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

[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.

 

[Loki sullivan]

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.

I was more interested in which angle the tilt of the Earth (celestial axis) is compared to the galactic plane. It would appear to tilt outward away from the galactic center more than it does north or south of the galactic plane. I assume this because the winter/summer solstice are at or near the galactic plane at this time and equinoxes are above and below the galactic plane. Does this sound correct? Just trying to give myself a relative perspective.

 

[sophiecentaur]

But, whatever it is, it's surely just arbitrary. There will be systems with an ecliptic that is almost at right angles to the galactic plane, systems that are right near the lateral edge etc. etc. so what would make their orientation of any interest at all?
This is a bit like numerology, which takes two items and tries to link them by some numbers.

 

[Bandersnatch]

I was more interested in which angle the tilt of the Earth (celestial axis) is compared to the galactic plane.

Don't the fig. 2 and fig. 3 in post #1 answer this question? The angles given are between axes or planes, but it's just a matter of subtracting them from 90 degrees if you want an angle between an axis and a plane.
In this particular case, it looks like you're looking at 90-62.9=27.1 degrees.

It would appear to tilt outward away from the galactic center more than it does north or south of the galactic plane.

I think you're describing the angle between the celestial axis (celestial north) and the galactic plane, but it's larger not smaller than the previous one (it's the 62.9 degrees from before).

If these are not the angles you mean, can you try and clarify which ones you have in mind?

 

[Janus]

Don't the fig. 2 and fig. 3 in post #1 answer this question? The angles given are between axes or planes, but it's just a matter of subtracting them from 90 degrees if you want an angle between an axis and a plane.
In this particular case, it looks like you're looking at 90-62.9=27.1 degrees.

I think you're describing the angle between the celestial axis (celestial north) and the galactic plane, but it's larger not smaller than the previous one (it's the 62.9 degrees from before).

If these are not the angles you mean, can you try and clarify which ones you have in mind?

I think the issue he has with the diagram is with where the nodes between the ecliptic and celestial planes are located. In the diagram, the Earth is shown as being near the winter solstice, but the direction the Earth's axis is shown as pointing, relative to the direction of the Sun, looks closer to what you would expect during an equinox.

 

[Loki sullivan]

I think the issue he has with the diagram is with where the nodes between the ecliptic and celestial planes are located. In the diagram, the Earth is shown as being near the winter solstice, but the direction the Earth's axis is shown as pointing, relative to the direction of the Sun, looks closer to what you would expect during an equinox.

Yes this was my intent. I may not have expressed that clearly. I was simply trying to discover the angle of the earth’s Celestial axis in relation to the direction of the travel of the sun. I may have complicated the question by using a inprecise variable such as galactic plane. I am assuming since the Earth is near galactic plane (as illustrated) at the time of summer solstice, then the axial tilt of the Earth would be in the direction of the sun and not to the galactic north. Is this a correct assumption? I’m not criticizing the great work. I am just trying to clarify my understanding of the earth’s orientation in relation to the sun’s direction of travel.

 

[Bandersnatch]

I am assuming since the Earth is near galactic plane (as illustrated) at the time of summer solstice, then the axial tilt of the Earth would be in the direction of the sun and not to the galactic north.

Ah. You're right. Well spotted. It does look like it's pointing in the wrong direction, and should indeed be deflected towards the reader rather than in the plane of the picture.
Maybe @fizixfan will stop by and take a shot at correcting it. Although I imagine it might be difficult to render it clearly in two dimensions.

 

[fizixfan]

I am assuming since the Earth is near galactic plane (as illustrated) at the time of summer solstice, then the axial tilt of the Earth would be in the direction of the sun and not to the galactic north.

I've been following this thread since Loki sullivan started posting, but I've been unable to fully understand what's being said. This may not answer your question, but the Earth's Axis of Rotation is tilted "away" from the Sun in the Winter, which is why it gets colder in the Northern Hemisphere during winter (you probably already know that), and tilted "toward" the Sun in the Northern Hemisphere in the Summer. The Earth is also situated between the Sun and the Galactic Center in the summer (Sun - Earth - Galactic Center). That's why we have such great viewing of the Milky Way in the summer months - because the Sun isn't in the way at night and we're looking toward the Galactic Center. In winter the Sun is between the Earth and Galactic Center (Earth-Sun-Galactic Center), so no viewing of the Milky Way in Winter from the NH.

