This is very perplexing.


  1. planets are moving around the sun (solar system)
  2. solar system is moving as part of the galaxy
  3. the galaxy is moving through space on some axis
  4. galaxy is part of a cluster, also moving through space
  5. other stars and constellations are also moving through space, presumably in the same direction and speed as the Milky way.

I appreciate the answer to Why is the Solar Helical (Vortex) model wrong? - but considering the question: how can we observe the solar system's Helical motion through space, I have a new question... please allow me to present both, since they are related.

Question 1:

Again, how can we observe the Helical motion of our solar system (and galaxy) from earth? Is it by observing the passing of planets across/through constellations?

Question 2:

If I can make an analogy: like different ticking clocks stored inside a grandfather clock - is the solar system following its own Helical motion and therefore are all other solar systems within the Milkyway also following their own Helical motion? AND is the galaxy also following its own Helical motion!?


Is the solar system's motion fixed (Helically speaking), and any such movement is tied to the motion of the parent, i.e. the galaxy? In other words, is the Helical motion of the solar system the same as the galaxy, or are all solar systems moving Helically through space, e.g. many clocks within a grandfather clocks analogy.

Initially, the question was simple: how can be observe the solar system's Helical motion?

But it seemed unintuitive to me that the solar system is moving Helically while the galaxy is also moving as such - both structures on their own trajectory and paths - seems like galactic chaos.

And, all the while planets maintain their fixed orbits.

I would have guessed that the solar system is NOT moving Helically, but the galaxy is, and the solar system is simply a fixed orbital passenger, as so to speak - in the same way 8 grains of sand (planets) are passengers in a bowl of jelly (galaxy), both being carried by another vehicle (cluster).

It seems to me then, the solar system's motion through space is limited by the galaxy's motion, in terms of axis, speed - and therefore the galaxy is also fixed like a passenger on a much larger galaxy cluster.

It seems reasonable to suggest the whole universe is moving Helically(?), and all galaxies merely inherit this, but are more like grains of sand in jelly than independently ticking clocks within a larger grandfather clock.

If our solar system is moving Helically independently from the galaxy, the solar system could move at its own speed, either quicker or slower depending on environmental factors. Could the solar system really skip-out of the galaxy, racing ahead on its own Helical path(?).

So many questions! But Question 1 is the one I wish to request help answering.

  • $\begingroup$ Relative to what? $\endgroup$ – ProfRob Mar 31 at 13:29
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    $\begingroup$ "seems like galactic chaos", yep, that's exactly what it is. There is some order at various levels, but yea. All sorts of things rotating on different axes, in different planes, in different directions. Look up videos of how the positions of stars will change over time in the night sky. Like bees around a hive. Fascinating stuff! $\endgroup$ – coblr Mar 31 at 19:41
  • $\begingroup$ @ProfRob - true. (it seems) everything is moving (solar system), as coblr explains - and then every object is bound together as a single collective object (galaxy). If there is one imaginary absolute point in space to observe the idea that we are not only moving through space, but 'we' form part of a larger structure that is moving (forward), within such an object is the clockwork and chaos of individual galaxies and solar system. $\endgroup$ – Dylan Mar 31 at 19:58
  • $\begingroup$ @coblr I think what is interesting about trying to observe the motion of the earth and moon and stars, is the consideration not only of the interconnection of objects (solar system) and considering how the motion of the night sky especially alludes to the fact we are rotating & moving through space - in addition to these, it is the consideration of the smallest objects responsible for structure and matter and their behaviour and motion, and of the the largest objects, such as galaxies and black holes. It is like the 'motion of matter, motion made and caused by matter'. Self-perpetuating $\endgroup$ – Dylan Mar 31 at 20:21

Question 1: "How can we observe the Helical motion of our solar system (and galaxy) from earth?"

Short answer:

We can't observe it directly. Instead, we can observe the position and velocity of other bodies in the Solar System and galaxy to deduce all of our collective relative motion, with help from Newton's law of gravitation.

Long answer:

Within our Solar System, from points on the Earth, we can track the locations of the other planets and moons with telescopes. We can also determine their distances using relative brightness measures and measures of apparent size in the night sky. We can also determine their speed and direction relative to the Earth by taking time differences of their positions. When we put all this together with Newton's law of gravitation we can create a model of the Solar System with the Sun near the center that has incredible descriptive and predictive accuracy.

For other stars in the Milky Way galaxy, we can determine their location with high accuracy telescopes. We can estimate their composition and mass using techniques from radio astronomy. We can partially estimate their direction and speed by taking a time series of measurements and by comparing their emitted frequencies to known emission spectra to derive Doppler measurements. Similar to the way we created a model of the Solar System, all the star velocity and position data allows us to create a model of the galaxy that is consistent with our measurements. For the galaxy, however, the rotation rate consistent with the Newton's law of gravitation (as captured by the virial theorem) requires a much higher mass then we have observed, so our model is only consistent with our measurements if we assume the presence of "dark matter."

