The Equinox

The approaching Equinox represents the product of two separate motions
- daily rotation which causes the day and night cycle and the slow
orbital turning of the Earth which brings the rotational axis in line
with the solar radiation/orbital shadow boundary and causes variations
in the daylight/darkness cycle at all points North and South of the
Equator -

http://www.youtube.com/watch?v=nwTrYVBcx9s]

The specific way the Earth orbits the Sun should be fascinating for
dynamicists and especially as it represents visual affirmation of
Kepler's view of orbital geometry and motion -

http://hubblesite.org/newscenter/arc...999/11/video/b

The original explanation by Copernicus for seasonal variations has
stood for many centuries,I suggest people treat the matter with the
respect it deserves and recognise just how important the modification
is.If they cannot then just leave it be.

On Sep 19, 6:12*pm, Sam Wormley wrote:
oriel36 wrote:
The approaching Equinox represents the product of two separate motions
- daily rotation which causes the day and night cycle and the slow
orbital turning of the Earth which brings the *rotational axis in line
with the solar radiation/orbital shadow boundary and causes variations
in the daylight/darkness cycle at all points North and South of the
Equator -

* *Many definition will say the equinox occurs when the sun crosses the
* *celestial equator, but the correct definition is when the apparent
* *longitude of the sun is 0° or 180°.


No offence Sam,about 500 years on from Copernicus,the least we can do
is explain the Equinox event in terms of the motions of the
Earth.Trust me,this is a major modification to the explanation for
seasonal variations in daylight/darkness with the added insight that
the daily rotational and orbital motions combine to generate the
variations in the natural noon cycle.



* *As Paul Schlyter pointed out on the 19th of last March, the difference
* *between the two is just a few tens of seconds at most, which most would
* *not notice.

* -Sam

This is the 21st century Sam and it is about time somebody actually
noticed that the Earth orbits the Sun in a specific way.The orbital
motion of the planet is slowly turning to the central Sun and is about
to split the rotational poles thereby generating global symmetry in
terms of daylight/darkness.For astronomers,the Equinoxes should far
more interesting than the hemispherical seasonal solstices but as yet
it requires astronomers to appreciate this.

Go ahead and celebrate your geocentric view of the Sun crossing the Ra/
Dec celestial Equator and be proud of it,I still have to find people
who have the aptitude and the dignity to approach the modification for
seasonal explanations using the motions of the Earth arising from the
original explanation of Copernicus -

"To this circle, which goes through the middle of the signs, and to
its plane, the equator and the earth's axis must be understood to have
a variable inclination. For if they stayed at a constant angle, and
were affected exclusively by the motion of the centre, no inequality
of days and nights would be observed. On the contrary,it day or the
day of equal daylight and darkness, or summer or winter, or whatever
the character of the season, it would remain identical and
unchanged." Copernicus

It is when the unequal natural noon cycles require an explanation that
the modification becomes apparent,specifically isolating orbital
motion and determining that a location slowly changes through 360
degrees with respect to the Sun in the absence of daily rotation.Your
colleagues are so focused on referencing daily rotational and annual
orbital motion off the astrological Ra/Dec framework that they cannot
see the subtle motion within a motion as a location slowly turns
through 360 degrees with respect to the central Sun or maybe it is
just that they do not want to see this 100% geometric and
observational certainty.

I am not surprised that you express the Equinox in geocentric terms
but it would be much better to see an attempt at explaining it in
terms of the motion of the Earth.

On Sep 19, 9:39*am, oriel36 wrote:
The approaching Equinox represents the product of two separate motions
- daily rotation which causes the day and night cycle and the slow
orbital turning of the Earth which brings the *rotational axis in line
with the solar radiation/orbital shadow boundary and causes variations
in the daylight/darkness cycle at all points North and South of the
Equator -

Yes, this is exactly right. Why do you think today's astronomers are
saying something that contradicts this?

The Earth rotates on its axis. It also moves around the Sun, which
changes the direction from the Earth to the Sun.

So, as a direct consequence of that (and these are things you seem to
dispute), the combination of those two motions gives the 24 hour
average length of the day, and departures of the Earth's motion around
the Sun from a perfect circle in the Earth's equatorial plane -caused
both by the tilt of the Earth's axis to the ecliptic, and the Earth's
orbit being an ellipse - lead to the Equation of Time.

The relation between "the solar radiation/orbital shadow boundary" and
the direction of the Earth's axis changes because the direction of the
Earth's axis, tilted with respect to the Ecliptic, does not change,
but the direction from the Earth to the Sun does change. This is what
causes the seasons.

We know this and do not deny this.

John Savard

On Sep 19, 10:12*am, Sam Wormley wrote:

* *Many definition will say the equinox occurs when the sun crosses the
* *celestial equator, but the correct definition is when the apparent
* *longitude of the sun is 0° or 180°.

The coordinate system in which the longitude of the Sun is zero at the
time of the Vernal Equinox, and 180 degrees at the time of the
Autumnal Equinox is *derived* from when the Sun crosses the celestial
equator.

Otherwise, precession means that there is one equinox in 1950.0
coordinates, and a different equinox in 2000.0 coordinates, and yet
another one in 2050.0 coordinates. Is that how you *really* want to
define the equinoxes?

You can buy star atlases in different coordinate systems; I did not
recall any IAU announcement of which coordinate system is currently
the "official" coordinate system for determining the equinox, but
maybe that's just me not paying attention.

John Savard

In article ,
Quadibloc wrote:
On Sep 19, 10:12=A0am, Sam Wormley wrote:

=A0 =A0Many definition will say the equinox occurs when the sun crosses t=
he
=A0 =A0celestial equator, but the correct definition is when the apparent
=A0 =A0longitude of the sun is 0=B0 or 180=B0.

The coordinate system in which the longitude of the Sun is zero at the
time of the Vernal Equinox, and 180 degrees at the time of the
Autumnal Equinox is *derived* from when the Sun crosses the celestial
equator.

This definition is in mean orbital elements though, not osculating
orbital elements. Because of that, the center of the solar disk
doesn't cross the RA=0,Decl=0 point exactly around the March
equinox, but passes slightly beside it. Only a fraction of an arcsecond
beside it, but there's still a discrepancy. Therefore the "apparent
longitude is zero" and the "apparent declination is zero" definitions
of the moment of the March equinox will yield slightly different results.

Otherwise, precession means that there is one equinox in 1950.0
coordinates, and a different equinox in 2000.0 coordinates, and yet
another one in 2050.0 coordinates. Is that how you *really* want to
define the equinoxes?

You can buy star atlases in different coordinate systems; I did not
recall any IAU announcement of which coordinate system is currently
the "official" coordinate system for determining the equinox, but
maybe that's just me not paying attention.

John Savard


--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stjarnhimlen dot se
WWW: http://stjarnhimlen.se/

On Sat, 20 Sep 2008 09:15:12 GMT, (Paul Schlyter) wrote,
in part:

This definition is in mean orbital elements though, not osculating
orbital elements.

Yes, that could work, as long as it is the celestial longitude for the
coordinates of date. But a definition in terms of celestial longitude is
still a "conventional" definition, whereas the one in terms of
declination is the fundamental definition - thus, for Oriel's purposes
in examining or re-thinking the root causes of celestial phenomena, he
is using the right definition, even if it isn't the current official one
by means of which our almanacs are written.

Of course a lot to criticize in his postings - but when off-base
criticisms are applied to that sort of thing, the confusion quickly
becomes hopeless.

John Savard
http://www.quadibloc.com/index.html