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I Orientation of the Earth, Sun and Solar System in the Milky Way

  1. May 21, 2017 #26
    60.2 is the angle between ecliptic and plane of the Galaxy. I am asking about the angle between radius of the Galaxy and axis of the ecliptic. The radius which connect center of the Galaxy and Sun. From many pictures this is 90 degrees but there is no information about that.

    To be more precise - what is the angle between axis of the Galaxy projected on the plane of the Galaxy and the radius of the Galaxy that connects center of the Galaxy and Sun.
    Last edited: May 21, 2017
  2. May 21, 2017 #27


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    96 degrees.
  3. May 21, 2017 #28
    Astonishing! Thank you. What is the source of this information?

    Is it true that axis of the ecliptic stays still like a gyroscope and therefore this angle will change over the movement around the Galaxy?

    I see that this angle is now in increasing mode. Am I correct?
  4. May 21, 2017 #29


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    The celestial coordinates to the X-ray source Sgr A identified as the supermassive black hole at the centre of our Galaxy are known (~18h RA, ~-29 dec). The axis of Earth's rotation happens to be angled in such a way w/r to the ecliptic, that it's just a matter of adding its inclination (~23 degrees) to get the 6 degrees between the ecliptic plane and the galactocentric radial direction (or 90+6 if you want an angle with the normal to the ecliptic).
    Best seen if you launch some planetarium software (I recommend Celestia) and turn on galactic, ecliptic, and celestial coordinate grids.
    That's also my understanding.
  5. May 21, 2017 #30


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    To add to Bandersnatch's reply, here's a screen shot taken from the Worldwide Telescope software. The blue grid are the galactic coordinates, where (0,0) marks the center of the galaxy. The white grid are the ecliptic coordinates, with the ecliptic marked out 260, 270, 280, etc. Note that the ecliptic crosses the plane of the galaxy ~6 degrees from the galaxy center coordinates.

  6. May 21, 2017 #31
    I made a mistake in my 'precision':

    Should be:
    To be more precise - what is the angle between axis of the ecliptic projected on the plane of the Galaxy and the radius of the Galaxy that connects center of the Galaxy and Sun.
    (can't edit post now)

    Anyway thank you for understanding and answers!
  7. May 21, 2017 #32
    I made a sketch about this angle (lets hope it's correct;)

  8. May 22, 2017 #33
    All this would be wonderfully easy to visualise in 3D, of course :sorry:
  9. Jul 11, 2017 #34
    Hello there! Thanks for the interesting conversation, I have another question:

    The solar apex has in consideration the movement of the surrounding context (respect to the so called local standard of rest). Because of that, it does no say what direction the sun is really going in respect to the milky way center.

    So what is the direction of the solar system irrespective of the local standard of rest? I'd say it should be near 90º from the center of the Galaxy (because the solar system should be moving over the tangent), so somewhere near Deneb.

  10. Jul 12, 2017 #35
    I redrew this (Figure 3 from my OP) from an illustration I found in the Wikimedia Commons https://commons.wikimedia.org/wiki/File:Ecliptic_equator_galactic_anim.gif
    It was one of the best visualizations of the relative orientations of the Celestial Equator, Ecliptic Plane, and Galactic Plane that I could find anywhere. Figure 2 was an attempt to show the angles between the Celestial, Ecliptic and Galactic North Poles and their respective planes. This is something that can't be done by simply adding or subtracting say, the angle of the Celestial Equator relative to the Ecliptic Plane (23.44°) and the angle of the Ecliptic Plane relative to the Galactic Plane (60.19°), to get the angle of the Celestial Equator relative to the Galactic Plane (which is 62.87°). You need spherical trigonometry for that, because the three planes don't intersect at a single point .

    Oddly enough, what started me off on this whole quest many years ago was that I wanted to know the angle between the Earth's axis of rotation (Celestial North Pole) and the Galactic Plane. Turns out it is 27.13°.

