Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

  1. Apr 27, 2017 #21


    User Avatar
    Gold Member

    @fizixfan Great Diagram!
  2. Apr 27, 2017 #22


    User Avatar
    Science Advisor
    Gold Member

    All stars, more like. Just to a varying degree.
  3. Apr 27, 2017 #23


    User Avatar
    Gold Member

    And even if a star is born perfectly on center it will be perturbed out of true in very short order by other masses it interacts with.

  4. May 20, 2017 #24
    What is the angle between axis of the ecliptic and radius of the Galaxy? Exact 90 degrees?
  5. May 20, 2017 #25


    User Avatar
    Science Advisor

    No, why should it be? They are just random orientations. Look at Figure 2 in Post #1 from this thread. It looks like the angle you are asking about is 60.2 degrees.
  6. 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
  7. May 21, 2017 #27


    User Avatar
    Science Advisor
    Gold Member

    96 degrees.
  8. 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?
  9. May 21, 2017 #29


    User Avatar
    Science Advisor
    Gold Member

    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.
  10. May 21, 2017 #30


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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.

  11. 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!
  12. May 21, 2017 #32
    I made a sketch about this angle (lets hope it's correct;)

  13. May 22, 2017 #33
    All this would be wonderfully easy to visualise in 3D, of course :sorry:
  14. 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.

  15. 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
  16. 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.
  17. 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"!
  18. 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.
  19. 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.
  20. Jul 14, 2017 #40


    User Avatar
    Science Advisor
    Gold Member

    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted