What's up, down, front, behind, left and right in the map?

[Shinji Ikari]

So we all know that Sol is the origin of our coordinate system in Elite, after all it's in the coordinates 0,0,0 but a good question that has been burning my head is, what other systems or zones in the Milky Way did they use to define the direccion of the coordinates?

An answer that I've thought of is that maybe they just set up Sol as 0,0,0 as we all know and then specify 6 arbitrary points all perpendicular to each other to define the coordinate system and from it build the procedural galaxy and then inside those coordinates just insert the real systems that we know of in real life.

 

[Jackie Silver]

Galactic coordinates for direction align the zero latitude plane with the plane of the galaxy, and align zero longitude with the centre of the galaxy (i.e. Sadge). Longitude goes round anticlockwise because... well, it just does.

In Cartesian Elite terms, if the coordinates are written (x,y,z) the xz plane is equivalent to the galactic longitude plane and z is perpendicular to that plane.
(I wouldn't have chosen that way of writing them; I would have used (x,y,z) with the xy plane equivalent to galactic longitude. Opinions vary on that one!)

Hand-authored stars are generally very close to their galactic coordinates in terms of direction (it's easy to measure the position of something in the sky) but can be far from their expected distance (it's not so easy to measure how far away something is.) On top of that, there are a number of oddities in the way stars were mapped from catalogues into the Forge, so some can be in unexpected directions and distances.
(Similarly it's easy to measure a star's spectrum and apparent brightness, but things like mass and radius and absolute brightness are less certain.)

To make things more complicated star positions are often given in RA / Dec coordinates (which is the easiest way of finding them in Earth's night sky), and translating those into galactic coordinates and into Cartesian galactic coordinates can be a pain. OTOH you can get star catalogues from places like Vizier and they will often include both RA / Dec and galactic coordinates.

 

[PanTast]

...
Longitude goes round anticlockwise because... well, it just does.
...

Anticlockwise because this is mathematically positive.

If you look from Sol to the core, you are looking towards the north. Right is east, left is west and ... you get it.

o7

 

[Shinji Ikari]

Galactic coordinates for direction align the zero latitude plane with the plane of the galaxy, and align zero longitude with the centre of the galaxy (i.e. Sadge). Longitude goes round anticlockwise because... well, it just does.

In Cartesian Elite terms, if the coordinates are written (x,y,z) the xz plane is equivalent to the galactic longitude plane and z is perpendicular to that plane.
(I wouldn't have chosen that way of writing them; I would have used (x,y,z) with the xy plane equivalent to galactic longitude. Opinions vary on that one!)

Hand-authored stars are generally very close to their galactic coordinates in terms of direction (it's easy to measure the position of something in the sky) but can be far from their expected distance (it's not so easy to measure how far away something is.) On top of that, there are a number of oddities in the way stars were mapped from catalogues into the Forge, so some can be in unexpected directions and distances.
(Similarly it's easy to measure a star's spectrum and apparent brightness, but things like mass and radius and absolute brightness are less certain.)

To make things more complicated star positions are often given in RA / Dec coordinates (which is the easiest way of finding them in Earth's night sky), and translating those into galactic coordinates and into Cartesian galactic coordinates can be a pain. OTOH you can get star catalogues from places like Vizier and they will often include both RA / Dec and galactic coordinates.

Does Elite use this galactic coordinates or are they used in RL?
I ask this because I think the rotation axis of the Earth isn't perpendicular to the galactic plane (although you can define the galactic plane with it as you mentioned) which would cause problems with the coordinate system far from Sol because then the game would either tell you are very high above the plane or very low even if you are well inside the Milky Way.

 

[Jackie Silver]

Does Elite use this galactic coordinates or are they used in RL?
I ask this because I think the rotation axis of the Earth isn't perpendicular to the galactic plane (although you can define the galactic plane with it as you mentioned) which would cause problems with the coordinate system far from Sol because then the game would either tell you are very high above the plane or very low even if you are well inside the Milky Way.

Yeah, absolutely that's a problem - on Earth people usually use equatorial coordinates (RA / Dec) for finding stars; those are tied to the Earth's rotation and so the Milky Way appears as a curved band on them.

Galactic coordinates are mainly used by scientists (as well as RA / Dec of course); Elite doesn't use the rotational galactic coordinates (galactic longitude and latitude) but uses Cartesian ones instead. To map something onto the Cartesian coordinates from rotational ones requires a distance measurement in addition to the longitude and latitude. That can be estimated from a star's parallax movement (difference in observed position of the star when the Earth is on different sides of its orbit); the HIP catalogue which you'll see a lot of stars from measured the parallax and hence distance of about a hundred thousand stars. It's still imprecise, especially for stars which are far away from Earth.

I don't know whether when translating things into Elite, they started with equatorial or rotational galactic coordinates before they combined them with distance measurements - could have been either, but in any case they had to add a distance measurement before they could translate them into the coordinates that Elite uses.

