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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 |
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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. |
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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. |
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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 |
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Direction of the Earth's axis tilt in the x-y ecliptic plane?
I believe you are refering to the summer solstice. - Canopus56
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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. |
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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 |
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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 |
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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
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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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