Galactic Coordinate System

[KAL 9000]

Let's say you're an advanced interstellar civilization. You need a galactic coordinate system to plot courses and know where everything is. Here's my idea:

(X,Y,Z) where X, Y, and Z are any number. X is forwrard-backward, Y is left-right, and Z is up-down. To determine galactic forward, backward, left, and right, imagine you are at the exact center of the Galaxy. You are facing the direction of the Galaxy's motion, relative to the Local Group (adjusted galactic up or down so it's in the plane of the galactic disk). That direction is galactic forward. To your left is galactic left, and to your right is galactic right. To your back is galactic backwards. The galactic disk divides the sphere with a radius of the Galaxy's dark matter halo into two hemispheres. Whichever hemisphere the Galaxy's non-adjusted motion is is galactic up (or at least the imaginary line going through it that is perpendicular to the plane of the Galaxy is), and vice verse for galactic down. Sag A*, the black hole in the center of the Galaxy, has coordinates (0,0,0). One coordinate "unit" is 500 light-years, i.e. a planet at coordinates (1,0,0) is 500 light-years from Sag A*. Just wondering, could anyone use this coordinate system to find where Earth and a few notable exoplanets are?

[Kerbart]

Putting the origin of your system in the center is a bad idea.

You're much better off putting the origin outside the system. Now you don't have to deal with dropping minus signs by accident, as all coordinates will be positive. Further more, you want to select the origin in such a way that x, y and z can never be confused (e.g. z is in the 0-1000 range, x in the 2000-15000 range, y in the 20000-35000 range or whatever).

That's how some countries choose their national coordinate system these days*

 

* with the origin outside their country, not outside the galaxy, just to be clear on that.

[KAL 9000]

21 minutes ago, Kerbart said:

Putting the origin of your system in the center is a bad idea.

You're much better off putting the origin outside the system. Now you don't have to deal with dropping minus signs by accident, as all coordinates will be positive. Further more, you want to select the origin in such a way that x, y and z can never be confused (e.g. z is in the 0-1000 range, x in the 2000-15000 range, y in the 20000-35000 range or whatever).

That's how some countries choose their national coordinate system these days*

 

* with the origin outside their country, not outside the galaxy, just to be clear on that.

With advanced computers (this is an interstellar civilization), accidentally dropping minus signs probably won't happen. That's your only problem? Yay, I did a good job!

[cantab]

In real life we use spherical polar co-ordinates. One distance from Earth, two angles to give the direction. That could work well for an interstellar civilization I think. At least in the relatively early days the homeworld is likely to be important, and distance from it a noteworthy figure.

An alternative I'm partial to galactic cylindrical coordinates. One distance which is how far you go in the plane of the galactic disk, an angle for the direction that distance is in (with 0 degrees being towards the galactic core), and a second distance which is how far you go at right-angles to the galactic disk. This system has a couple of benefits - it's more immediately obvious whether a location is in the disc or in the halo, and an easy-to-understand map can be made by just plotting the first two numbers; not precise enough for navigation but suitable for teaching and general display.

Of course any polar system, spherical or cylindrical, has a couple of drawbacks. Giving the angles to a fixed precision means the position is less certain the further from the origin the point is, and calculating the distance between two coordinates is rather harder.

Regardless of the details, I think an obvious choice of reference direction is the line linking the homeworld and the galactic centre, and an obvious choice of reference plane is the galactic disc or something chosen to nearly match it.

Edited December 21, 2015 by cantab

[Temstar]

I've seen it defined this way:

Set the origin of your coordinate system as the singularity of the supermassive black hole at the galactic center.
Set your homeworld (say, Sol) as the zero degree line.

Then all other stars can be defined using three numbers:

1. Distance (in the length unit of your choice) from the center singularity
2. Angle relative to your homeworld, eg "the Susan System is 23 degrees, 47 minutes and 26 seconds counter-spinwards" (relative to your homeworld)
3. Distance from the galactic plane (above or below, with above as defined as looking down at the galaxy and see spin in clockwise direction), where the galactic plane is defined as the disk where the highest concentration of the galaxy's mass is located. Or alternatively the plane of your home system's orbit around the galaxy, if it's relatively close to the plane of the highest mass concentration.

Due to stellar drift the accuracy of the coordinates will graduate degrade so they have to be updated from time to time. It's probably not a serious problem though since our own GSP coordinate also degrade over time due to continental drift, but we can live with it.

Still, it won't work well for extreme cases like S2, but I suppose those stars are not so interesting anyway as they are in a region that would be extremely hostile to life.

