NavList:

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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 <glap...@pacbell.net> 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 0º to 360º 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 0º to 180º 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 0º to 90º and then back the other way, 90º to 180º. My
> > implementation of the Bygrave eliminated the second set of markings on
> > the scale, 90º-180º, to eliminate clutter so I added an extra step to
> > bring hour angle into the range of 0º to 90º 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 90º, H
> > = LHA; if LHA is greater than 90º but less than 180º, H = 180º - LHA; if
> > LHA is greater than 180º but less than 270º then H = LHA - 180º; and if
> > LHA is greater than 270º but less than 360º, H = 360 º- 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 ºto GHA prior to

subtracting ALon.)
>
> > If your AP is in east longitude: LHA = GHA + ALan. (if LHA then exceeds
> > 360º, subtract 360º from the result.)
>
> > Using the first formula for your first two examples, GHA (55º + 360º) -
> > 77º = LHA = 338º.
>
> > GHA 95º - 77º = LHA = 18º
>
> > 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 -