The Earth, in a physical sense, is about 50 light years north of the Galactic Equator, so change in position of 186 million miles from over the course of a year isn't going to change our position with respect to the Galactic Plane. What does change is our point of view, since we are on a tilted, spinning sphere orbiting the sun.

The axial tilt of the Earth does not vary relative to the Galactic North on a seasonal basis or even during a human lifetime - although it does precess (Google that term if necessary). But for the purposes of this discussion let's just say the North Celestial Pole (Earth's Axis of Rotation) and the North Galactic Pole (Milky Way's Axis of Rotation) do not vary. The angle between the NCP and NGP is 62.9°, although this can only be determined using spherical trigonometry, since the Celestial Equator, Ecliptic Plane and Galactic Plane do not intersect at a single point. See Figures 2 and 3 in my original post. I'm including another crude drawing I did that may help. Words are really kind of hopeless in explaining three different celestial coordinate systems - which is why I prefer pictures.

It might also help if you read all the posts in this thread, but especially, I would encourage you to go to a place where there are dark skies during the summer months and look up. I recently purchased a telescope for my camera, and have found some amateur astrophotographers to hang out with. I would also recommend uploading Stellarium - a free planetarium program for your PC, and Skywatch for you mobile device.

 

[Loki sullivan]

I've been following this thread since Loki sullivan started posting, but I've been unable to fully understand what's being said. This may not answer your question, but the Earth's Axis of Rotation is tilted "away" from the Sun in the Winter, which is why it gets colder in the Northern Hemisphere during winter (you probably already know that), and tilted "toward" the Sun in the Northern Hemisphere in the Summer. The Earth is also situated between the Sun and the Galactic Center in the summer (Sun - Earth - Galactic Center). That's why we have such great viewing of the Milky Way in the summer months - because the Sun isn't in the way at night and we're looking toward the Galactic Center. In winter the Sun is between the Earth and Galactic Center (Earth-Sun-Galactic Center), so no viewing of the Milky Way in Winter from the NH.

The Earth, in a physical sense, is about 50 light years north of the Galactic Equator, so change in position of 186 million miles from over the course of a year isn't going to change our position with respect to the Galactic Plane. What does change is our point of view, since we are on a tilted, spinning sphere orbiting the sun.

The axial tilt of the Earth does not vary relative to the Galactic North on a seasonal basis or even during a human lifetime - although it does precess (Google that term if necessary). But for the purposes of this discussion let's just say the North Celestial Pole (Earth's Axis of Rotation) and the North Galactic Pole (Milky Way's Axis of Rotation) do not vary. The angle between the NCP and NGP is 62.9°, although this can only be determined using spherical trigonometry, since the Celestial Equator, Ecliptic Plane and Galactic Plane do not intersect at a single point. See Figures 2 and 3 in my original post. I'm including another crude drawing I did that may help. Words are really kind of hopeless in explaining three different celestial coordinate systems - which is why I prefer pictures.

View attachment 227104

It might also help if you read all the posts in this thread, but especially, I would encourage you to go to a place where there are dark skies during the summer months and look up. I recently purchased a telescope for my camera, and have found some amateur astrophotographers to hang out with. I would also recommend uploading Stellarium - a free planetarium program for your PC, and Skywatch for you mobile device.

Let me first say, thank you for sharing your work. I have learned much from your illustrations. As far as my inquiry, I hope you understand I am not criticizing your work but seeking clarification for my own understanding. I DO understand the axial tilt of the Earth and its affect on seasonality. My question was referring to the direction of axial tilt of the earth. In the illustration, the angle of the Earth axis in relation to galactic plane is apparent, but the axial tilt direction appears to be towards the sun when near the winter solstice. If I am understanding that the northern hemisphere is shown to the left, then wouldn’t the direction of axial tilt be away from the sun at winter solstice? It may just be a misinterpretation due to the nature of 2D renderings, so I ask for clarification for my own understanding.

