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Paul Wally
2011-Aug-29, 09:31 AM
What is the best coordinate system to use for a 3-D stellar map. I'm interested in compiling a stellar map of the solar systems discovered by Kepler space telescope and it would be great to actually have an idea of where these stars are located in relation to the sun and in relation to the galaxy.

astromark
2011-Aug-29, 09:58 AM
" Through Orion to Sirius and left and down a tad.. about 6.5 ly.,on, yes that's it Sol.. and there, the small third planet."

No that will not do will it ? So we draw a grid.. and name each quadrant. A1 or Alpha for this area..

Each area only ten ly across. The whole Galaxy could be mapped fine..

Trouble is we can not yet see it all.. there will be gaps of unknowns.

The greater the distance from us, the less we know of it.. Some of it is obscured from us.

That infra-red James Web scope might have shed some light... :razz:

Jeff Root
2011-Aug-29, 09:59 AM
You'll have to get the data from somewhere. I expect
the easiest thing to do is to use the same coordinate
system used by your data source.

-- Jeff, in Minneapolis

Robert Tulip
2011-Aug-29, 10:33 AM
Check out http://www.atlasoftheuniverse.com/

Paul Wally
2011-Aug-29, 11:14 AM
You'll have to get the data from somewhere. I expect
the easiest thing to do is to use the same coordinate
system used by your data source.

-- Jeff, in Minneapolis

On the Kepler website they give coordinates [right ascension], [declination], J2000, and of course they give distance in parsecs. I looked up the meaning of these things in Wikipedia, so these are celestial equatorial coordinates, right? J2000 they say is Julian 2000 epoch or something to do with precession. Now this is a nice system to get the positions in the sky and also the relative positions. Now it's just a matter of rotating this whole thing in relation to the galaxy.

I'm not clear on how this coordinate system translates to galactic coordinates. Doesn't the celestial equatorial plane make some funny angle relative to galactic plane and are we on the galactic plane? I'm just trying to orientate myself spatially within the galaxy.

Paul Wally
2011-Aug-29, 11:31 AM
Check out http://www.atlasoftheuniverse.com/

Thanks for that link. Now in those pictures, especially the 12.5ly radius one, what is the inclination of the ecliptic plane and also the orientation of the equatorial plane relative to that plane?

Robert Tulip
2011-Aug-29, 12:58 PM
Thanks for that link. Now in those pictures, especially the 12.5ly radius one, what is the inclination of the ecliptic plane and also the orientation of the equatorial plane relative to that plane?
From http://www.bautforum.com/showthread.php/80827-Solar-System-Angle-to-Galaxy?p=1357025#post1357025


The ecliptic is inclined at around sixty degrees to the galactic plane.
Here (http://www.ghutchison.pwp.blueyonder.co.uk/orbits/galaxy.jpg) is a picture of the solar system, looking past the sun towards the galactic centre. Galactic plane horizontal. Orbits of the planets in blue. Movement of the sun pretty much leftwards, though rising a little.
Current coordinates of the galactic north pole in ecliptic coordinates are: latitude 29.81, longitude 180.02.

Grant Hutchison

Have a look at that thread. As I understand it, it shows the inclination of the ecliptic plane is 60 degrees. Given the 23 degree difference between the ecliptic plane and the celestial equator, based on the tilt of the earth, I assume that would mean the equatorial plane is tilted 37 degrees from the galactic plane, but I might be wrong.

From http://www.atlasoftheuniverse.com/sattelit.html you can work out our solar system orientation by seeing that the Large Magellanic Cloud is located at the South Ecliptic Pole.

Correction: http://www.atlasoftheuniverse.com/250lys.html shows Vega, which was the pole star about 12,000 years ago. We are now at the opposite end of the precession cycle, so the earth's orbit would now be in a very different alignment to the galactic plane. Polaris is outside this map, at 434 LY, but the former pole star alpha draconis (Thuban) and beta ursa minor are in it. http://en.wikipedia.org/wiki/Galactic_coordinate_system states the Galactic North Pole is in Coma Berenice (http://en.wikipedia.org/wiki/File:Coma_Berenices_IAU.svg), at 27.4 north.

neilzero
2011-Aug-29, 10:37 PM
I don't think anyone has devised an easy to use plan = co-ordinate system. Four dimensions is complicated, and time, the 4th dimension is important as every thing is moving with respect to everything else with very rare exceptions, plus the speed of light causes us to observe long after the light was emitted. Intergalactic Navigation will be one of the more difficult subjects, if we ever learn to travel between galaxies. Am I thinking correctly that 8 "quadrants" is more sensible than 4 quadrants? Neil

Hornblower
2011-Aug-30, 02:22 AM
From http://www.bautforum.com/showthread.php/80827-Solar-System-Angle-to-Galaxy?p=1357025#post1357025



Have a look at that thread. As I understand it, it shows the inclination of the ecliptic plane is 60 degrees. Given the 23 degree difference between the ecliptic plane and the celestial equator, based on the tilt of the earth, I assume that would mean the equatorial plane is tilted 37 degrees from the galactic plane, but I might be wrong.

