Re: Direction of the Earth's axis tilt in the x-y ecliptic plane?

On 2006-03-19, canopus56 <canopus56@xxxxxxxxx> wrote:
Tomorrow is the spring equinox. This got me to thinking about
orientatoin of the Earth's axis with respect to its direction of travel
in its orbit around the Sun. Although the Earth's axis is tilted at 23
1/2 degrees in the ecliptic z-plane, what is the axis's orientation
with respect to the ecliptic x-y plane? Does it point:

a) in the direction of Earth's direction of revolution;
b) perpendicular to Earth's direction of revolution;
c) opposite Earth's direction of revolution; or,
d) some other angle?

I assume this changes over geologic time as the Earth wobble's around
it's center on a 28,000 year cycle.

What easily observed characteristics of the solar daily or yearly cycle
would indicate the direction of the tilt in the x-y ecliptic plane?

The north pole of the Earth points toward RA 0h Dec +90,
near Polaris. The north pole of the ecliptic points toward
RA 18h Dec +66.5 roughly. There's a planetary nebula,
NGC 6543 that's near to the spot. The celestial pole moves
counterclockwise in a circle around the ecliptic pole, taking
26,000 years for a complete turn. The poles of the ecliptic
move a little, owing to perturbations of the Earth orbit.

The ecliptic longitudes of the stars, measured from the
point in the sky where the Sun crosses the equator in March,
slowly change, which is how Hellenistic astronomers discovered
precession. They interpreted it as a slow rotation of the
celestial sphere on the axis of the ecliptic, since in their
planetary system the Earth was fixed and the rest of the cosmos
revolved around it in various ways. The daily motion of the
stars and planets was interpreted as a faster rotation around
the axis of the Earth. The other effect of precession is that
the sideral year is different in length from the tropical year.

--
The night is just the shadow of the Earth.
.

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