 

[fizixfan]

Let me first say, thank you for sharing your work. I have learned much from your illustrations. As far as my inquiry, I hope you understand I am not criticizing your work but seeking clarification for my own understanding. I DO understand the axial tilt of the Earth and its affect on seasonality. My question was referring to the direction of axial tilt of the earth. In the illustration, the angle of the Earth axis in relation to galactic plane is apparent, but the axial tilt direction appears to be towards the sun when near the winter solstice. If I am understanding that the northern hemisphere is shown to the left, then wouldn’t the direction of axial tilt be away from the sun at winter solstice? It may just be a misinterpretation due to the nature of 2D renderings, so I ask for clarification for my own understanding.

It's the nature of the 2D rendering, I'm afraid.

 

[alantheastronomer]

Looking at the Sun's direction of travel arrow on fizixfan's 'Motion of Earth & Sun around the Milky Way' diagram, does this mean then that the solar system is orbiting the Galaxy in a clockwise direction? Also, which hemisphere of Earth's is (mostly?) facing the direction the solar system is taking during its orbit round the galactic disc? North or South? Or do they each take turns during the course of a terrestrial year? I can't quite put it all together it somehow.

You'll be interested to know that, since the Earth's equator is inclined so steeply (60.2 degrees) to the plane of the Milky Way, that there are features of the Milky Way, such as the Galactic Center and Galactic Bulge, that can only be seen from the Northern Hemisphere, and features such as the Coalsack Nebula and the small and large Magellanic Clouds, that can only be seen from the Southern Hemisphere!

 

[davenn]

that there are features of the Milky Way, such as the Galactic Center and Galactic Bulge, that can only be seen from the Northern Hemisphere

you sure about that ?

the galactic centre is in Sagittarius which comes well above the horizon in the southern hemisphere.
In fact at this time of the year, it passes right overhead
Maybe you take a trip "down under" one day and I and some other Australian astronomers will treat you to the delights of the southern skyDave

 

[davenn]

there you go one of my own pix looking straight into the core complete with bulge

Dave

 

[alantheastronomer]

Obviously I'm mistaken - Thanks for letting me know! I guess I'm not as familiar with the southern hemisphere as I'd like to be!

 

[Janus]

You'll be interested to know that, since the Earth's equator is inclined so steeply (60.2 degrees) to the plane of the Milky Way, that there are features of the Milky Way, such as the Galactic Center and Galactic Bulge, that can only be seen from the Northern Hemisphere, and features such as the Coalsack Nebula and the small and large Magellanic Clouds, that can only be seen from the Southern Hemisphere!

Here's the relationship between the Celestial Equator, Galactic Equator, and Ecliptic.

The white grid are celestial coordinates and the light blue grid galactic coordinates.
Note that the Galactic Center is ~29 degrees South of the Celestial equator. This means that it is visible in the entire Southern Hemisphere and not visible for points above ~61 degrees North Latitude. The ecliptic crosses the Galactic equator ~6.5 degrees from the Galactic center and the Celestial equator crosses the galactic equator 33 degrees from the galactic center along the galactic equator.

 

[alantheastronomer]

Thanks, Janus - Are there any parts of the Milky Way that aren't observable from the Southern Hemisphere?

 

[davenn]

Are there any parts of the Milky Way that aren't observable from the Southern Hemisphere?

Not really, as you can see from my pic. you can see it from one thin end on the left, through the centre and out to the thin end on the right side

What we don't see from the southern hemisphere are the constellations that are well away from the galactic plane ... ie. close to the north celestial pole

Ursa Major and minor, Cassiopeia, Draco, Cepheus, Hercules ... to name a fewDave

 

[Ovidiu]

Hello

Thanks for all the posts. I would like to ask for some clarifications, if possible.
Here some images:

So, from Europe during summer, if I look at SagittA, the solar system is moving toward left (as do all the spiral arms). So, if I take a view-angle like in the second image, and clear my perception from local stars, voila, the "clouds" remain. They are in fact "curved", but we see them from inside the "tunnel", so to say. But, I can almost say that the curvature is perceivable, after I have trained my attention.

