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Sun's Movement in Milky Way

  1. Mar 27, 2013 #1
    I have seen a depiction of the Sun traveling in it's orbit in the Milky Way. In this depiction, the Sun was bobbing up and down.

    1. From what does the Sun get its movement orders from? (The black hole at the center of our galaxy?)

    2. How come it bobs up and down, but none of the other stars do? Or is this just because showing the movement of all the other stars would be a nightmare?

    3. If the Sun gets it's movement orders from something bigger than it, why doesn't Earth get its orders from that same object?

    Thanks in advance!
  2. jcsd
  3. Mar 27, 2013 #2
    Obviously the Sun is not the only Star to "bob" up and down because the forces that dictates the sun's movement are the same as those dictating the orbits of the rest of the stars.
  4. Mar 27, 2013 #3


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    Also, keep in mind that the period of the bobble is over 60 million years so whatever it is, it is very gradual. Why would you think that the Earth does not "get its orders from the same object"? Do you think that as the sun bobbles, it leaves the earth behind?
  5. Mar 27, 2013 #4
    Well...what does the Sun get its orders from? Why wouldn't the Earth get its orders from that same object? Why doesn't every object in our galaxy get its orders from the black hole at the center of our galaxy.

    Thank you guys very much for the responses.
  6. Mar 27, 2013 #5
    So does every star "bob" up and down then?
  7. Mar 27, 2013 #6
    Earth does get its orders from that very same object; however in addition it also orbits the sun in the same way that the moon orbits Earth and thereby also the sun since the Earth orbits the sun.
  8. Mar 27, 2013 #7
    Yes, or every solar system does.
  9. Mar 27, 2013 #8


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    Most stars in the spiral arms do in fact travel up and down while in their orbits. Stars in other areas of the galaxy experience different conditions and their orbits may vary greatly.

    Let's drop the "get its orders" as that is very inaccurate. Both the Sun and Earth are affected by the combined gravitation of the entire galaxy. This ends up making us orbit around the center. However the supermassive black hole in the center of the galaxy is NOT the primary source of this gravity. The combined pull from everything else in the galaxy greatly overwhelms it.
  10. Mar 27, 2013 #9

    One should specify that the "center" is the galaxy's center of mass, right?
  11. Mar 27, 2013 #10


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  12. Mar 27, 2013 #11
    Why do the stars travel up and down in their orbits, and the Earth the other planets do not orbit in that manner?

    I only said "get its orders" because that is how this book I am reading refers to gravity. Why is it bad?

    What is the primary source of the gravity then? In my simple understanding, I am accustomed to seeing it only as one object, i.e. moon orbits Earth...Moons or other planets, etc.

    Thanks for the responses!
  13. Mar 28, 2013 #12
    'Everything' is a source of gravity and some might say 'zero'(mathematical breakdown) with reasons we don't fully understand. We only know or we think we knew of it's effect. It is dependent/proportional on both the mass of the bodies and the distance between them. Imagine your holding an apple. An apple has a gravitational field but it is so tiny that we are unaware of it. Drop the apple and it falls to Earth. This is because the gravitational field of the planet is millions of times more than that of the apple. On a much larger scale it appears that a diff effect is at play.

    Common understanding is that the moon orbits the Earth. This is not strictly true as they actually orbit each other. The point around which they orbit is about a mile below the surface of the moon.

    The order that we knew of is that "the larger/denser the mass and the nearer you are to the center the greater/stronger the pull effect (distance). Each effect can be relative or absolute.

    The planets/stars/etc appear to be moving up and down due to fact of retrograde effect being observed by a moving body by assumption of a stationary observer and predefined center of reference. It also constitute to the elliptical orbit in the case of such planets. It looks that way relative to a local vantage point.
  14. Mar 28, 2013 #13


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    Why do you KEEP saying that? I ask again, do you think that as the sun "bobbles" it leaves the Earth behind? Does it leave the other planets behind?

    The entire solar system is affected as a unit because the center of mass of the galaxy, which is what causes the bobble, is to a huge number of decimal places exactly the same distance from all of them.

    EDIT: by the way, I suspect it's an oversimplification to say that the center of mass of the galaxy is the entire cause, but my point is still valid since all of whatever is causing it is basically just as far from Earth as it is from the sun and is very far from both and so affects both together.
    Last edited: Mar 28, 2013
  15. Mar 28, 2013 #14


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    I'm sure you intended to say "a mile below the surface of the EARTH"
  16. Mar 28, 2013 #15
    Does this involve dark matter as well? I have heard that dark matter is necessary for holding galaxies together.