    Celestial, Ecliptic & Galatic Poles-Planes - 11Jul2017.jpg
  11. Jul 12, 2017 #36
    According to Wikipedia, the Solar Apex refers to the direction that the Sun travels with respect to the mean motion of material in the Milky Way in the neighborhood of the Sun. I find these terms confusing, and don't use them much. But you can look here for more info: https://en.wikipedia.org/wiki/Solar_apex and https://en.wikipedia.org/wiki/Local_standard_of_rest

    Basically, these terms refer to the "local motion" of the Sun with respect to its neighboring stars. The speed of the Sun towards the solar apex is about 20 km/s. The solar apex is located in the constellation of Hercules, southwest of the star Vega. So it's closer to Vega than Deneb.


    However, the Sun and its neighboring stars are collectively moving around the center of the Milky Way in a clockwise motion (with Galactic North as "up") at about 230 km/s. This is perhaps what you mean by "the direction of the solar system irrespective of the local standard of rest."

    local motion of stars in solar neighborhood.png

    The Sun is roughly 50 light years above (north of) the galactic plane, and passed northward through it about 3 million years ago in its undulating path around the galactic center. It might help to think of the stars in our galaxy as a kind of colloidal suspension, with the individual particles jostling each other randomly, but still moving around a common center.
  12. Jul 13, 2017 #37
    Yes, but you did give any hint on the present direction of the sun in that "clockwise motion (with Galactic North as "up") at about 230 km/s"!

    That direction should not vary much in our current life-span, as it turns very little, only 360º / (~230* 10^6) years = ~1,56 * 10^-6 degrees per year.

    I think it is it should not be difficult to assess this direction (I've given my hunch) and, at least for me, it's much more interesting than the "solar apex" direction, because it's not so "local"!
  13. Jul 13, 2017 #38
    "Present direction" is a pretty vague term. It seems you meant "degrees per year," which refers more to a rate than a direction. In any case, it appears you have answered your own question.
  14. Jul 13, 2017 #39
    Lapsus calami: *didn't

    Present direction in respect to the centre of he galaxy, if you care to understand; the direction the sun when considering it's the velocity of "about 230 km/s" which you've mentioned. Doesn't seam a vague term.

    Last but not the least, I've only given my hunch, which is open for discussion. I wouldn't ask if I had the answer.
  15. Jul 14, 2017 #40


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    If I understand the conventions right, the stated direction of orbital motion is exactly at 90 degrees to the galactocentric radial direction, and that's what LSR motions are measured against.
    So it's a bit ahead of Deneb:
    If you want the 'real' direction of travel in galactic-rest coordinates, just add the velocity vectors w/r to the LSR.

    By the way, I don't think the 20 km/s value and direction given by Wikipedia is a good one. At the very least it shouldn't be taken as anything more than a first approximation. The source for this value given in the article does not seem to actually have it, and in any case it's from 1993. Recent papers give a rather wildly varying (approx. 5-15 km/each component) velocity estimates (see here: https://arxiv.org/abs/1411.3572, including the referenced values).

    The combined motion should be in the direction of somewhere around Lyra-Cygnus, and any more accuracy than that doesn't seem justified at the present time.
    Last edited: Jul 14, 2017
  16. Oct 10, 2017 #41
    A beautiful depiction of everything I've been taught over the years brought together very nicely... and yet it appears to be missing part of the "3D" aspect of motion. While you show the "orbital wobble" around the axis of the Sun and Earth, it's missing on the Moon (as well as the Moon's North Pole, a minor issue yet still incomplete), but on the bigger scale, you've not included the wobble on the Galactic North Pole. I cannot imagine that it would be totally stationary and locked when everything that it's comprised of isn't stationary at all, even if we don't yet know the amount of the angle of the galactic wobble (however small that may be). Also not shown is the effect of that wobble in regards to the path of the Sun around the Milky Way, or the effect that it would have on the Galactic Plane of the Milky Way itself. Perhaps a "thickening/thinning" of the path lines to show the "to/fro" of the motion that can't be shown with up/down pathing, and provide a 3D view of the Earth/Moon orbits? It all really simply depends on just how accurate you want it to be, but even if we don't know the numbers, we can show the motion with a "?", and let someone fill that in at some future date. I'm sure at some point, some mathematician will figure out ALL the numbers and win some kind of award for it, but since it takes 240Million years for a galactic revolution, it may be a while.
  17. Oct 12, 2017 #42
    Thanks for your kind words.

