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



 
 
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  #1  
Old March 20th 06, 12:19 AM posted to sci.astro.amateur
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Default Direction of the Earth's axis tilt in the x-y ecliptic plane?

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?

- Canopus56

  #2  
Old March 20th 06, 01:36 AM posted to sci.astro.amateur
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Default Direction of the Earth's axis tilt in the x-y ecliptic plane?

First of all in the AMerican south it is the Equinox but up here in
Canada it is not. I think June 26th is our Equinox. You should expain
your post better.

  #3  
Old March 20th 06, 01:46 AM posted to sci.astro.amateur
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Default Direction of the Earth's axis tilt in the x-y ecliptic plane?

On 2006-03-19, canopus56 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.
  #4  
Old March 20th 06, 01:53 AM posted to sci.astro.amateur
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Default Direction of the Earth's axis tilt in the x-y ecliptic plane?

The Earth's rotational axis points towards the North Celestial Pole,
90°N (indepensent of Right Ascension, althought there might be a
conventional RA used when describing the pole that I'm unaware of).

The Earth's axis of revolution (around the Sun) points towards the
North Ecliptic Pole, at 18h 00m 00s +66° 33' 39" (epocj 2000).
So, "d".

This is in constant change, and the Earth's rotational axis traces out
a cone whose axis aligns along the line towards the NEP over the
course of about 26kyrs.

Some easily observed characteristics are the change of seasons, and
the cycle of the Sun appearing to rise and set at different places
along the east and west horizons, respectively, over the course of a
year.

??

On 19 Mar 2006 15:19:04 -0800, "canopus56"
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?

- Canopus56


=============
- Dale Gombert (SkySea at aol.com)
122.38W, 47.58N, W. Seattle, WA
http://flavorj.com/~skysea
  #5  
Old March 20th 06, 01:54 AM posted to sci.astro.amateur
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Default Direction of the Earth's axis tilt in the x-y ecliptic plane?

I believe you are refering to the summer solstice. - Canopus56

  #6  
Old March 20th 06, 02:15 AM posted to sci.astro.amateur
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Default Direction of the Earth's axis tilt in the x-y ecliptic plane?

SkySea wrote:
snip

Thanks to you and WIlliam H. Thanks for tips. So if I put the "North
Celestial Pole" (NCP) into a coordinate converter, the ecliptic
coordinates of the North Celestial Pole are at:

Ecliptic Long Eclipitc Lat
90 00 00.0 66 33 38.6

So, if at the spring equinox, the ecliptic longitude and latitude of
the Sun is:

Ecliptic Long Ecliptic Lat
00 00 00.0 00 00 00.0

The x-y axial orientation of the Earth's axis relative to the ecliptic
is
perpendicular to the direction of the Sun. (???)

But is the 90 degs ecliptic latitude in the direction of Earth's travel
in its orbit or tilted opposite the Earth's orbital direction of
travel?

- Canopus56

P.S. - At the summer solstice, when the Sun is at 90 degs ecliptic
longitude, the north pole points at the Sun at the 270 deg ecliptic
long winter solstice is points away.

  #7  
Old March 20th 06, 02:40 AM posted to sci.astro.amateur
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Default Direction of the Earth's axis tilt in the x-y ecliptic plane?

canopus56 wrote:
But is the 90 degs ecliptic latitude in the direction of Earth's travel
in its orbit or tilted opposite the Earth's orbital direction of
travel?


After playing around with the JPL Planetica
http://space.jpl.nasa.gov/ (Earth view from above, counterclockwise
orbit travel), I guess I've convinced myself that the x-y plane axial
orientation of the Earth's axis at the equinoxes is in the direction of
the Earth's travel in its orbit. If someone could confirm, it would
appreciated. - Canopus56

  #8  
Old March 20th 06, 03:05 AM posted to sci.astro.amateur
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Default Direction of the Earth's axis tilt in the x-y ecliptic plane?

On 2006-03-20, canopus56 wrote:
SkySea wrote:
snip

Thanks to you and WIlliam H. Thanks for tips. So if I put the "North
Celestial Pole" (NCP) into a coordinate converter, the ecliptic
coordinates of the North Celestial Pole are at:

Ecliptic Long Eclipitc Lat
90 00 00.0 66 33 38.6

So, if at the spring equinox, the ecliptic longitude and latitude of
the Sun is:

Ecliptic Long Ecliptic Lat
00 00 00.0 00 00 00.0

The x-y axial orientation of the Earth's axis relative to the ecliptic
is perpendicular to the direction of the Sun. (???)


At the March equinox, yep, that sounds true to me,
since the Earth-Sun line then lies on both the celestial equator
and (as always) also on the ecliptic.

And, if I'm thinking rightly, at March equinox the south pole
of the earth's rotation axis is closer than the north pole is,
to Earth's direction of motion about the sun.

Why? In March, we see the Sun near right-ascension 0 hours in
ecliptic coordinates; and the Sun sees Earth near 12 hours RA.
Earth's orbit carries us eastward, toward higher right-ascension.
If the orbit were circular (almost true), Earth's motion vector at
March equinox would be 90 degrees (6 hours) further east along
the ecliptic, at 18 hours RA, -23.5 declination, in Sagittarius:
clearly closer to the south pole of Earth's axis.

Of course at other times of year the answer would be different!


The Earth's motion vector spends about half the year in the
northern hemisphere, about half in the southern, but the durations
aren't quite equal because of its elliptical orbit. So, in which
hemisphere does it spend more time?

Stuart in (clear!) Champaign, IL
  #9  
Old March 20th 06, 03:18 AM posted to sci.astro.amateur
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Default Direction of the Earth's axis tilt in the x-y ecliptic plane?

granite stone wrote:
First of all in the AMerican south it is the Equinox but up here in
Canada it is not. I think June 26th is our Equinox. You should expain
your post better.



Astronomy is country independent.

Equinox - March 20, 2006 18:26 UTC
Solstice - June 21, 2006 12:26 UTC
Equinox - Sept 23, 2006 04:03 UTC
Solstice - Dec 22, 2006 00:22 UTC

  #10  
Old March 20th 06, 03:25 AM posted to sci.astro.amateur
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Default Direction of the Earth's axis tilt in the x-y ecliptic plane?

Stuart Levy wrote:
snip
Why? In March, we see the Sun near right-ascension 0 hours in
ecliptic coordinates; and the Sun sees Earth near 12 hours RA.
Earth's orbit carries us eastward, toward higher right-ascension.
If the orbit were circular (almost true), Earth's motion vector at
March equinox would be 90 degrees (6 hours) further east along
the ecliptic, at 18 hours RA, -23.5 declination, in Sagittarius:
clearly closer to the south pole of Earth's axis.


Yeah, that's the way to think about it in RA and dec.

If the Sun is at RA 0h Dec +0, and,
if the NCP is at RA 18h Dec +66.5 (perpendicular or the Earth-Sun
line), and,
if the Earth moves towards increasing RA,

then, at the equinoxes, the NCP is tilted towards the direction of
Earth's travel in its orbit around the Sun.

- Canopus56

P.S. - As you note, this x-y plane orientation relative to the
direction of travel changes throughout the year.

 




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