A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Frank Reed
Date: 2017 Aug 1, 14:53 -0700
Tony Oz, you wrote:
"Why the definition of Equinoxes demands the Sun's longitude to be equal to either 0° or 180°?"
Well, probably nothing more complicated than deep history. The season definitions are really "astrological" in origin, from a modern perspective. The Sun enters the various "signs" at exact 30° increments of ecliptic longitude. Four out of those dozen sign-entry dates yield the equinoxes and solstices. Another way to look at it: a declination-based definition of the equinoxes would be relatively easy to implement, but this would be problematic for the solstices. The standard definition works equally well for all of these events.