    I'm sure exactly what you mean by "orbital wobble," since it's not a standard term. Perhaps you mean rotational wobble, ie, precession. My diagram doesn't show precession, but it does show the relative inclination of the orbits of the moon, earth and sun, and the undulating path of the sun around our galaxy. Those circular arrows don't show "wobble" if that's what you mean - they just indicate the direction of rotation. Our solar system has a negligible effect on the axis of rotation of the Milky Way - it's just one of hundreds of billions of stars. It's hard to show everything in 3D in a 2D model. The orbits coming "out" of the diagram show the rotation arrows in front of and behind the axes of rotation.

    Attached Files:

  18. Oct 12, 2017 #43
    CORRECTION: To my post #42 above:

    I'm NOT sure exactly what you mean by "orbital wobble"
  19. Oct 13, 2017 #44
    Okay, I'm terrible at getting my thoughts out. I know this, so please bare with me as I get stupidly simple in my thought process here (me, not you).
    First, let me say that I had incorrectly seen your "direction of rotation" as the rotational wobble of the planet, which is why I wondered why you didn't show it on everything else. My bad. However, in response to your not quite understanding what I was referring to... It's much like the rotational wobble (i.e. precession) you're referring to, but delegated up and down to the various next level, then the next, then the next...etc. Singular precession, orbital precession (as objects orbit each other), Stellar percession (a system orbiting a star), galactic precession... ... ... see how the scale gets bigger and bigger?

    Now for me to get REALLY stupid... If you'd like, you can let me know where I'm wrong in all this.

    Let's say the Earth were all alone floating around in space with nothing else close enough to it to have any kind of gravitational effect on it what-so-ever. Thanks to its own internal gravitational torque, it would end up spinning around on its axis just as pretty as you please, like a top that never quits.... assuming that the mass and composition isn't a perfectly formed ball of iron too small to liquefy the central core, it should also begin going around in a little circle while it spins and the mass is constantly flung around 360 degrees. The same goes for the top/bottom of the Earth which would cause the tilt. So now we have a tilted, rotational and circular motion on this one Earth (a motion transition that can be shown with any decently spun top floating in a vacuum). There's the percession of motion with just one single Earth.

    Now stick a Moon around that Earth. The Moon would have all of the same aspects as the Earth when it comes to motion, but now they begin to orbit each other. Due to the difference in mass between the two and the difference in the angular momentum and so on, they wouldn't orbit in a perfect plane or in a perfect circle (unless they had equal mass). They would circle each other and begin to go up and down on that plane just as your Suns path around the Milky Way shows... but this motion would be between the Earth and Moon. Up and Down, Left and Right, To and Fro in a never ending circular circle that in time would hit every angular degree possible and start all over again.
    Now add in the Sun. A MASSIVE element that would bring a bit of stability to the motion of the Earth/Moon's orbital rotating dance. The Sun has its own polar axis and rotational motion, as well as it's own tilt and wobble just as the Earth and Moon do. The plane of motion would settle down for the Earth/Moon, but it would still go up and down on the Suns equilateral plane and therefore the orbit isn't perfectly circular around the Sun. This gives the Sun an orbital wobble to the rest of the bodies orbiting.

