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

Solar System's orbit around galaxy, a concise 3D description?

  1. Dec 3, 2012 #1
    Hello, this is my first topic/question, thanks for reading thus far...

    It is very easy to find explanations about the general path that the solar system takes when it orbits the center of our galaxy, when viewed in 2D from above, a circle around the center some distance away from that center, and how fast we're moving relative to other distinctive galactic features.

    But I've also read that the path takes us up and down relative to the galactic equator, in kind of a sine wave patterh when viewed in 2D from the side.

    I've also read that the "bobbing up and down" is the result of a spiraling path taken by our solar system. So the 2D path viewed from above like a bicycle tire around the hub, but then the 3D path is as if you replaced the bicycle tire with a slinky wrapped around the rim.

    Is this depiction correct? If so, why a spiral? Are are we spiraling around something, some larger thing that is moving near the galactic equator that our sun and its system is orbiting?

    I'm really curious about this, particularly if our angle on the up or downswing ever gets us high enough above the dust cloud at galactic equator to see the center.

    thanks again for reading and any answers.
  2. jcsd
  3. Dec 3, 2012 #2


    User Avatar
    Science Advisor

    The Apex of the Sun's Way, or the solar apex, is the direction that the Sun travels through space in the Milky Way. The general direction of the Sun's Galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 60 sky degrees to the direction of the Galactic Center. The Sun's orbit around the Galaxy is expected to be roughly elliptical with the addition of perturbations due to the Galactic spiral arms and non-uniform mass distributions. In addition, the Sun oscillates up and down relative to the Galactic plane approximately 2.7 times per orbit. This is very similar to how a simple harmonic oscillator works with no drag force (damping) term. These oscillations were until recently thought to coincide with mass extinction periods on Earth.[84] However, a reanalysis of the effects of the Sun's transit through the spiral structure based on CO data has failed to find these correlations.[85]

    It takes the Solar System about 225–250 million years to complete one orbit around the Galaxy (a Galactic year),[86] so the Sun is thought to have completed 18–20 orbits during its lifetime and 1/1250 of a revolution since the origin of humans. The orbital speed of the Solar System about the center of the Galaxy is approximately 220 km/s or 0.073% of the speed of light. At this speed, it takes around 1,400 years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 AU (astronomical unit).[87]

    Above from:http://en.wikipedia.org/wiki/Milky_Way

    I hope it helps.
  4. Dec 3, 2012 #3
    Yes, thanks. I have seen this before, but it gives me something to help focus my question.

    Is there any information on the oscillation, the maximum distance on either side of the midpoint?

    It seems that the path is something like the edge of a warped phonograph record where the oscillation is more or less straight up and down with respect to the general plane of the orbit, not a spiral where movement also occurs back and forth with respect to the center.

    Just trying to visualize this is a straightforward way.
  5. Dec 4, 2012 #4
    A "spiral" does not have movements "back and forth" with respect to center - a "spiral" has one way movement only inside or only outside.

    Compare Moon.

    Moon does NOT return to the same location relative to Earth and fixed stars on each orbit. For one thing, the Moon does not return to apogee or perigee in the same time it returns to the same constellation - Moon returns to apside slightly slower. The result is that Moon´s orbit looked from a pole looks like a daisy of overlapping elliptical petals. The apside line returns to the initial position in 8,85 years, which is about 120 orbits.

    Moon ALSO does not orbit Earth in any plane. The result is that when Moon passes the ecliptic in Dragon´s Head or Tail, the next time the Moon meets the Dragon in the same direction is after the Moon has passed the point of ecliptic where the previous encounter was. The Dragon returns to initial position in 18,6 years, which is about 250 orbits.

    Now, the Milky Way is much less spherical than Earth is!

