# NavList:

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

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Re: CP-300/U Star Finder
Date: 2015 Mar 14, 21:50 -0400

Hello Ed Popko

While waiting for the CP-300/U star finder to arrive, I carefully examined to sequential azimuthal equidistant overlays from the 2102D.

I aligned the zenith points for 35° and 45°.  Further the LHA Aries arrows were aligned.  This would, I believe, be the equivalent usage of the CP300 if it were to be used for 40° latitude.

The error term along the meridian is quite small.  I will not quantize the value until I get to the maths.  Remember, this is just an analog device.

Yet when I examine the error term at 90 degrees azimuth, right at the observers horizon, I find ~10° of azimuth error and ~10° of elevation error.  Its slightly more at the observers 120° of azimuth.  Of course, this is mirrored at observer's 240°.

This error reduces in both azimuth and elevation as we approach the observers zenith, wherein it goes to zero.  That is where we aligned the azimuthal overlays after all.

This may be thought of in a different way.  The stars on the base are plotted in polar fashion.  The azimuth is the stars SHA and the declination is the radial distance from the north (or south) pole.  When superimposing your celestial hemisphere on that base, the azimuthal equidistant overlay must be squeezed together towards the North Pole and spread out to the south, due to azimuth away from the meridian.  Yet the distance N/S is linearly proportional to declination.

I look forward to examining the CP300 and detailing the expected error.  Further, with the electronic 2102E, I expect to be able to perfectly quantize the error term

```On 2015-03-09 10:01, Russ Keller wrote:
> Are these limited to a particular span of years.  Or in other words are these still useful for sight planning?

Yes, they still work. The precession of the celestial equator causes a
constant movement of the SHA/dec coordinate system. However, this
amounts to only one degree (in great circle measure) every 72 years for
a star on the ecliptic, less elsewhere.

Even if the coordinate system maintains a fixed orientation, the space
motion of the stars with respect to our solar system gives them slowly
changing coordinates. That's called "proper motion". At 3.7 arc seconds
per year, Rigil Kentaurus has the greatest proper motion of the
navigational stars. But that's almost 1000 years per degree, so the
effect on a star finder is negligible compared to precession.

```

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