# NavList:

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

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Re: Please help me with the math Re: learning sight reduction
From: Bill B
Date: 2006 May 5, 01:19 -0500

> Would you please be able to give me a sample problem and work through the
> answer so that I can repeat the steps and know I'm getting to the correct
> Guy

Not sure about the scope of the area(s) in question, so let's start with the
trickiest of the three initial inputs, LHA.  (I am not certified to explain
the universe and give three examples ;-)

For the sun:

GHA minus Assumed Position (AP) longitude = LHA

West longitude is positive, east longitude is negative (so minus, minus a
minus, is a positive in the case of east longitude)

From your initial questions, it seems you have done some homework and have a
grasp of the concepts, but dlo vs LHA was your area of concern.  Let's work
these examples as even hours, for this exercise in even degrees, and ignore
interpolating for minutes past the hour, v & d corrections.

Example 1:

GHA = 110d
AP lon = 90d W
110 - 90 = LHA 20

Example 2:

GHA = 70d
AP lon = 90d W
70- 90 = LHA -20

For tabular methods LHA (local hour angle) MUST be a number from 0 to 359d
59.XXX'.  Not 361 or -1.  (Calculators can handle that, tables don't).
Therefore we will add -20 to 360 (one lap around the circle) to get it in
range.

-20 + 360 = 360 - 20 = 340 LHA

Example 3:

GHA = 350
AP lon = 100 E  (Note east, so LHA = GHA - (-100)

350 - (-100) = 350 + 100 = 450

Again, for tabular methods LHA (local hour angle) MUST be a number from 0 to
359d 59.XXX'.  Not 361 or -1.  Therefore we will *subtract* one lap around
the circle (360) from 450.

450 - 360 = 90 LHA

======================================

I chose 229 to start, and like Susan Howell's book as a guide.  She has
included a great many examples, works you through them, and then gives you
test questions (answers in the back). Step by step.  I am not married to
229, and perhaps other members can suggest texts that will both help you
work through the the basics, as well as providing tests questions on
isolated areas and the whole of site reduction.

No being clear about what portion of the site-reduction process you wish
test questions and answer for, I hope the above addressed your immediate
needs.

Bill

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