    Accordingly, the Sun returns to apsides faster than full orbit - and much faster. Sun meets each apside about 1,5 times per orbit (compared to Moon which has each apside about 0,992 times per orbit). Sun also meets each node 2,7 times per orbit - compared to Moon which meets each node 1,004 times per orbit.
    Last edited: Dec 4, 2012
  6. Dec 8, 2012 #5
    Thanks for this answer. When I said "back and forth" I meant that, looking at the path from the side, if it was a spiral path, the system would move toward the center of the galaxy, then away.

    If the solar system was orbiting something that was in an elliptical orbit around the edge of the galaxy, but the solar system orbited that something more towards the perpendicular to the general plane of the galaxy, the solar system would move up and down with respect to the plane of the galaxy, and then in and out with respect to the center.
  7. Dec 9, 2012 #6


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Hi Mathman, thanks for quoting a potentially useful Wikipedia article, however it contains a serious error. The red statement is false or misleading. The "solar apex" is NOT the direction that the sun and other neighboring stars are moving in their orbit around galactic center. That orbit has a slight eccentricity (say 10%) but very roughly speaking it is CIRCULAR. The roughly circular orbit speed is estimated about 220 km/s as you say. The general direction of that orbital velocity is about 90 DEGREES FROM GALACTIC CENTER.


    The motion towards the "solar apex" is different. It is only about 16 km/s, it is a small deviation from the orbital motion of 220 km/s that is typical of stars in our neighborhood. the collective average orbital motion of our neighborhood is used to establish what astronomers call "Local Standard of Rest" (LSR).

    Our sun's individual motion relative to LSR is what is 16 km/s in the direction of star Vega.
    There's also the slow bobbing up and down etc that the Wikipedia article talked about. But those are just relatively small add-ons to the main roughly circular orbit around Center.

    the blue is right, the red is wrong because motion towards the "solar apex" (i.e. the star Vega) is only a small deviation added to the main 220 km/s orbital motion.

    Above from:http://en.wikipedia.org/wiki/Milky_Way
    Last edited: Dec 9, 2012
  8. Dec 9, 2012 #7


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    The sun doesn't need to be "orbiting something" (which is itself in orbit) in order to bob up and down.

    I never heard of such. You are trying to understand why it bobs up and down, which is good. Well that has a simple explanation: most of the visible mass of the galaxy is either in the central bulge or in a relatively thin pancake---the galactic plane. Because of the concentration of mass in the plane, whenever the sun sails up ABOVE it is pulled by all those thousands of stars below and it starts to fall back down into plane. then its momentum carries it thru and it goes out BELOW the plane. and the concentration of mass, thousands of stars above it, pull it back up towards the plane again.
    It is like a pendulum or springbok. Once that kind of motion gets started there is no reason it should stop. But it is very slight and slow compared with the overall circular orbit motion around the huge central mass.
    Last edited: Dec 9, 2012
  9. Dec 9, 2012 #8
    Wrong explanation!

    If the Milky Way disc had negligible mass compared to a huge central mass then the orbit of Sun would be unperturbed planar ellipse, a closed orbit.

    But since the plane of Sun´s orbit would be completely independent of the orbit of the disc, it would generally be in a different plane - intersecting the Milky Way plane in the central mass and along a fixed line of nodes through the centre.

    So relative to an observer on exactly circular orbit exactly in the plane of Milky Way, Sun would be bobbing up and down - exactly once each orbital period, returning to the nodes in the same fixed direction.

    But the Milky Way disc does have significant mass. This mass attracts the Sun, and causes the Sun to bob up and down much faster than the Sun would naturally be doing - about 2,7 times each orbit, not just 1 time.
  10. Dec 9, 2012 #9


    User Avatar
    Staff Emeritus
    Science Advisor

    Snorkack I believe Marcus explained it pretty much exactly the way you did. When he says the motion is very slight and slow compared with the orbital motion around the central bulge he means that our velocity up and down is much less than it is forward. But since the disk is very very thin we can traverse across it a little quicker compared to the time it takes to orbit the galactic center.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook