Welcome to the NavList Message Boards.

NavList:

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

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Re: Formulas to Compute LHA
    From: Gary LaPook
    Date: 2009 Jun 27, 22:56 -0700

    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 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 -
    >
    >
    --~--~---------~--~----~------------~-------~--~----~
    Navigation List archive: www.fer3.com/arc
    To post, email NavList@fer3.com
    To unsubscribe, email NavList-unsubscribe@fer3.com
    -~----------~----~----~----~------~----~------~--~---
    
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site