Edited December 21, 2015 by Temstar

[PB666]

3 hours ago, KAL 9000 said:

Let's say you're an advanced interstellar civilization. You need a galactic coordinate system to plot courses and know where everything is. Here's my idea:

(X,Y,Z) where X, Y, and Z are any number. X is forwrard-backward, Y is left-right, and Z is up-down. To determine galactic forward, backward, left, and right, imagine you are at the exact center of the Galaxy. You are facing the direction of the Galaxy's motion, relative to the Local Group (adjusted galactic up or down so it's in the plane of the galactic disk). That direction is galactic forward. To your left is galactic left, and to your right is galactic right. To your back is galactic backwards. The galactic disk divides the sphere with a radius of the Galaxy's dark matter halo into two hemispheres. Whichever hemisphere the Galaxy's non-adjusted motion is is galactic up (or at least the imaginary line going through it that is perpendicular to the plane of the Galaxy is), and vice verse for galactic down. Sag A*, the black hole in the center of the Galaxy, has coordinates (0,0,0). One coordinate "unit" is 500 light-years, i.e. a planet at coordinates (1,0,0) is 500 light-years from Sag A*. Just wondering, could anyone use this coordinate system to find where Earth and a few notable exoplanets are?

The problem here is that the stars in the galaxy orbit at different angular velocities.

The issue is that your system is not physics based, the angular reference has to be a vector reference frame that if everything is at zero velocity they fall into the GBH.

I would argue that zero angle should be the a plane section that starts of the point of inflation through the gbh, but since the origin is uncipherable, the I woukd chose the mass center of say those seven pulsars, it could also be the gbh of andromeda, but this is subject to future problems as the two galaxies. Now that i have a zero point line, a comoving space-time based on the gbh, then every non-nertial body can have an x,y,z,t relative to the gbh along with an dx, dy, dz per dt. Of course this is a rough system in which objects move at 100,000s of meters per second, and another roughness is that the plane of the galaxy which zeros the z axis is also an estimate. Therefor it would predominantly be used to centering the oosition of local groups stars of the clusters would be have to be mapped according to that reference frame, obviously is angular reference and z plane as the same as the galazy. 

[KAL 9000]

7 minutes ago, PB666 said:

The problem here is that the stars in the galaxy orbit at different angular velocities.

The issue is that your system is not physics based, the angular reference has to be a vector reference frame that if everything is at zero velocity they fall into the GBH.

I would argue that zero angle should be the a plane section that starts of the point of inflation through the gbh, but since the origin is uncipherable, the I woukd chose the mass center of say those seven pulsars, it could also be the gbh of andromeda, but this is subject to future problems as the two galaxies. Now that i have a zero point line, a comoving space-time based on the gbh, then every non-nertial body can have an x,y,z,t relative to the gbh along with an dx, dy, dz per dt. Of course this is a rough system in which objects move at 100,000s of meters per second, and another roughness is that the plane of the galaxy which zeros the z axis is also an estimate. Therefor it would predominantly be used to centering the oosition of local groups stars of the clusters would be have to be mapped according to that reference frame, obviously is angular reference and z plane as the same as the galazy. 

I did not understand what you just said.

[YNM]

@PB666 : OP is trying to make a coordinate that'll work for any layman in the galaxy, not just for those interested in the physics.

@ OP : A cylindrical coordinate should be better. While the "longitude" component can be problematic, after all, you _do_ expect them to be problematic out of galactic rotation, which is not a rigid body rotation (so, no reason to say that galaxy is best modeled after rigid disk). Not counting systems where inclination plays in and their height in the cylinder sloowly changes.

 

(oh really, bring back bb codes, the old editor ! WYSIWYG editor works ridiculously bad in mobile for me)

 

 

[kunok]

4 hours ago, YNM said:

@PB666 : OP is trying to make a coordinate that'll work for any layman in the galaxy, not just for those interested in the physics.

@ OP : A cylindrical coordinate should be better. While the "longitude" component can be problematic, after all, you _do_ expect them to be problematic out of galactic rotation, which is not a rigid body rotation (so, no reason to say that galaxy is best modeled after rigid disk). Not counting systems where inclination plays in and their height in the cylinder sloowly changes.

 

(oh really, bring back bb codes, the old editor ! WYSIWYG editor works ridiculously bad in mobile for me)

 

 

This, a Cylindrical coordinate system, the origin being the core of the galaxy, the origin of the longitude axis being the galactic plane. Other option it's what @cantab says, use Earth as the origin of the coordinate system, more useful for a "local" human expansion

[Kerbol Macrosystems]

14 hours ago, KAL 9000 said:

I did not understand what you just said.

