Galactic Coordinate Question

[ThunderAeroI]

If you set 0 degrees heading to be the direction the galaxy is headed towards "The Great Attractor" which is itself out towards the Hyrda constellation, and this area itself lies within the "Zone of Avoidance" what degree heading would the earth be in such a coordinate system?

I'm thinking it would be somewhere on the backend of the galaxy around 180 degrees but does someone with more knowledge have a better guess?

Its not homework, it is for a short story I am writing and I need the coordinates to better answer a plot question I am working on.

Thank you.

 

[Crazy Eddie]

I'm thinking it would be somewhere on the backend of the galaxy around 180 degrees but does someone with more knowledge have a better guess?

My guess is you're writing a fiction story, in which case you might as well just make something up because 99% of your readers won't know the difference and the 1% that does won't even care.

This is a scientific concept so esoteric that you can literally write it however the plot needs it to be and you'd get away with it just fine.

 

[Christopher]

My guess is you're writing a fiction story, in which case you might as well just make something up because 99% of your readers won't know the difference and the 1% that does won't even care.

This is a scientific concept so esoteric that you can literally write it however the plot needs it to be and you'd get away with it just fine.

On the contrary, science fiction has a lot of readers who are scientifically savvy and who appreciate it when writers take the time to do their research, and who notice when they get it wrong. There's a whole subgenre of hard science fiction that's all about getting it right, or as close as possible, and readers of that genre certainly do care. If you're writing for that market, then you're definitely better off doing the research.

If you set 0 degrees heading to be the direction the galaxy is headed towards "The Great Attractor" which is itself out towards the Hyrda constellation, and this area itself lies within the "Zone of Avoidance" what degree heading would the earth be in such a coordinate system?

I'm thinking it would be somewhere on the backend of the galaxy around 180 degrees but does someone with more knowledge have a better guess?

I can't fault your reasoning. The galactic center's in Sagittarius, so Hydra and Centaurus (the area of the Great Attractor) would be just a bit north of the galactic plane and a little off to the side.

It's tricky to get specific, though. Since we're moving along with the rest of the galaxy, it's not easy to compute its direction of motion precisely, and there's some uncertainty there. The best estimate I can find is that it's toward the Norma Cluster (Abell 3627) at RA 16h 15m 26.7s and dec -60° 53' 17". We can treat the difference in angle between our POV and the galactic center as trivial on this scale. The galactic center is at RA 17h 45m 40.04s and dec -29° 00' 28.1". That gives an angular separation of roughly 35.2534 degrees, or 35° 15' 12.24". But if we look at it from the galactic center's POV, we'd be in the opposite direction, so we need to subtract that from 180°, giving 144° 44' 47.76".

That just tells you the raw angle between the Galaxy's direction of motion and the vector connecting earth to the galactic center, and I'm not sure it would be enough to specify its position with only one angle. You'd need to split that into a right ascension and declination relative to your coordinate axis. But as it happens, the Norma Cluster is just about in the plane of the Milky Way -- actually about 7 degrees off the plane. I'm not really sure how to use that to correct that figure above; I think the angle within the galactic plane would be a little bit smaller, but I'm not sure of the formula. But 7 degrees is small enough that it wouldn't make too much difference, and there's a pretty large error in the actual galactic direction of motion. If you gave it as something in the ballpark of 140 degrees (or 9 h 20 m) by 7 degrees, that might work.

 

[Crazy Eddie]

My guess is you're writing a fiction story, in which case you might as well just make something up because 99% of your readers won't know the difference and the 1% that does won't even care.

This is a scientific concept so esoteric that you can literally write it however the plot needs it to be and you'd get away with it just fine.

On the contrary, science fiction has a lot of readers who are scientifically savvy and who appreciate it when writers take the time to do their research, and who notice when they get it wrong.

"I've suffered for my art and now it's your turn."

How many science fiction readers actually research the plot points of the books they're reading for scientific accuracy? And in this case, in dealing with an unbelievably subtle aspect of astronomy that most people have never even heard of, the answer can be as convenient as plot contrivance.

Just saying.

 

[B.J.]

How many science fiction readers actually research the plot points of the books they're reading for scientific accuracy?

You're asking that on *this* board?!?

A lot of SF fans will research the science, especially if it piques their interest a bit. And if it proves to be popular, you can bet that it will get researched. Case in point, Ringworld. Niven actually had to address science/engineering errors people found in the first book in his second.

BTW, the webcomic Schlock Mercenary does an amazing job at getting the science right!

 

[Crazy Eddie]

And Ringworld was only one of the most popular science fiction stories ever written; even then, fewer people noticed the errors than are actively posting on this board.

But Larry Niven, being the ascendant demigod of scifi that he is, made the fix to keep their heads from imploding.

 

[rhubarbodendron]

If you set 0 degrees heading to be the direction the galaxy is headed towards "The Great Attractor" which is itself out towards the Hyrda constellation, and this area itself lies within the "Zone of Avoidance" what degree heading would the earth be in such a coordinate system?

