# How would a Universal Coordinate system work?

We have a variety of ways of describing our location here on Earth. We can refer to a nearby landmark (eg "by the City Hall"), we can describe our location relative to an area (eg "in the South of the island") and we can describe our location with Longitude and Latitude. It is the later of these methods I am interested in, but on the scale of the Universe.

Obviously the Lat/Long system works because we have identified the North Pole, the South Pole and the Equator of the Earth. This allows us to give 2 arbitary numbers to locate any point on the globe. I am imagining that to have a similar coordinate system in the Universe we would also need to mark out some absolute points.

Given that everything in space is always moving, what could be used as an absolute point in the Universe? The Big Bang perhaps? I considered having the observer as an absolute point, but I am unsure how this would create a reliable coordinate system that could be transfered to another observer.

Whereas the Lat/Long system only requires 2 coordinates, I can imagine a Universal Coordinate system would require at least 3 coordinates. I have been considering adding time as a 4th coordinate, as everything is in a state of motion, knowing the time might help work out relative points if no absolute points can be found.

In general I am struggling with how a Universal Coordinate System would work. Simple X, Y, Z coords (with 0,0,0 being Earth?) seem insufficient in a medium that is in a constant state of change.

Has anyone else tackled such a system?

• The Big Bang didn't have a center, nor does the accelerating expansion of the universe have a center. – kleer001 Mar 6 '20 at 17:17
• Astronomy is five thousand years old. What do you think, have astronomers thought about ways to describe the positions of distant stars and galaxies? Look up celestial coordinate systems, especially (for large scales) the galactic coordinate system and the supergalactic coordinate system. – AlexP Mar 6 '20 at 17:20
• Why though? Absolute coordinates serve no practical function. Coordinate systems make the assumption that there is a fixed reference point, and the coordinates are just the distances along a particular axis (also assumed) from that reference point. The universe has no fixed points, absolute coordinates do not exist. In order to travel from one "thing" to another "thing" you will have to calculate their trajectories anyways to accomplish anything practical. – Nate White Mar 6 '20 at 17:53
• Quasars can serve as lighthouses for analogous navigation, by the way. They shine bright enough and distinctly enough to navigate a galaxy. – SRM Mar 6 '20 at 19:06
• what could be used as an absolute point in the Universe? The Big Bang perhaps A common misconception, but the Big Bang did not happen at a point as that Physics SE Q&A explains. – StephenG Mar 6 '20 at 22:25

Observe the Pioneer Plaque:

See that bunch of radiating lines on the left? That's a remarkably good way to identify where (and perhaps even when) Earth was when the space probe was launched.

The patterns on each line represent the pulses emitted by a particular pulsar. There's some additional fiddliness here caused by the need to encode it in binary and include a way to describe the time period being encoded, but that's for communicating with aliens and you don't need to worry about that.

Because each pulsar is uniquely identified by its period, you can describe your location in terms of the angles between various specific pulsars which will effectively pin you down in space. You don't even need to know how far away the pulsars are, which is nice.

Pulsar positions will drift over time, and their pulse rates will decrease, but those timescales are long and if you only need to have a location that's good for a few thousand years you'll be just fine. Beyond that it'll still be traceable by anyone who has got a good model of pulsar spindown and orbital motion about the galactic centre, though without also knowing your velocity and heading (which you'd need to encode separately) they wouldn't know where you ended up. Multiple solutions may crop up as one pulsar slows down to the point where its period precisely matches that of an ancient pulsar used to define a location, but handling that is a Simple Matter Of Mathematics, of course.

The system can be made universal, but the position will be in terms of the pulsars in a particular galaxy. You'd need some completely different way to describe the position of your galaxy in terms of other galaxies... SRM suggested Quasars which could work, though they lack the signature pulse rate that makes identifying specific pulsars relatively straightfoward.

Honestly though, if you can manage easy intergalactic travel and communication, you'll be able to find someone or something who can think up a better navigation scheme, as near-godlike powers will be required to cross intergalactic distances in any reasonable length of time.

• Voyager's 'Cosmic Map' Of Earth's Location Is Hopelessly Wrong – Alexander Mar 6 '20 at 21:43
• @Alexander ...over the timescale of millions of years. That's a deeply misleading headline. – Starfish Prime Mar 6 '20 at 21:56
• if it wasn't for the second point in that article (which pulsars point their pulses at Earth change over time in an unpredictable fashion) I would have marked this as correct. – Jimmery Nov 13 '20 at 14:19

Orbital Mechanics!

It's not a simple topic, but it's worth your time. They're basically the 6 numbers required to describe the position in time and space of an object orbiting another object.

So, the takeaway from that is if you want to describe the position of an object in space it'll need to be in reference to another object. That could be the galatic center if you're describing a solar system (or a spaceship in interstellar space), or the closest planet if you're describing a moon. And they'll need to be nested if you're describing the position of a moon around a planet from another galaxy.

Why orbital mechanics?

## Because there are no privileged frames of reference in space.

The Orbital Elements...

To mathematically describe an orbit one must define six quantities, called orbital elements. They are

Semi-Major Axis, a
Eccentricity, e
Inclination, i
Argument of Periapsis, ω
Time of Periapsis Passage, T
Longitude of Ascending Node, Ω


Their details require a bit of book learning I won't copy-paste here, but they're available lots and lots of places (wikipedia, your local library, your local astronaut, "SevenEves" from Neil Stephenson).

• You make the assumption that a particular object has it's path dominated by a single gravitational mass. This is not necessarily the case for all things. For example, stars moving around the galactic center are also influenced by their neighboring stars. N-body problem en.wikipedia.org/wiki/N-body_problem – Nate White Mar 6 '20 at 17:56
• @NateWhite Nest that in your coordinates when describing that system. Easy. Anyways, there can never be a perfect coordinate system (to the Nth decimal point) when projected onto the real world. At some point we need to accept a level of rough granularity and throw out some information. – kleer001 Mar 6 '20 at 18:41
• @NateWhite for any unstable system of orbits (stars around galaxy over long term), you have to have a published map update when deflection occurs. Ultimately, any place that drifts that you want to get back to either needs to drift deterministically (orbital mechanics) or has to have a person/beacon to announce its updated location. There’s no other logical option, though there are many techs that fulfill the role of beacon/map updater. – SRM Mar 6 '20 at 19:10
• @kleer001 n-body problems are not "Easy" :). Also the notion of "nesting" assumes a hierarchical configuration of the gravitational influences. ie. the star orbits the galactic center, and the planet orbits the star, and the moon orbits the planet. This fails in the case of multiple stars that are gravitationaly bound (globular cluster), they interact with each-other, but are not in orbit around common barypoint that they orbit in a regular period. – Nate White Mar 6 '20 at 19:39
• @SRM The problem is that solving N-body systems are hard, and they tend to be simulated rather than actually solved. None of these exercises actually answer the question, since it is not possible to define absolute coordinates, since there are not fixed locations in the universe. – Nate White Mar 6 '20 at 19:42