He's talking about inertial vs. Non-inertial reference frames. Your coordinate system moves with the local group, making it a non-inertial reference frame. In order to predict where things will be we would need to invent some imaginary forces to counter-act the motion of your coordinate frame.

This is similar to what happens if I try to do calculations centered on a spacecraft in orbit. I need to invent centrifugal force to explain why the spacecraft doesn't fall down to the ground. 

We typically prefer inertial reference frames because it simplifies the physics calculations. This is a coordinate system that doesn't move. It's easier to account for the motion of all bodies in the inertial system. 

[PB666]

13 hours ago, YNM said:

@PB666 : OP is trying to make a coordinate that'll work for any layman in the galaxy, not just for those interested in the physics.

@ OP : A cylindrical coordinate should be better. While the "longitude" component can be problematic, after all, you _do_ expect them to be problematic out of galactic rotation, which is not a rigid body rotation (so, no reason to say that galaxy is best modeled after rigid disk). Not counting systems where inclination plays in and their height in the cylinder sloowly changes.

 

(oh really, bring back bb codes, the old editor ! WYSIWYG editor works ridiculously bad in mobile for me)

 

 

If you are taveling between local groups you better understand the physics of space time you are travelingin, different local groups age at different rates. An example is that you need to travel between stars at say 1/100th speed of light,mfor this we imagine that the travelers enter stasis at their destination they awake. They are traveling to a planet in a star in a local group. It leaves 1 month before it scheduled arrival the crew is awakened, howver because it did not take time dilation effects cause by differences and changes in velocity due to orbits within local groups and galaxies the shipbarrives at the wrong place and months two early,mthe crew die as thier fuel is exhausted and the run out of food and water in the lifeless emptyiness of interstellar space. 

[Bill Phil]

Polar coordinates!

But it'll likely be spinward/antispin and coreward/anticoreward, plus a vertical system of some sort.

[Findthepin1]

How would you know where an object is in the coordinates? Stars move, planets move. If two people attempt to teleport to the Earth's coordinates at a specific time, but one person does it one second late, they'll be hundreds of kilometers from their destination point.

Edited December 30, 2015 by Findthepin1

[MarvinKitFox]

4 minutes ago, Findthepin1 said:

How would you know where an object is in the coordinates? Stars move, planets move. If two people attempt to teleport to the Earth's coordinates at a specific time, but one person does it one second late, they'll be hundreds of kilometers from their destination point.

Space coordinates for moving stuff is easy, as long as the movement follows some predictable pattern. (such as orbits around the sun, for local stuff, orbit around the galactic center for stars, etc..)

 

You need 3 coordinates for WHERE it is

You need 3 coordinates for MOVEMENT where it is headed

You need one coordinate for WHEN this data was valid

 

All the rest can be extrapolated, although you may need a *lot* of contextual info about surrounding object too, to make any sensible future prediction come out accurately.

[Pixel of Life]

On 12/22/2015 at 3:26 AM, Bill Phil said:

Polar coordinates!

But it'll likely be spinward/antispin and coreward/anticoreward, plus a vertical system of some sort.

 

How about:

• 0 to 180 degrees spinward or antispinward, zero being the direction in which the galaxy is moving.
• Distance from the galaxy's center, in any unit of distance.
• Distance from the galactic plane, again in any unit of distance. Positive values are in the direction from which the galaxy appears to rotate clockwise, negative values are in the opposite direction.

Not sure how that last one would work for irregular galaxies.

 

E: Assuming irregular galaxies also have a black hole somewhere (Do they? I don't know) and that said black hole is rotating, the "reference plane" would be perpendicular to the black hole's axis of rotation and its center point would be at the center of the black hole.

Edited December 30, 2015 by CaptainKorhonen

[Mazon Del]

Incidentally, this topic is likely to be relevant in the next decade or so, not because we will be navigating outside of the solar system any time soon, but because of NASA's work on the annoyingly named "GPS" or Galactic Positioning System. This setup works much like the normal GPS (Global positioning system), except that it uses pulsars as the "satellites". In theory, if you have really fine azimuth and elevation detection, you can do trig on the positions of various known pulsars (all of which have distinctive features about their signals, so you can tell one from another) to find your position. Last I heard, they are confident that with today's technology we can get the position localization down to within an inch. The cool part is that it doesn't matter how far away from the pulsars you are provided that you can see any 3+ cataloged pulsars. IE: If the catalog of pulsars includes several in other galaxies (it does), then the system is equally valid in those galaxies relative to any origin of your choice. Of course, if you DO get FTL engines working, then you are going to need to do something to account for the "sudden drift" of the stars positions given light lag.

Edited December 31, 2015 by Mazon Del