From http://www.atlasoftheuniverse.com/sattelit.html you can work out our solar system orientation by seeing that the Large Magellanic Cloud is located at the South Ecliptic Pole.

Correction: http://www.atlasoftheuniverse.com/250lys.html shows Vega, which was the pole star about 12,000 years ago. We are now at the opposite end of the precession cycle, so the earth's orbit would now be in a very different alignment to the galactic plane. Polaris is outside this map, at 434 LY, but the former pole star alpha draconis (Thuban) and beta ursa minor are in it. http://en.wikipedia.org/wiki/Galactic_coordinate_system states the Galactic North Pole is in Coma Berenice (http://en.wikipedia.org/wiki/File:Coma_Berenices_IAU.svg), at 27.4 north.

I think you are mistaken on some of these inclination details. On a time scale of tens of thousands of years the orientation of Earth's orbit with respect to the galactic plane is nearly fixed, wandering only slightly from gravitational perturbations, mainly from Jupiter. A glance at my charts shows roughly 60 degrees of inclination.

At the present time the celestial equator has nearly the same inclination to the galactic plane, because it is nearly parallel to the ecliptic around the solstices, which is where the ecliptic crosses the galactic plane now. If we go back some 6,500 years we find the September equinox point in Sagittarius, and the celestial equator inclined less than 40 degrees to the galactic plane. At a similar time in the future the March equinox point will be precessed into Sagittarius, and the celestial equators galactic inclination will be over 80 degrees.

eburacum45
2011-Aug-31, 03:15 PM
I recommend Winchell Chung's pages on this subject
http://www.projectrho.com/starmap.html
these are very comprehensive.

I prefer galactic x-y-z coordinates myself, but your opinion might vary.

eburacum45
2011-Aug-31, 03:22 PM
The Earth's polar axis varies with respect to the ecliptic, due to precession; but the angle of the ecliptic to the galactic plane remains constant, at least on any scale we can measure. (as far as I know).

Hornblower
2011-Aug-31, 04:29 PM
In general, I would say that the best coordinate system for a particular task is one which makes calculations as simple as possible, provided there are no significant adverse side effects.

Map projections are cases in point. There is no one projection that is "the best". A cartographer needs to know the characteristics of all of them to make good choices for particular locations and tasks at various scales. A choice that gives minimal visual distortion for a tract the size of Africa could have ghastly distortion toward the edges if we try to show the entire surface of the Earth. The Mercator projection is useful for navigation because a constant bearing relative to true north projects as a straight line, but it gives hideously distorted views of Alaska, Canada, Scandinavia and Russia, not to mention Antarctica.

Robert Tulip
2011-Sep-07, 12:02 PM
I think you are mistaken on some of these inclination details. Sorry, where I said "the earth's orbit would now be in a very different alignment" I meant the earth's axis. Thanks for pointing out the mistake.
At the present time the celestial equator has nearly the same inclination to the galactic plane, because it is nearly parallel to the ecliptic around the solstices.No, that is not right. The celestial equator maintains a constant angle to the ecliptic equal to the tilt of the earth's axis.

chornedsnorkack
2011-Sep-07, 02:15 PM
Does the equatorial plane of galactic coordinates go parallel to the plane of the disc and therefore pass at some distance from the centre, or does it connect Sun and Milky Way centre, thus intersecting the disc plane at a small angle?

Hornblower
2011-Sep-07, 04:30 PM
snip...
No, that is not right. The celestial equator maintains a constant angle to the ecliptic equal to the tilt of the earth's axis.

My bold. That is correct, and I did not assert otherwise. What I said is that where they cross the galactic equator, some 90 degrees of ecliptic longitude away from the equinox points, the celestial equator and the ecliptic are nearly parallel for a short stretch, and thus cross the galactic equator at angles near 60 degrees and differing from each other only slightly. A glance at my Norton's Star Atlas shows this clearly. One quarter of a Great Year before or after the present, the equinox points will be near the galactic equator, and the inclination of the other two great circles with the galactic equator will differ by the full 23+ degrees.

