On 2009.03.20, at 11:00 , HR-CALNDR-L wrote:
The vernal equinox is defined only by the heliocentric ecliptic longitude of
earth being zero. The ecliptic latitude, which is not always exacly zero, is
completely ignored. Earth's nutation is also ignored, so the moment when the
sun is exactly in the equatorial plane is usually not the very same moment as
that of the vernal equinox.
Irv replies: My comments concerned rather crude observation of the equinox.
Definitions based on celestial mechanics can be much more precise, but then one runs into controversies as to exact definitions of the moment.
Isn't the "heliocentric ecliptic longitude of earth being zero" undefined in isolation?
There is no heliocentric longitude in space which points to a fixed zero, because of precession of the equinoxes.
The only "nail to hang one's hat on" is the solar declination crossing zero, and at that moment one can define the ecliptic solar longitude as 0° at the northward equinox, corresponding to a heliocentric ecliptic longitude of Earth = 180°.
If the equinox is the moment of solar declination zero, then how can that be distinguished from a zero ecliptic latitude?
With respect to nutation in this context, it would be included in typical solar longitude calculations, but generally not in the solar declination calculation.
-- Irv Bromberg, Toronto, Canada
