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Comment by echoangle | Hacker News Reader

> It would be like asking for the GPS coordinates of the moon

No problem at all, just give the location where the moon is at the Zenith and use the distance as the altitude.

> Reason this works for the ISS is because of the subsatellite point. It can receive GPS signals regardless of altitude above the Earth’s surface.

No, wether the object can actually receive GPS signals is completely irrelevant to wether its location can be described in the GPS coordinate system.

You could describe the location of the Sun in GPS coordinates too, the altitude value would just be very large.

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You can use GPS to describe a point on Earth. To use the moon or sun is kind of weird because of their size to use GPS coordinates for this

I was referring to finding your position on the moon using Earth referenced GPS signals.

> You can use GPS to describe a point on Earth.

No, you can describe any point in the universe using GPS coordinates. You just lose some resolution the further away from earth you are because it's basically spherical coordinates (like polar coordinates but for 3D). And the system isn't inertial but earth-fixed, of course, so you would have to give the coordinates together with a time.

And if you're describing the location of the moon and the sun, you would probably pick their center of gravity.

I believe this isn’t true otherwise NASA would be doing this

- Earth isn’t a universal reference

- GPS uses WGS84

- GPS is bound to the Earth’s surface and center

- It’s Geodetic

- There's no universal “equator” or “prime meridian” beyond Earth

- Space uses inertial frames or celestial coordinate systems (right ascension and declination, or galactic coordinates)

That's exactly what I said. It isn't very practical for space ops, but you can absolutely give a current GPS position for every object you want.

That’s conceptually misleading.

They are meaningless for things not near Earth because they’re tied to Earth's shape, rotation, and gravity field

I wouldn't call it meaningless if it can be converted back and forth with a (non-linear) transformation.

You can do a lot of things…

There are a handful of Earth centered, geocentric standard reference frames. The most used today is the Geocentric Celestial Reference System (GCRS). It should be obvious that if you want to compute where to point a telescope, a transformation of coordinates will involve a step through such a coordinate system. GPS is it's own system, but there are transformations to and from the GCRS and GPS frames. Which one makes sense depends a lot on your application.

I agree that GCRS and other Earth-centered inertial systems are fundamental for astronomy, tracking, and spaceflight.

And yes, you can transform between GPS coordinates and GCRS (or ECI/ECEF), depending on what you’re trying to do. The key distinction I’m making is about contextual meaning and application. GPS (WGS-84) coordinates are geodetic.

Once you're transforming positions millions of kilometers away (eg. L2, the Moon, or Mars) into lat/lon/alt, you're applying a system optimized for geodesy to a domain where the altitude is arbitrary, and "longitude" rotates with Earth.

The person is simply saying you can. That’s it! As they said, it isn’t useful for things not near Earth. Perhaps not meaningless, but the “meaning” of the coordinate would decrease the further away from earth the object is.

If you’re saying it physically isn’t possible, then you’re basically saying the universe has some topological structure that cannot be projected onto the surface of a sphere. Which now that I’m thinking about it, is probably the case because of black holes? Lol maybe if the object is “in” a black hole then the coordinate actually is completely meaningless, but any other object would have some amount, even if low, of “meaning”

You're right that any 3D point can be mapped to a spherical coordinate system like GPS. I don’t disagree.

GPS coordinates are geodetic, tied to the WGS-84 ellipsoid and Earth’s rotating reference frame. They were designed for terrestrial navigation and near-Earth orbit, not for interplanetary space. Once you're dealing with the Moon, the Sun, or Lagrange points, you're so far outside the system's intended domain that projecting lat/lon/alt onto those locations introduces more confusion than clarity.

You're also right to point out that it's not physically impossible to describe a faraway object in that coordinate system.

There is just no practicable utility.

NASA and others use inertial frames, barycentric coordinates, and RA/Dec for a reason.

Also, love the black hole tangent - that’s where coordinate systems get seriously weird :-)

I could build a house with my pinkie if I excuse the fact I’ll use a team of laborers to do the work and accept that they are so inaccurate that I would be lucky to end up with a shed… if I only cared about technicality.

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