 

[Wedgetail]

Does Elite use this galactic coordinates or are they used in RL?
I ask this because I think the rotation axis of the Earth isn't perpendicular to the galactic plane (although you can define the galactic plane with it as you mentioned) which would cause problems with the coordinate system far from Sol because then the game would either tell you are very high above the plane or very low even if you are well inside the Milky Way.

The Celestial plane is based on an imaginary plane through Earth's equator, and centered on earth - and as you point out, is offset based on Earth's axial tilt and solar orbit - this is what the declination and right ascension are based on (Equatorial coordinate system)

The galactic plane is based on the line from Sol to the center of the galaxy, which avoid the earth-orbital plane discrepancy you describe - in this system galactic latitude and longitude are used as per Jackie Silver's post above (Galactic coordinate system)

Wikipedia has a pretty handy article that explains the different galactic coordinate systems

 

[Rupert Pupnick]

Relative to your diagram, you can say that if you position yourself at Sol at the origin,
you look in the +y direction to find the galactic core?

 

[Shinji Ikari]

The Celestial plane is based on an imaginary plane through Earth's equator, and centered on earth - and as you point out, is offset based on Earth's axial tilt and solar orbit - this is what the declination and right ascension are based on (Equatorial coordinate system)

The galactic plane is based on the line from Sol to the center of the galaxy, which avoid the earth-orbital plane discrepancy you describe - in this system galactic latitude and longitude are used as per Jackie Silver's post above (Galactic coordinate system)

Wikipedia has a pretty handy article that explains the different galactic coordinate systems

I read it and it makes sense but in Elite Dangerous, Saggitarius A* is about 40ly below the plane so it doesn't explain too much in the game.

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Relative to your diagram, you can say that if you position yourself at Sol at the origin,
you look in the +y direction to find the galactic core?

The problem here is that the galactic core is a region and not an specific point so it's kind of ambiguous.

 

[Wedgetail]

I read it and it makes sense but in Elite Dangerous, Saggitarius A* is about 40ly below the plane so it doesn't explain too much in the game.

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The problem here is that the galactic core is a region and not an specific point so it's kind of ambiguous.

You kinda answered your own question the center of the galaxy is the mass-center, the closest discrete body to which is (we assume) a supermassive black hole. Similar to the way we count sol as the center of our solar system, but the mass distribution puts the barycenter not quite at the same point (gotta point out, i am not an authority in this, just my interpretation of some of the stuff i've read)

 

[Shinji Ikari]

You kinda answered your own question the center of the galaxy is the mass-center, the closest discrete body to which is (we assume) a supermassive black hole. Similar to the way we count sol as the center of our solar system, but the mass distribution puts the barycenter not quite at the same point (gotta point out, i am not an authority in this, just my interpretation of some of the stuff i've read)

At least in our system we know with quite a lot of precision where the barycenter lies because the Sun is quite round but the galactic core is certanly not very spherical so it's hard to tell where does the center of the core lies, not only that but the boundaries of the core are also very ambiguous.

What question did I answer? My reply to you only said that Sag A* isn't the reference for our coordinate system because it's below (or above I don't remember) from the plane not what system (stellar system) defined our coordinate system.

 

[Wedgetail]

Yeah i didn't say it very well (i blame a mix of late night, netflix and trying to fly between twin neutron stars distracting me). Basically, Sol is the reference (0,0) for our coordinate system. If you imagine a disk centered on sol that also passes through the mass-center of the galaxy (which isn't sag a*, it is just a mathematically determined region of space, although the two are relatively close), you have defined the galactic plane.

Although there are bodies closer to the center of the galaxy, the closest notable landmark happens to be sag a*, and people have adopted it as the titular center, even though it actually sits below the plane. (Apparently, it is also a few thousandths of a degree east of the galactic center too)

Basically it just came down to the definition of the galactic plane. If they had decided to define it as "a plane centered on sol that passes through sag a*" then sag a* would always have a galactic latitude of zero, and the y(or z) coordinate would always be zero. Unfortunately, we went for the actual centroid of the galaxy

 

[Shinji Ikari]

Yeah i didn't say it very well (i blame a mix of late night, netflix and trying to fly between twin neutron stars distracting me). Basically, Sol is the reference (0,0) for our coordinate system. If you imagine a disk centered on sol that also passes through the mass-center of the galaxy (which isn't sag a*, it is just a mathematically determined region of space, although the two are relatively close), you have defined the galactic plane.

Although there are bodies closer to the center of the galaxy, the closest notable landmark happens to be sag a*, and people have adopted it as the titular center, even though it actually sits below the plane. (Apparently, it is also a few thousandths of a degree east of the galactic center too)

Basically it just came down to the definition of the galactic plane. If they had decided to define it as "a plane centered on sol that passes through sag a*" then sag a* would always have a galactic latitude of zero, and the y(or z) coordinate would always be zero. Unfortunately, we went for the actual centroid of the galaxy

Then that asnwers the question! +rep