Brian Tung wrote:
SkySea wrote:

2) The sun's RA at the solstice should be very close to 6 hours, but
since the RA is defined by the vernal equinox rather than the
solstice, the solstice may not occur at exactly 6 hours.

True for times off a chart's given epoch. By definition though, 0h is
the intersection of the equator and the ecliptic, and the
intersections of the ecliptic with the 6, 18, and 12 hour circles mark
the equioxes and solstices.


No, I think the original poster was right: The vernal equinox is at 0h,
but the autumnal equinox and the solstices are all based on properties
of the ecliptic and the celestial equator. So the autumnal equinox is
when the ecliptic crosses the equator going southward, and the solstices
are the two points where the Sun's declination is at one of its two
extremes.

I have no explanation for the bizarre behavior of SNP, though.

Sorry I didn't see this post before I finished replying to the original
one.

The vernal equinox point on the celestial sphere is indeed exactly at
RA 0h, Dec 0 deg in equatorial coordinates; latitude 0 deg, longitude
0 deg in ecliptic coordinates.

The center of the sun will NOT go through this point in general, because
its geocentric latitude is zero only at the instant when the moon is at
one of the nodes of its orbit. If the sun's latitude happens to be
positive around March 20, it will cross the celestial equator before it
hits longitude 0.

If the moon's "argument of latitude" is between 90 and 270 deg, it is
moving southward in ecliptic coordinates, the earth is moving
northward, and the sun's apparent latitude is decreasing. If this
situation holds around June 21, the sun's declination will max out
before the solstice. Remember, the solstice is defined by ecliptic
longitude, not by RA, not by maximum Dec.

-- Bill Owen