# NavList:

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

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Re: Zodiacal coordinates
From: John Huth
Date: 2012 Sep 15, 08:20 -0400
Hi, All -

A few updates I've found, and yet another question.    First up, I've used the RA's in the table to help identify planets, and usually just convert it immediately into GHA.   I sometimes do this with a calculator, but there's a nice online calculator here:

Second, I asked a local astronomer to help me understand the table better, Owen Gingerich, here's his answer to me:

"John, I'm a little surprised to see a modern calculation given in the
Pre-Flamsteed convention used in all the ephemerides up into the 18th
century.

All of Ptolemy's planetary calculations and tables are given in ecliptic
coordinates because the planets go around near the ecliptic.  The
coordinate system begins where the ecliptic intersects with the equator.
The ecliptic is divided into 12 30-degree intervals, named according to
the zodiacal constellations beginning with Aries.  These signs remain
constant even though the stars in the constellations precess out of their
named signs.  The latitudes (+ or -) are measured along great circles
going through the ecliptic poles.  The latitudes of stars remain constant,
while precession takes place in longitude."

Finally, a bit of a puzzle for me.   If you take this 12 30-degree interval seriously, I come up with a few head scratchers.   It may just have to do with the projections onto the ecliptic.   If I go back to 1960, the time of the vernal equinox is 14:00 on March 20th, roughly.   But, the boundary between Virgo and Libra is typically listed as Sept. 22nd, which doesn't align with the 12*30 degree projections by about 4 degrees.  I'm not sure why the 'traditional' boundaries of the tropical zodiac don't seem to correspond to this.   Again, I might be missing the projection.   There are more days between the vernal equinox and autumnal equinox than between the autumnal and vernal, which I assume has to do with the eccentricity of the earth's orbit.   I guess I'm trying to think like Ptolemy and can't quite make the shift.

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