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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: Star Sha & Dec changes.
Date: 2013 Feb 1, 12:03 -0500

Just for giggles, I went ahead and did the 1849 Bowditch Perpetual Almanac extrapolation for Spica, so as to compare with modern sources and some of the precision data correspondents of the NavList.

Table VIII states the data as epoch 1830.

Spica
R.A.  13.16.15  (hr.min.sec)
annual variation 3.15 seconds, add for each year forward

Dec 10.16 (deg.min)
annual variation 18.9 seconds add for each year forward

Convert RA to decimal hours, we obtain 13.27083333 hr

Convert annual variation to decimal hours, we obtain 8.75e-4.

Multiply by 330 years, we obtain 0.28875

Subtract (since we are going backwards in time) we obtain 12.982083333 hrs

Convert to deg, multiply by 15, we obtain 194.73125

Subtract from 360, we obtain 165.26875

Therefore, the SHA for Jan 1500 is 165 deg 16 min 7.5 seconds.

As the declination is in degrees and minutes, I will jump to the solution of 8 deg 32 min 3 sec.

+++++++++++

As a comparison, we have Kermit's result of 165 deg 15 min SHA and 8 deg 31 min.  In both cases Bowditch's 1849 Perpetual Almanac is with 2' for a 330 year delta.

As a practical matter, when plotted on a 2102D star base, none of these solutions will be distinguished from another.  All will be plotted as the same dot.

On Jan 31, 2013 11:58 PM, "Peter Hakel" <pmh099@yahoo.com> wrote:

I also used Simbad; for Spica on January 1, 1500 I get:

SHA: 165d 14.9'
Dec: S 8d 30.4'

Peter Hakel

From: Antoine Couëtte <antoine.m.couette---fr>
To: pmh099---com
Sent: Thursday, January 31, 2013 8:47 PM
Subject: [NavList 22217] Re: Star Sha & Dec changes.

Re : [NavList] Re: Star Sha & Dec changes.
From: Sean C
Date: 31 Jan 2013 10:54
Byron,
As an example: For Spica in the year 1500, the SHA was ca. 165°15.0' and the Dec. was ca. S8°31.0'. Today's values are SHA: 158°31.4' Dec.: S11°13.8'. I used Stellarium (the program Frank recommended) to find these values.
Regards,
Sean C
******* ******* ******* *******
From an independent Software, using the latest IAU theories for Precession Nutation and from the SIMBAD Star catalog I can find :
Jan 01st, 1500 at 00:00 UT1 (with TT-UT=198.3 seconds of time : I know .... this one is quite totally superfluous ....)
SPICA (Apparent Equatorial Coordinates)
GHA = 274d11'0
SHA = 165d14'8
RA = 12h59m00.987s
DEC = S-8d30'25.880"
AP = 0.01306"
Paul, could you please crosscheck the results here-above, if you have time ? :-)
In view of the accuracy permitted by a Star-Finder, there are differences of some significance when compared with the Stellarium results : almost 3 degrees for SHA .... Any typo from your side, Sean ?
Best Regards to all,

Kermit
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