    The Sun doesn't sit still in it's place just as the Earth/Moon combination, or even the Earth by itself doesn't. There's the "orbital wobble" I'm referring to. The not quite so circular path that everything ends up taking due to the forces of the gravitational torque that builds and builds as you add more and more. This motion isn't just applied in the top down view, but also on a side view as well which is why you get the up and down path of the Sun to the Ecliptic Plane of the Galaxy your drawing shows. Up and Down, In and Out, Left and Right... all motion in all directions all the time. Nothing is static. Of course once you get to a point of view large enough, then that motion becomes negligible, but it's still there. That's the 3D motion I was saying was too bad wasn't depicted. Of course nothing would be to scale, but when you're talking about orbits around galaxies, putting it to scale is impossible.
  20. Oct 13, 2017 #45
    Check out this link:
    The focus in my OP was to illustrate the “Orientation of the Earth, Sun and Solar System in the Milky Way,” so perturbations in multiple-body orbits were not considered.
  21. Oct 15, 2017 #46
    Excellent diagram! Still not clear how we are able to distinguish the Sun's forward motion around the Galactic center from the forward motion of the "Orion Arm"? If we assume one rotation of Sun around Galactic center at 26K light year radius we are looking at a distance of 164K light years over say 226 million years. So one degree of forward motion would take approx. half a million years. Now math is not my strong point so happy to be "slapped down".
    Also the "declination" cycle of Sun above and below the Galactic plane has been estimated at 70 millions years to complete.and this seems to be the most
    Influential short-term cycle, so interesting to speculate on its cause. Within this cycle we get the Precession cycle of 26k years, as seen from the Earth which suggests constellations are moving with the Sun. Point I'm making is the time scales and distances are so vast how do we separate out observed fact from assumption.
  22. Oct 17, 2017 #47
    Barycenter!!! THAT's the word I was looking for! (slaps forehead). Anyway, as I was saying, I love your diagram... if you decide to do one that goes out to a larger scale (say... galactic?), I'm sure you would use 3D styled lines to show the to/fro motion as the bodies revolve around their barycenter. Can you imagine the detail that would have to be placed into that one?
  23. Oct 17, 2017 #48


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    I get the impression from these discussions that you are picturing the orbit of the sun around the Milky Way as a closed curve. This is almost certainly not the case. Orbits in 1/r potentials like the solar system are closed curves, but in a potential like the potential of a galaxy, the orbits are almost never closed curves. This site discusses some of the complexities. In addition, the galaxy potential changes with time. So the sun's orbit around the galaxy center probably looks more like one strand in a bowl of spaghetti than a classical Keplerian orbit.
  24. Oct 18, 2017 #49
    No such thing as a closed orbit (except in possibly extremely rare cases, and then only when greatly limiting ones viewpoint). Gravitational motion tends to eliminate that possibility from the beginning. Even limiting our view all the way down to the earth/moon orbit, we find that the moon is moving away from the earth just a tad bit every year. The Earth is moving away from the sun as well, and again, just a tad bit each year. Not enough to make any real difference in a thousand thousand lifetimes, but because that movement is there, the possibility of a closed orbit is impossible. Even the solar system doesn't end up back in the exact same place relative to the galaxy when it's completed an entire revolution. Expand the view to the local cluster, and any thought of any kind of closed orbit just goes right out the universal window. :)
  25. Oct 21, 2017 #50
    One thing is very clear we are still not sure about the Structure of the Milky-way as no one has ever seen it from outside (see www.skyandtelescope.com article
    Seeing Far side of Milky Way) Two spiral arms or Four? Gaia Mission also raises interesting questions about the trajectory of stars. Stars primary motion around the Galaxy would seem to be the result of the rotation of the Spiral arms in which they reside. The Gaia mission on the other hand would suggest that all stars have a secondary orbital trajectory within their host spiral arm i.e. Sun's 70 million year declination cycle. The apparent erratic trajectory of some stars would suggest collisions are probable so Solar system not closed and some researchers believe our Solar system is a "fruit salad" of captured planets and Red dwarf stars (Velikovsky). That would certainly better explain Venus environment than "Greenhouse warming".
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