Using a coordinate system with the supermassive black hole SGR A* at the center, we create a model of motion consistent with our measurements for both the Solar System and the Milky Way galaxy. Further, this model is consistent with Newton's law of gravitation, dark matter, and just a bit of Einstein's relativity. In this model, the movement of planets in the Solar System around the Galactic center form something similar to a bent helix, since the angular momentum vector of the Solar System is inclined about 60 degrees from the angular momentum vector of the galaxy.

It may not make much sense to think about the motion of the galaxy as a whole, since there isn't an advantage of one spatial frame of reference over another at that scale, according to special relativity. If you choose something like the Great Attractor as your inertial reference frame origin, the motion of stars in the Milky Way will also trace out bent helixes.

Question 2: "Is the solar system following its own Helical motion and therefore are all other solar systems within the Milkyway also following their own Helical motion?"

Yes, the helical motion of our Solar System is independent of the helical motion of other systems in the galaxy and the galaxy's helical motion is independent of the helical motion of the stellar systems within it. Your analogy of a grandfather clock with many independent small clocks inside of it is more apt than your analogy of grains of sand in jelly.

Perhaps a better analogy yet is the carnival ride tilt-a-whirl, with the children's toys, pin-wheels affixed to the cars. If you put LED lights on the ends of the pinwheels and took a time lapse photo of the tilt-a-whirl running, you would see independent combinations of helical motion from both the pinwheels and cars!

  • $\begingroup$ Yes, I see - reminds me of the illustration often used to explain relativity, with objects moving independently within other moving objects, and the light trace (visual effects) often used to show continuous motion. We can observe the rotation of the earth around the sun, and rotation of moon around earth. We can also observe the spherical property of earth by noticing the shape of sunrise of the horizon (like a half waning moon shape). But the greatest observation when noticing the motion of stars is that earth is spinning and moving forward. Time-lapse is a a great way to notice motion. $\endgroup$ – Dylan Mar 31 at 19:29

All motion is relative. There is no absolute space which can be said to be the correct reference frame in which we must measure all motion. Hence, for any object you're free to construct any reference frame that does, or does not, give that object a helical motion.

Reference frames

Of course, some reference frames make more sense than others. When you cycle to work, it makes sense to measure your speed relative to the bike lane. The reference frame of the Sun makes sense if you're concerned with planets' orbit. In this frame, Earth has no helical motion, only circular motion. Or you could call it helical motion with $v_z = 0$, if you wish.

On the other hand, the Sun and its neighbors follow a path through the Milky Way; the so-called Local Standard of Rest, measured by observing nearby stars and star-forming regions. In this frame, Earth has a (non-$v_z\!\!=\!0$) helical motion.

Similarly, in the reference frame of Milky Way's path toward, say, Andromeda, the Sun has a helical motion. Another star will have a similar, but different, motion.

Comoving coordinates

That said, there is one reference frame which has a special status, namely the Universe's comoving frame. These are the coordinates that expand along with the Universe, and they are the coordinates in which everything on average is at rest. If you hear the phrase "Milky Way's motion through space", this is probably what is referred to. Since time flows differently in different frames, it's also the frame of choice when you talk about the age of the Universe. Likewise, since lengths are contracted in moving frames, the comoving frame is the frame in which we measure cosmological distances.

It's important to realize, however, that motions, positions, etc. measured in the comoving frame are no more "correct" than in any other frame.

The cosmic microwave background

But how can we measure a velocity in this reference frame, if all motion is relative?

One way would be to measure the motion of a million galaxies with respect to us, and take the average. But a much easier method, which is in fact how we do it, is to measure the dipole of the cosmic microwave background (CMB). This radiation was emitted when the Universe was very young, and has the special property that it was emitted isotropically (i.e. the same in all directions) with a near-perfect Planck spectrum.

Hence, if you standing still in the comoving frame, you will see exactly the same CMB spectrum in all directions, whereas if you're moving you will observe the CMB to be blueshifted in front of you, and redshifted behind you. This difference between the two opposite directions is called the dipole, and its value was measured by the Planck satellite (Planck Collaboration 2020) to imply a total velocity of the Sun in the comoving reference frame of $$ v_\odot = 369.82\pm0.11\,\mathrm{km}\,\mathrm{s}^{-1}, $$ toward the Galactic coordinates $$ \{\ell,b\} = \{264.021^\circ\pm0.011^\circ, 48.253^\circ\pm0.005^\circ\}. $$

Note the amazing precision, with an uncertainty in speed and direction of only 400 km/h and less than an arcminute, respectively!

This velocity is important to know, since it is used to correct all measured cosmological velocities.

  • $\begingroup$ Amazing how scientists have been able to rationalise, and develop technology to gain greater insight into the structure of the universe. Before the rational, calculating, mathematical explanation, there is also the visualisation happening in our intelligent mind as we try to comprehend ideas around vast space and structures. The step from visualisation and imagination through to science is a micro-moment of discovery, which is a more accessible way to learn about everything you've outlined. Comoving and considering the motion of objects forming part of moving objects: galaxy & solar system. $\endgroup$ – Dylan Mar 31 at 19:43
  • $\begingroup$ @Dylan Yes, for some people (e.g. me) visualization is an important part of understanding. Others are able to understand from pure math, but I often find it helpful to make little drawings of my problems. $\endgroup$ – pela Mar 31 at 21:46

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