line up the 90 mark on the S scale with the cursor, see -00.

Fourth, move the cursor to the declination, 20, on the S scale and read out on the D scale the product of sine latitude and sine declination, .1915. see - 02.

Fifth, line up the S and D scale again, see -88.

Sixth, move the cursor to the latitude, 34, on the cosine scale, red numbers on the S scale increasing right to left, see -04.

Seventh, line up the 90 mark on the S scale with the cursor, see -05.

Eighth, move the cursor to declination, 20, on the cosine scale, see -07.

Ninth, line up the 90 mark on the S scale with the cursor, see -09.

Tenth, move the cursor to LHA, 14, on the cosine scale and read out the product of cosine latitude, cosine declination and cosine LHA on the D scale, .755, see -11.

Eleventh, add the two products together, .1915 plus .755 equals .9465.

Twelve, line up the S and D scale again and locate .9465 on the D scale and read out Hc on the S scale, 71.1º , see -13.

Next we calculate Az using the formula (sin LHA cos Dec)/ cos Hc rearranged as (sin LHA/cos Hc) cos Dec. Az will be in the range of 0 to 90.

Thirteen, move the cursor to LHA, 14, on the S scale, see -15.

Fourteen, line up Hc, 71.1, on the cosine scale with the cursor, see -26.

Fifteen, move the cursor to declination, 14, on the cosine scale, see -27.

And finally, sixteen line up the S and D scales and read out Az, 44.5º, on the S scale under the cursor, see -28.

gl

,

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The information transmitted by this electronic mail (and any attachments) is being sent by or on behalf of Tactronics; it is intended for the exclusive use of the addressee named above and may constitute information that is privileged or confidential or otherwise legally exempt from disclosure. If you are not the addressee or an employee or agent responsible for delivering this message to same, you are not authorized to retain, read, copy or disseminate this electronic mail (or any attachments) or any part thereof. If you have received this electronic mail (and any attachments) in error, please call us immediately and send written confirmation that same has been deleted from your system. Thank you."

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