NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Longitude by Time Sight Using a Slide Rule
From: Lu Abel
Date: 2012 Dec 16, 12:18 -0800
From: Lu Abel
Date: 2012 Dec 16, 12:18 -0800
I've had a 6" plastic Pickett in the nav desk of my boat for 30 years. Still sliding nicely.
I'll admit I'm a coastal sailor, not an offshore one, so I've never taken a breaking wave with my companionway hatch open (thereby dousing my nav desk).
What do I use it for? Primarily for converting seconds (found on some harbor charts) to tenths of a minute (or vice-versa) for use with my GPS. Put the one over the six and just slide the cursor, try that you hand calculator/iPhone freaks!
Lu
I'll admit I'm a coastal sailor, not an offshore one, so I've never taken a breaking wave with my companionway hatch open (thereby dousing my nav desk).
What do I use it for? Primarily for converting seconds (found on some harbor charts) to tenths of a minute (or vice-versa) for use with my GPS. Put the one over the six and just slide the cursor, try that you hand calculator/iPhone freaks!
Lu
From: Hewitt <hhew36@gmail.com>
To: "NavList@fer3.com" <NavList@fer3.com>
Sent: Sunday, December 16, 2012 10:46 AM
Subject: [NavList] Re: Longitude by Time Sight Using a Slide Rule
Speaking of slide rules, does anyone know how a wood one fares over time on a small boat? I never wanted to trust my beloved K&E on any trip, so I used to carry a 6" circular aluminum one.
Hewitt
Sent from my iPad
On Dec 15, 2012, at 8:09 PM, "Greg Rudzinski" <gregrudzinski@yahoo.com> wrote:
> For those that are game for trying the time sight reduction by slide rule there are a few ways to gain some accuracy. A means to magnify the slide rule scale goes without saying.
>
> 1. Use a four place SinHo look-up from the Bowditch trig table.
> 2. Use the Bowditch trig table for the final ArcCos look-up to get LHA (t) meridian angle.
> 3. Slide rule alignment on the two multiplications can be improved if a whole latitude is used.
>
> This will result in two table look-ups, two slide rule multiplication moves, one slide rule division move, and one conventional sum.
>
> Greg Rudzinski
>
>
>
> [NavList] Re: Longitude by Time Sight Using a Slide Rule
> From: Greg Rudzinski
> Date: 14 Dec 2012 10:18
>
> K&E slide rule accuracy seems very good for LHA(t) meridian angles greater than 50*. There is no need for small meridian angle calculations when the ex-meridian method would be used. See attachment for this mornings time sight by slide rule. The plot exactly matched an azimuth and intercept by computer reduction.
>
> Greg Rudzinski
>
> [NavList] Longitude by Time Sight Using a Slide Rule
> From: Greg Rudzinski
> Date: 14 Dec 2012 09:03
>
> The K&E 10" has been dusted off to perform some time sights using the formula:
>
> LHA(t)=ArcCos[ (SinHo ~ SindSinL) / (CosdCosL)
>
> ~ add if contrary name
> ~ subtract if same name
> t = meridian angle
> Ho = true altitude
> d = declination
> L = Latitude
>
> This seems to be accurate enough. If Paul Hirose is watching then maybe he can do a slide rule accuracy analysis for the time sight using the 10".
>
> Greg Rudzinski
>
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