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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Equinoxes: when Lon=0 (or 180) but not when Dec=0?
From: Lu Abel
Date: 2017 Aug 2, 01:07 +0000
From: Frank Reed <NoReply_FrankReed@fer3.com>
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?
From: Lu Abel
Date: 2017 Aug 2, 01:07 +0000
I'm a bit confused. One of my absolutely all-time favorite graphic diagrams is the annalemma, the figure-eight often found on globes. If it's on the 180th meridian, then it shows the location on earth where the sun is directly overhead at midnight GMT each day of the year.
However, the lines of the eight cross considerably north of the equator and very close to the 180th meridian. That says if we consider meridian crossing as defining the equinoxes, they should occur in April and August, not March and September!
The Wikipedia article on the annalemma clearly labels the equatorial crossings as defining the equinoxes, not the meridian crossings.
From: Frank Reed <NoReply_FrankReed@fer3.com>
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?
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.
"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.
Frank Reed
