# NavList:

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

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