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    Re: Formulas to Compute LHA
    From: Will Ross
    Date: 2009 Jun 28, 15:13 +0000
    Correct, except its the south pole your looking at.         Chempro...Will Ross, PhD

    ----- Original Message -----

    From: glapook---.net

    To: NavList

    Sent: Sun, 28 Jun 2009 05:56:13 +0000 (UTC)

    Subject: [NavList 8854] Re: Formulas to Compute LHA





    The "time diagram" is found in many navigational texts. It consists of

    a circle with the north posle as the center. You draw lines across the

    circle representing the Greenwich meridian, your meridian, and a line

    representing the GHA of the body (or of Aries if using H.O 249 vol.1).

    This diagram makes clear the relationship between all of these lines.



    gl



    On Jun 27, 6:38 pm, Greg Rudzinski wrote:

    > 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 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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