This is what I am uncertain of:

1. What do we really see in the left side of the Milky Way, are these light clouds the spiral arms?
2. If we move approx. toward Vega, how is that the SagittA and Vega are so close to each other in the sky? I suppose this is because Vega is so close, and galactic center is so so far away?

Thank you

Ovidiu

 

[Bandersnatch]

Hi, Ovidiu. Welcome to PF.

2. If we move approx. toward Vega, how is that the SagittA and Vega are so close to each other in the sky? I suppose this is because Vega is so close, and galactic center is so so far away?

They're roughly 70 degrees apart. Sgr A and the nearby Deneb are full 90 degrees apart. Deneb roughly points to the direction at a right angle with the direction towards the centre of the galaxy. 70 degrees is not that far from being half way through to the other side of the sky from Sgr A, so I'm not sure it counts as being close to each other.
If you're only looking at it through a planetarium software, then the separations on the sky may appear warped, especially if you zoom out. Try going out at night, and pointing towards both with each arm. Note the angle your arms make.

The radial distance to Vega vs Sgr A doesn't play any part in their angular separation. If you imagine looking at Vega, and then - in your mind - pushing it ten or a hundred times farther along the line of sight, it would not move from the same point in the sky. So distance can't play a role.

Btw, the difference between 90 degrees towards Deneb and 70 towards Vega comes from Sun's peculiar motion - i.e., its local velocity, unrelated to its orbital velocity.

Also, keep in mind these are not precise directions. Best to think of Cygnus/Lyra/Hercules as a 'roughly there' direction of motion, rather than pointing to anyone star as the direction.

 

[Ovidiu]

Hello Bandersnatch

Thank you for your clarifications. Indeed, I perceive a difference, but seemed less than 70 degrees as I remember from last nights.
This is what I see, then? (just for orientation)

Looks like I have to wait until next clear night to look at this angle again, but I do remember this angle as 45 sort of, that's why it puzzled me.

Now, regarding my other question, about what we see.
I still cannot visualise this fact - so, we are moving toward Deneb-Vega
that means
- left of this area is just Perseus Arm? or?
- and in the right of Vega, then those "clouds" are in fact... 4-5-6 layers over layers?

I wonder if there is somewhere a scientifically correct image, using the perspective as in the image above?
I need to perceive-understand what I see, with arrows to show more reference points regarding the "spiral arms name", and also to show "the curvature" and the "layering" and the "Sun direction of movement"?

This is the best video I found, but it doesn't show the curvature of the spiral arms


Thank you
O.

 

[Bandersnatch]

I'm having a bit of a hard time understanding your pictures.
For example, in the first picture in your post #89, the approx 70 degree angle is between the directions that you marked with the wide red arrow towards the galactic centre, and the thin red arrow towards Vega. Not the much smaller angle between the two blue lines - which I'm not sure what are indicating.
The angle between the centre and Deneb should be a right angle.

Judging by how you've superimposed the image of the galactic arms onto the snapshot of the sky in the second picture in that post, it looks like you're making the mistake of not placing yourself inside the galaxy. You would not see all the arms (the entire galaxy) contained within the span of one night-sky (<180 degrees), since you are observing it from the inside. You'll see the galaxy all around you - including the parts of the sky below the horizon.

The band of the Milky Way is made of stars and dust contained mostly in the arms, but it is unlikely you'll be able to discern which arm you're seeing. In rough terms, if you look away from the galactic centre, you see the material in our arm, and in the Perseus arm. Looking towards the galactic centre, you'll see some combination of material in all the arms along your line of sight. There is no real way of separating material from one arm from another, just by eyeballing it, while sitting inside the galactic disc.

I'm still not sure we're not talking past each other, and if we do, then I apologize.