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Herbert wrote:

Conceptually, equinox implies that the declination of the sun equals
zero. Hipparchus supposed this to happen in the equinoctial point, i.e.
at zero longitude. When it was realized after Newton that the apparent
Sun does not necessarily have to move through the equinoctial point each
year, one had to make a decision whether to choose zero longitude or
zero declination for the definition of equinox. Longitude was more
practical.

Thanks for the explanation and drawing.  I had been wrestling with the
problem.

For a rough estimate I supposed declination 0 and via the almanac came up
with 18:24

Then I worked it as equal GHA's.  I averaged the 24 hour velocities of the
sun and Aries, and determined the difference to be 0d 02' 17", and the
difference in GHA's to be 0d 01' at 18:00:00.  Using that input GHA's
matched at 18:26:17.  I also figured worst-case rounding error for GHA and
velocity and came up with +/- 00:02:40.

Thanks to your explanation, I now know I was solving the wrong problem, but
at least was in the ballpark ;_)

Bill

   

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