
NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Lu Abel
Date: 2017 Aug 2, 01:07 +0000
From: Frank Reed <NoReply_FrankReed@navlist.net>
To: luabel@ymail.com
Sent: Tuesday, August 1, 2017 3:20 PM
Subject: [NavList] Re: Equinoxes: when Lon=0 (or 180) but not when Dec=0?
"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.