Robert Tulip
2011-Sep-09, 12:43 PM
Still struggling with this one. Where Hornblower said "the celestial equator has nearly the same inclination to the galactic plane" I read it as meaning that the equator has the same inclination as the ecliptic. But that can never be the case. The earth's axis is always about 23 degrees tilted from the ecliptic. Looking at the North Celestial Pole (http://upload.wikimedia.org/wikipedia/commons/1/16/Precession_N.gif), the ecliptic pole, orthogonal to the ecliptic, is constant in the middle of Draco while the celestial pole, orthogonal to the equator, rotates around the ecliptic pole. The celestial pole wanders from 23 degrees north to 23 degrees south of the ecliptic plane, plus variation of obliquity, but this is a three dimensional movement, so at the half way point it is 23 degrees east or west of the ecliptic pole.

Going back to Paul Wally's question (http://www.bautforum.com/showthread.php/120190-What-is-the-best-coordinate-system?p=1929113#post1929113), the map of the Pole shows that 14000 years ago the celestial pole pointed to Vega, then 3000 years ago it pointed to Alpha Draconis, and now it is in Ursa Minor. This galactic map (http://www.atlasoftheuniverse.com/250lys.html) gives the appearance that the pole has been drifting north, but the map may be misleading. Our coordinate system is based on the earth's axis, so it is hard to tell, for me at least, at what point the celestial pole reached its northernmost tilt with respect to the galaxy. Declination tables of stars with respect to the galactic plane would answer this question.

Looking further at this question, the North Galactic Pole (http://en.wikipedia.org/wiki/Galactic_coordinate_system#Definition) is in Coma Berenices (http://en.wikipedia.org/wiki/Coma_Berenices), which is adjacent to Virgo, suggesting the earth's axis was at its northernmost tilt with respect to the galaxy 6000 years ago, in about 4000 BC, when the celestial pole was between the ecliptic pole and Virgo.

This picture (http://www.bautforum.com/attachment.php?attachmentid=15378&stc=1&d=1315572046) shows Coma Berenice on the line from the Milky Way through the ecliptic pole to Virgo.

Paul Wally
2011-Sep-09, 02:37 PM
I try to picture the tilt of the celestial equator with respect to the ecliptic plane in the following way. Tilt a Compact Disk 23 degrees with a vertical rod fixed through the central hole, so if I turn the rod axially, the CD should kind of swivel around. This swiveling represents precession. Now the line where the Celestial equator intersects the ecliptic, is that the line joining the two equinoxes? So that line is rotating on a 60deg inclined ecliptic plane, so in what direction is this line currently pointing (e.g. Sep looking towards March equinox). Let's say the the direction towards the galactic center is 12 o'clock where is March equinox currently?

Hornblower
2011-Sep-09, 10:02 PM
I try to picture the tilt of the celestial equator with respect to the ecliptic plane in the following way. Tilt a Compact Disk 23 degrees with a vertical rod fixed through the central hole, so if I turn the rod axially, the CD should kind of swivel around. This swiveling represents precession. Now the line where the Celestial equator intersects the ecliptic, is that the line joining the two equinoxes?
Yes, by definition.


So that line is rotating on a 60deg inclined ecliptic plane, so in what direction is this line currently pointing (e.g. Sep looking towards March equinox). Let's say the the direction towards the galactic center is 12 o'clock where is March equinox currently?

I would call it 9 o'clock. It is about 90 degrees around the ecliptic from the galactic equator and is precessing toward it. About 6,500 years from now the March equinox point will be in Sagittarius near the galactic equator.

Robert Tulip
2011-Sep-09, 10:28 PM
Linking this discussion to the speculation about 2012, it is an interesting fact that the point at which the great circles of the ecliptic and the galaxy intersect is close to the center of the galaxy. The disc of a solar system could line up to the galactic plane at any point, but ours just happens to cross the galaxy near the center. However, the solstice point passed the galactic center in 1998, so the popular myth that this will occur next year is empirically incorrect.

Jerry
2011-Sep-14, 05:01 AM
There is not a 'best' coordinate system because the universe is so uncoordinated:)

Depending upon what you want to know, a frequency domain plot can be much more useful than a time/space coordinate system. Very often, the most usefuly coordinates are purely differential: where is the rocket compared with its planned trajectory.