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

[fizixfan]

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.

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."

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.

 

[Delphinus]

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."

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"!

 

[fizixfan]

Yes, but you did [sic] 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.

"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.

 

[Delphinus]

"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.

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.

 

[Bandersnatch]

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.

 

[JDL1964]

@fizixfan
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.

 

[fizixfan]

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.

 

[fizixfan]

CORRECTION: To my post #42 above:

I'm NOT sure exactly what you mean by "orbital wobble"

 

[JDL1964]

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.

 

[fizixfan]

Check out this link:
https://en.m.wikipedia.org/wiki/Barycenter
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.

 

[Dahook4444]

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.

 

[JDL1964]

Check out this link:
https://en.m.wikipedia.org/wiki/Barycenter
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.

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?

 

[phyzguy]

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.

 

[JDL1964]

@phyzguy
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. :)

 

[Dahook4444]

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 http://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".

 

[Janus]

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 http://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 present model for the spiral arms is that they are caused by density waves moving through the galaxy independently of the orbits of the stars. Stars on the inner part of the galaxy orbit faster than the density waves move and those far out orbit slower. Thus stars move in and out of the spiral arms over time. The relative brightness of the arms is caused by the increased density causing greater star formation. By the time the stars formed in the density wave leave it, the bright massive stars have died, leaving only the cool dim stars with longer lifetimes. So the brightness of the spiral arms isn't about a difference in the number of stars in the spiral arms but rather that there is a greater population of young bright stars

 

[fizixfan]

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.

I agree that the solar system doesn’t come back to its starting place in its orbit around the galaxy. It’s in a sort of loose colloidal suspension with neighboring stars, gas clouds and other interstellar material, even passing through various spiral arms in its journey around the Milky Way. But comparing its orbit to a strand in a bowl of spaghetti is pushing the analogy a bit far, I think. I’d compare it to a molecule of milk in the very top layer of a stirred cup of coffee, but even that’s inadequate.

 

[phyzguy]

Here is a recent paper on the arxiv about orbits in a potential with a bar like the Milky Way. I've pasted in Figure 3 Below, which shows the orbits over 1 Gy on
the left and 10 Gy on the right. I'll let you judge, but think my spaghetti analogy is apt..

 

[fizixfan]

The problem with a strand of spaghetti is that it's static, and doesn't convey a sense of motion. A strand of spaghetti can loop back in on itself, and follows no particular direction. Maybe a strand of spaghetti wrapped around a fork. I'm not a fan of the spaghetti strand model. If you're going for something that is static, maybe the tangled fishing line model or the ball of yarn model. I prefer the dynamic colloidal cream-in-coffee model.

 

[Drakkith]

The problem with a strand of spaghetti is that it's static, and doesn't convey a sense of motion.

Huh. It conveys a sense of motion for me. *shrug*

 

[fizixfan]

Huh. It conveys a sense of motion for me. *shrug*

This is getting seriously off-topic. In my original diagram, which took me many hours and a lot of research to complete, I wasn't thinking about whether or not spaghetti was an appropriate analogy.

I was thinking about how to portray the "Orientation of the Earth, Sun and Solar System in the Milky Way" in a 2-D diagram. It seems to please some people to pick things apart and dwell on small details.

 

[Drakkith]

It seems to please some people to pick things apart and dwell on small details.

My apologies if it seemed like I was picking apart your post. That certainly wasn't my intention.

 

[fizixfan]

My apologies if it seemed like I was picking apart your post. That certainly wasn't my intention.

No problem! Cheers.

 

[sophiecentaur]

This is getting seriously off-topic. In my original diagram, which took me many hours and a lot of research to complete, I wasn't thinking about whether or not spaghetti was an appropriate analogy.

I was thinking about how to portray the "Orientation of the Earth, Sun and Solar System in the Milky Way" in a 2-D diagram. It seems to please some people to pick things apart and dwell on small details.

I think you misinterpreted the reaction you got. When you introduce a useful contribution like your animation, you trigger a lot of thoughts in a lot of heads and you can expect all sorts of technical comments which can read like adverse criticism when they aren't. People (me too) tend to forget to compliment a contributor and that can be a bit off-putting to a newcomer to PF. (We are dealing with the Nerdy end of the market here and the niceties are often ignored; on balance, it works very well, though.)

 

[Drakkith]

(We are dealing with the Nerdy end of the market here and the niceties are often ignored;

Got a valid reference for that, sophie?

 

[sophiecentaur]

Got a valid reference for that, sophie?

Haha. There are worse places on the Web, I know.

 

[Rohan]

Thanks for the great diagrams, working back through the traditional astronomical jargon was going to be tedious!
Perhaps the diagram could be made less'busy' using an "exploded view" where the plane of the Earth-Moon system ,
is shown as a smaller part of the plane of the Solar System , it self a smaller part of the plane of the Milky-Way !

When I started googling "galactic co-ordinate system", my naive idea was for a co-ordinate system,
with origin based on the multi-million solar-mass black-hole at the'centre' of the Milky-Way Galaxy (ie ours);
Since the diameter of the event-horizon of this is less than the orbit of Jupiter ;
it is effectively a point relative to the Milky-Way's diameter of 100x10^6 light-years.
The natural co-ordinate system which suggested itself was actually a cylindrical one with the our star (Sol ?)'s;
distance to that centre as one co-ordinate, it's hieght above or below the galactic plane (or angle subtended at the origin) another;
and finally the whole co-ordinate system rotating with the Milky-Way by setting Sol's 'longitude' to zero degrees !
Perhaps I have been unconsciously influenced by "Star-Trek" with it's 'alpha-quadrant' etc ?
regards Rohan

 

[Drakkith]

When I started googling "galactic co-ordinate system", my naive idea was for a co-ordinate system,
with origin based on the multi-million solar-mass black-hole at the'centre' of the Milky-Way Galaxy (ie ours);
Since the diameter of the event-horizon of this is less than the orbit of Jupiter ;
it is effectively a point relative to the Milky-Way's diameter of 100x10^6 light-years.

