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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: Formulas to Compute LHA
From: Greg Rudzinski
Date: 2009 Jun 27, 18:38 -0700

```Andrew,

I believe the time diagram refered to is the Weems & Plath celestial
slide rule. Google search this item for a description then look one on
eBay.

Greg

On Jun 27, 5:27�pm, Andrew Corl  wrote:
> I am interested in this diagram,� where can I find it?
>
> Andrew
>
> ________________________________
> From: chempro
> To: NavList
> Sent: Saturday, June 27, 2009 11:00:31 AM
> Subject: [NavList 8849] Re: Formulas to Compute LHA
>
> Why dont you learn to use a TIME DIAGRAM? It simplfies ALL of the
> concepts.It can universally used for all reduction methods from Ageton
> to NASR. Chempro-Dr Will
>
> On Jun 24, 8:07�am, Gary LaPook  wrote:
>
> > To see that you can use hour angle and LHA equally well, you just have
> > to look at H.O. 249. Look at the LHA columns at each edge of the page
> > and you will see that you get the same Hc for two values of LHA. Look at
> > LHA 10 and LHA 350 on this example page:
>
> >http://www.fer3.com/arc/img/106576.2007-page%20152.pdf
>
> > LHA 10 is the same as hour angle 10 west., LHA 350 is the same as hour
> > angle 10 east.
>
> > This relationship is not so evident if you just look at H.O 229 due to
> > the arrangement of the tables.
> > gl
>
> > Gary LaPook wrote:
> > > Remember, LHA is in the range of 0� to 360� and is always measured to
> > > the west from the assumed longitude (ALon). It is the angle measured
> > > westward from the meridian of the ALon to the meridian containing the
> > > body's grographic position (GP). There is no such thing as easterly LHA.
>
> > > In the past, various computations methods and tables (e.g H.O. 214) used
> > > "hour angle," (abbreviated "H.A." or "H" or "t"), which is the angle
> > > measured between the meridian of the ALon and the meridian containing
> > > the body's GP measured the shortest way, either west or east. Using this
> > > notation, hour angle ends up in the range of 0� to 180� only and is
> > > denoted "east" or "west." Because of the the way the trig formulas work,
> > > using either method computes the same Hc and the same "azimuth angle"
> > > ("Az" or "Z"). The only thing affected by choice of notation is the
> > > method used for the final determination of Zn,(azimuth used for plotting
> > > the LOP.)
>
> > > The original Bygrave used hour angle, not LHA, and the scales were
> > > marked from 0� to 90� and then back the other way, 90� to 180�. My
> > > implementation of the Bygrave eliminated the second set of markings on
> > > the scale, 90�-180�, to eliminate clutter so I added an extra step to
> > > bring hour angle into the range of 0� to 90� only and provided the
> > > necessary rules for the final computation of Zn. This is what is
> > > happening on the top of the form I provided. If LHA is less than 90�, H
> > > = LHA; if LHA is greater than 90� but less than 180�, H = 180� - LHA; if
> > > LHA is greater than 180� but less than 270� then H = LHA - 180�; and if
> > > LHA is greater than 270� but less than 360�, H = 360 �- LHA.
> > > Conceptually, this is the smallest angle measured from either the upper
> > > branch or from the lower branch of the observer's meridian to the
> > > meridian containing the body's GP.
>
> > > See the revised form at:
>
> > >http://www.fer3.com/arc/img/108719.revised%20form%206-18-09.pdf
>
> > > The formulas for calculating LHA are:
>
> > > If your AP is in west longitude: LHA = GHA - ALon. (If necessary, add
> > > 360 �to GHA prior to subtracting ALon.)
>
> > > If your AP is in east longitude: LHA = GHA + ALan. (if LHA then exceeds
> > > 360�, subtract 360� from the result.)
>
> > > Using the first formula for your first two examples, GHA (55� + 360�) -
> > > 77� = LHA = 338�.
>
> > > GHA 95� - 77� = LHA = 18�
>
> > > The third example you bring up makes no sense since GHA is never
> > > measured to the east, it is always measured west from Greenwich.
>
> > > gl
>
> > > Andrew Corl wrote:
>
> > >> All,
>
> > >> I need some help. �I am attempting to work the problem in Ocean
> > >> Navigator using the Lapook-Bygrave Slide Rule. �I am uncertain how to
> > >> compute the Local Hour Angle (LHA).
>
> > >> In the following cases I am assuming my longitude to be 77 degrees
> > >> west of Greenwich.
>
> > >> 1. The GHA of the Sun is 55 degrees west of Greenwich
> > >> 2. The GHA of the Sun is 95 degrees west of Greenwich
>
> > >> In the following cases I am assuming my longitude to be 120 degrees
> > >> west of Greenwich
>
> > >> 1. The GHA of the sun is 170 degrees east of Greenwich.
>
> > >> Also what is the formula if my position is east of Greenwich and the
> > >> sun has a GHA of more than 180 degrees?
>
> > >> Thanks
> > >> Andrew- Hide quoted text -
>
> > - Show quoted text -
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