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    Re: Formulas to Compute LHA
    From: Greg Rudzinski
    Date: 2009 Jun 28, 16:38 -0700

    Ken,
    
    I have never used one of the W&P slide rules but it did look like
    something a beginner might find useful when I read the internet
    description. Not something NavListers would need.
    
    Greg
    
    On Jun 28, 4:16�pm, Ken Gebhart  wrote:
    > Greg,
    >
    > I don't think this is it. �The W&P "slide rule" is not a slide rule �
    > at all. �It has no logarithmic scales on it. �You can only add or �
    > subtract hour angles on it. Not very useful. �I post this so that no �
    > one will waste their money buying one on eBay.
    >
    > Ken Gebhart
    > On Jun 27, 2009, at 8:38 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 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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