Unfortunately the location of the exact center of the Milky Way is not known due to a number of issues. From wiki:

An accurate determination of the distance to the Galactic Center as established from variable stars (e.g. RR Lyrae variables) or standard candles (e.g. red-clump stars) is hindered by countless effects, which include: an ambiguous reddening law; a bias for smaller values of the distance to the Galactic Center because of a preferential sampling of stars toward the near side of the Galactic bulge owing to interstellar extinction; and an uncertainty in characterizing how a mean distance to a group of variable stars found in the direction of the Galactic bulge relates to the distance to the Galactic Center.

The supermassive black hole is almost certainly not at the center though, but probably lies a few thousand light-years off from the center. It's a bit like how the Sun isn't always the center of the solar system (as defined as the barycenter, or center of mass).

 

[Dr Wu]

That 3D TV & Film never really worked out - at least in its latest iteration - is a topic beyond the scope of this thread. Nevertheless, it would be an ideal way to represent the orientation (and motion) of the solar system in relation to the Milky Way, especially if it included 'zoom' controls. Such a fully immersive apprehension-at-a-glance technology will probably be a visual treat reserved for the next generation. Saying that, I did once undertake a slow tour of the solar system via Oculus VR, and that was tremendously impressive. Sticks in the memory, even now.

 

[fizixfan]

Thanks for the great diagrams, working back through the traditional astronomical jargon was going to be tedious!
Perhaps the diagram could be made less'busy' using an "exploded view" where the plane of the Earth-Moon system ,
is shown as a smaller part of the plane of the Solar System , it self a smaller part of the plane of the Milky-Way !

When I started googling "galactic co-ordinate system", my naive idea was for a co-ordinate system,
with origin based on the multi-million solar-mass black-hole at the'centre' of the Milky-Way Galaxy (ie ours);
Since the diameter of the event-horizon of this is less than the orbit of Jupiter ;
it is effectively a point relative to the Milky-Way's diameter of 100x10^6 light-years.
The natural co-ordinate system which suggested itself was actually a cylindrical one with the our star (Sol ?)'s;
distance to that centre as one co-ordinate, it's hieght above or below the galactic plane (or angle subtended at the origin) another;
and finally the whole co-ordinate system rotating with the Milky-Way by setting Sol's 'longitude' to zero degrees !
Perhaps I have been unconsciously influenced by "Star-Trek" with it's 'alpha-quadrant' etc ?
regards Rohan

I made this diagram using the drawing tools Microsoft Word believe it or not. The reason I made it was to avoid using a lot of words, which wouldn’t really give people a clear idea idea of what I was trying to convey. And in all honesty, I don’t really understand what you’re saying. I would suggest, if you’re trying to get your idea across, that you use pictures and drawings (worth thousands of words).

 

[ifmihai]

Amazing thread! Thank you very much for the pictures. They are great! They say a lot more than reading.

I have a question.
Is there something like an equinox, as seen from sun? When galactic center crosses sun's equator?

Hard to imagine if this is even a pertinent question, but i couldn't answer it myself. What seems to confuse me is the large spans of time and space involved.

From what i got from this thread, it appears solar system crossed galactic equator around 3Mil years ago. Was that the moment that could be considered as an equinox?

Is there such a point or area in the sky, representing this equinox moment?

Ps.
Also, to me it seems a little strange, that ~3 million years number. Kind of coincides with homo erectus location in time, more or less.

 

[rootone]

The term 'equinox' means that for Earth, both North and South hemispheres are receiving the same amount of sunlight.
This of course happens regularly for Earth, twice every year.
The galactic center does not emit any radiation that makes any difference for the circumstances of Earth, or the Sun.

 

[ifmihai]

Ok, but i wasn't asking about any radiation. I was not interested in that.
I am interested in the geometry of the situation.
I presume that wavey trajectory of the sun is waving around a center line that matches galactic disc.
If this is so,
Then sun's equatorial plane would cross galactic equatorial plane, right?
That geometry would be the same as sun-earth equinox, right?
The two planes creating an axis, or 2 nodes.

If this is so,
Then, where would this axis point to?

 

[Loki sullivan]

Is it correct in diagram 1 that the summer and winter solstice occur closest to the center of the galactic plane and equinoxs occur when the Earth is farthest from the galactic plane? If this is the case, then wouldn’t the North Celestial Pole be misrepresented in this 2d model?

 

[sophiecentaur]

Is it correct in diagram 1 that the summer and winter solstice occur closest to the center of the galactic plane and equinoxs occur when the Earth is farthest from the galactic plane? If this is the case, then wouldn’t the North Celestial Pole be misrepresented in this 2d model?

It's important to bear in mind the scale of what you are describing. The Galaxy is around 1000LY thick and the solar system is only about 30 AU across (1AU is around 1/6000 LY). So the solar system is minute in terms of the layout of the galaxy. The "Galactic Plane" is far too fuzzy to use the geometry that you seem to be using. The 'tilt' of the plane of the ecliptic relative the the galactic plane is a pretty random and irrelevant quantity.