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

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Re: Weems and Plath Celestial Slide Rule
From: David Pike
Date: 2020 Apr 12, 03:21 -0700

Weems and Plath Celestial Slide Rule – epikdadIV's Final Analysis

The white scale has 360 equal divisions increasing anticlockwise.  It is used to add SHA Star to GHA or LHA Aries to give GHA or LHA star to be read from the white scale

The blue scale has 360 equal divisions increasing anticlockwise.  It is used in conjuction with the amber disc to to give LHA Sun (or planet) to be read from the blue disc, or in conjuction with the amber disc and the white disc to give LHA star on the white scale

The outer amber scale has 180 equally spaced divisions increasing either side of zero, one clockwise, and one anticlockwise.  The writing around zero and at 90 east and west labels the anticlockwise increasing scale ‘West’ and the clockwise increasing scale ‘East’.  This scale is used to apply longitude to GHA to obtain LHA or in conjunction with the inner amber scale to convert LHA to meridian angle or vice versa.

The inner amber scale has 360 equal divisions increasing anticlockwise.  It can be used as a fast way of converting LHA to meridian angle.  This function is not necessary to complete modern celestial calculations from tables.  It can be useful in spherical geometry to obtain the smaller angle between the observer's and star or planets meridians. It is the angle P in the PZX triangle.

To obtain Meridian Angle from LHA: Move the cursor over LHA on inner amber scale.  Read meridian angle from the outer amber scale.

To obtain LHA Sun (or Planet):  Align GHA Sun (or Planet) on the blue scale with zero on the outer amber longitude scale.  Move cursor to the observer’s longitude on the outer amber scale and read off LHA Sun (or Planet) from the blue scale.

To obtain LHA Star:  Using the line on the cursor, align zero on the blue scale with SHA star on the white scale and place thumb over both discs to keep them aligned during the remaining operations. Align the cursor with GHA Aries on the blue scale.  Move the amber disc until its zero is aligned with the line on the cursor and GHA Aries on the  blue scale. keeping all three discs together by intelligent use of your thumb, align the cursor with lonitude on the outer amber scale.  Read LHA star from where the line on the cursor crosses the white scale.

Apologies for any errors and typos; it's a long time since I was at Nav-School. DaveP

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