Welcome to the NavList Message Boards.


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

Compose Your Message

Add Images & Files
    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?

    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 -

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

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    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 Settings

    Posting Code:

    Custom Index

    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