# NavList:

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

**Re: Formulas to Compute LHA**

**From:**Andrew Corl

**Date:**2009 Jun 27, 17:27 -0700

I am interested in this diagram, where can I find it?

Andrew

**From:**chempro <snav-coxn---.net>

**To:**NavList <NavList@fer3.com>

**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...---.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 -

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