I'm thinking it would be somewhere on the backend of the galaxy around 180 degrees but does someone with more knowledge have a better guess?

Wouldn't the planet's coordinate change constantly? After all our planet rotates, our sun rotates and the whole galaxy does so as well. That would lead to us being only occasionally at 180°, relatively to the general direction in which our galaxy moves.

Very generally speaking, we are on the inner side of a spiral arm in the outer third of the galaxy, currently at the back right, about 140°-150° from the direction in which the galaxy moves. (Basically, we're on the Galaxy's right buttock )

 

[Christopher]

A lot of SF fans will research the science, especially if it piques their interest a bit. And if it proves to be popular, you can bet that it will get researched. Case in point, Ringworld. Niven actually had to address science/engineering errors people found in the first book in his second.

Exactly. Know your audience. A lot of people out there do want the science to be right, do know enough about science to be disappointed or frustrated when fiction gets it wrong, and hard SF is written for them. Hal Clement, one of the great hard-SF writers, liked to say that hard SF was a friendly competition between the writer, who tried to construct a plausible and consistent fictional world, and the readers, who tried to find the flaws and inconsistencies in that world. No, that's not how most SF fans (particularly not mass-media SF fans) approach it, but there are definitely people who do, and authors have been writing hard SF for such audiences since the days of Jules Verne.

Wouldn't the planet's coordinate change constantly? After all our planet rotates, our sun rotates and the whole galaxy does so as well. That would lead to us being only occasionally at 180°, relatively to the general direction in which our galaxy moves.

As I interpret the original question, the idea is to place the Sol system in a coordinate system defined with the center of the galaxy as its origin point and the galaxy's direction of motion as its axis. In other words, a system that isn't Earth-centric but more objective and universal. It takes about 200-250 million years for the Sun to orbit the center of the galaxy, so it's going to take a century and a half or more for the Sun's coordinates in such a galactic system to change by even one second of arc. And the diameter of the Earth's orbit is about half of a billionth (or one two-billionth) of the Sun's distance to the galactic center, so the Earth's orbital motion is going to have no significant effect on its galactic coordinates.

 

[ThunderAeroI]

Thank you Christopher that was very helpful. What I find totally odd about the current galactic coordinate system is that it is so earth centric when such a process would seem to do more harm to understanding our environment then helping it.

The thought experiment here is that if the galaxy has its own coordinate system where x,y,z all meet at Sig A* and 0 degrees is the direction of total galactic movement then at any given time the value of (x,y,z) will be different as they do rotate the galactic body. The actual relationships between stars does not change 'much' due to the way Dark Matter affects 'planetary' movement around the galactic core. The principal is the same as GPS, GPS is always changing due to Plate techtonics but the rate is understood and can be countered - the same would be true more or less for solar system movement through the galaxy. Some base Quasar or 'thing' is used to calibrate the relationships every so many 'cycles' and the system stays in order.

As said the Sun will come back to 144 in 200 to 250 million years but our orbit isn't on an even plane with the galactic center plane and in fact moves 'freely' up and down 2.7 times per complete orbit of the galaxy. The total distance covered by the sun moving around the core isn't totally based on top POV circumference as there is movement up a hill and then down a hill to consider.

 

[rhubarbodendron]

Good point, Christopher. That's exactly what I tried to cover in the second paragraph of my post (the one you omitted to quote), where I made an attempt to explain the situation not from the astrophysical/mathematical point of view but from a common sense /layperson angle:

Very generally speaking, we are on the inner side of a spiral arm in the outer third of the galaxy, currently at the back right, about 140°-150° from the direction in which the galaxy moves. (Basically, we're on the Galaxy's right buttock )

 

[ThunderAeroI]

On the same token, has there been any research into how the movement of the galaxy would impact the rotation of the bodies into the galaxy? For example, are they able to determine rotation of the bodies based on red and blue shift?

would the blue and red shift be incorrect since everyone is rotating relative to each other?

 

[Christopher]

On the same token, has there been any research into how the movement of the galaxy would impact the rotation of the bodies into the galaxy? For example, are they able to determine rotation of the bodies based on red and blue shift?

would the blue and red shift be incorrect since everyone is rotating relative to each other?

I don't think it matters. Everything in the galaxy is orbiting its center of mass. The whole thing as an aggregate is moving toward the Great Attractor, but you can easily enough set the galactic center as the origin of your coordinate system and then it's the GA that's drifting toward us. All motion is relative.

And by the same token, there is no "incorrect" red or blue shift, because those are not absolute quantities. They're measures of how something is moving relative to Earth, at least the radial component of its motion. It's one of the data points we take into consideration in extrapolating the motions of stars, galaxies, etc., but it's not the whole picture all by itself.

 

[rhubarbodendron]

it's the red and blue shift (Doppler Effect), that gave us an idea in the first place of how fast an in which direction the galaxy rotates and exactly where our current position is, relatively to other objects in it and to the galaxy's center.