    I guess my question is really silly. I am wondering why since the Sun bobbles, why aren't the earth and the other planets in bobbles of their own simultaneously?

    In other words, I see the orbit of the planets as an ellipse, and the orbit of the sun like a sine wave, slowly oscillating back and forth over 60 million years. Why are these orbits not the same in behavior? I understand that the planets bob back and forth with the sun but it just seems really odd to me that orbits would differ in behavior. Are the orbits of the planets not as simple as the 2D figures I make them out to be?
  17. Mar 28, 2013 #16
    It can go both ways in barycentric interactions but definitely the greater/dominant effect goes to the denser mass out of 2 orbiting bodies. ^^

    As far as we can tell about Dark matter. Yes and maybe to some.

    Let's say it gives that impression for now.
  18. Mar 28, 2013 #17
    Indeed they are not as simple.

    For a simple comparison, look at the orbit of Moon.

    It is like an ellipse. And it resembles an ellipse in a plane inclined to ecliptic at about 5 degrees, so intersecting ecliptic twice.

    But this is merely a resemblance. If Moon followed a planar orbit then eclipses could only happen twice a year at fixed dates. This is not the case - eclipses of Sun and Moon occur at all seasons, over extended period of time.


    Because Moon does NOT follow a planar orbit. Moon does cross ecliptic about twice a month. But actually, a bit more often! The nodes of Moon make a full circle around ecliptic in 18,6 years - which means that in about 240 orbits of Moon, Moon will bobble one extra cycle.

    Likewise, Moon does not follow an elliptical orbit. Moon experiences perigees and apogees - but each successive apogee is in a different direction. The apside line makes a full circle in 8,85 years - about 120 orbits of Moon.


    Moon would follow a planar, elliptic closed orbit if Earth and Moon were alone as a two body system, with Sun very, very far away.

    But this is not the case.

    The gravity of Sun perturbs the movement of the Moon. Even though the semimajor axis, eccentricity and inclination of Moon stay relatively constant, the direction of apsides and nodes change rapidly.

    How about planets?

    Remember that the planets are very small relative to Sun. Jupiter is 1050 times lighter than Sun.

    The orbit of Earth is also not planar, nor a closed ellipse. But it is very much closer to these than the orbit of Moon is.

    For example, the apsides of Earth make a full rotation in 112 000 years, which is 112 000 orbits - compare the apsides of Moon making a full rotation in 120 orbits. This is caused mostly by Jupiter and Venus, and the rest from other planets and general relativity.

    So the open orbits happen because there is mass elsewhere than in Sun, but they are near to closed ellipses because there is not much mass there.

    Now look at the orbit of Sun, and all other stars in Milky Way.

    If the mass of Milky Way were concentrated in the central black hole and the rest of Milky Way were of comparatively negligible mass then Sun and other stars should follow closed, plane elliptical orbits.

    If the bulk of the Milky Way mass were in spherically symmetric crown, whether of dark matter or of dim old stars and globular clusters, with Milky Way disc being shiny but lightweight gas and young stars, then Sun would have a changing direction of apsides - but there would be no reason to change the plane of orbit.

    Instead, the Sun bobs up and down relative to Milky Way plane - not 1 crossing each way per orbit, as would happen if Sun followed a planar orbit ignoring the Milky Way plane, but I have heard the number of 2,7 crossings each way.

    So 1,7 extra crossing pairs each orbit - compared to Moon which has just 0,004 extra crossing pairs each orbit.

    The reason is that Milky Way disc does have a lot of mass attracting the Sun - its influence is strong compared to black hole, masses of stars and dark matter in the centre. Again compare Moon - it crosses the ecliptic 13 times per year each way, simply because it is orbiting Earth, and only then, along with Earth, is orbiting the Sun. Sun, likewise, is mostly attracted by the nearby parts of Milky Way disc, and only more weakly by the various stuff defining the middle of Milky Way.

    What I could not find in a hurry is the actual speed of the precession of Earth´s ecliptic plane. Does anyone know?
  19. Mar 28, 2013 #18


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    The reason the Sun travels up and down several times per orbit is that while in the middle of the disk of the galaxy, the Sun experiences no net force from gravity up or down. As it passes up through the disk gravity from the disk pulls it back down and it follows through until it starts to leave the disk at the bottom, at which point the disk pulls it back up again. This is purely due to mass being "underneath" or "above" us as we start to leave the disk.

    The planets have negligible mass in the solar system pulling them up or down so they don't behave the same way.
  20. Mar 28, 2013 #19
  21. Mar 28, 2013 #20


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    That's not bob, that's due to the inclination of the planets orbits being different. AKA the orbits are tilted.
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