# B Is there a standard mapping of celestial coordinates to geo-coordinates

Sorry, I'm not an astronomer. This question relates to the book "S." by Doug Dorst.

I understand that the celestial coordinates have a zero-point at the vernal equinox. (0h, 0m, 0s RA, 0⁰, 0", 0' Dec.)

I also understand that it's possible to map these coordinates to spherical, or geo-coordinates.

My question: Is there a standard mapping of celestial coordinates to the surface of the Earth? I bet that if there is, the vernal equinox would be pinned to 0⁰ N, 0⁰W. (The equator at GMT.)

For instance, Sagittarius, which is at 19h, 0m, 0s RZ, -25⁰, 0", 0', would map to 75⁰ W, 25⁰ S.

Is there such a correspondence, or did I just make that up?

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#### phyzguy

Of course, the Earth is spinning, so any correspondence can only occur at an instant in time. But yes, if you convert RA into degrees then RA corresponds to longitude, and declination corresponds to latitude. This is basically where the celestial coordinates come from. You can think of them as taking the Earth longitude and latitude lines and projecting them onto the plane of the sky.

Of course, the Earth is spinning, so any correspondence can only occur at an instant in time. But yes, if you convert RA into degrees then RA corresponds to longitude, and declination corresponds to latitude. This is basically where the celestial coordinates come from. You can think of them as taking the Earth longitude and latitude lines and projecting them onto the plane of the sky.
I understand that the Earth is rotating. I'm asking if there's a standard, fixed mapping of celestial coordinates to the surface of the Earth. Say, 0⁰ N, 0⁰ W would correspond to the vernal equinox. So that any given constellation always corresponds to a location on Earth's surface.

#### phyzguy

Yes. As I said, this is how the celestial coordinate system was created.

Oh, OK. Thanks. Is there a name for this map? A Website I can go to and see the constellations mapped onto Earth?

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#### phyzguy

Well, you could try overlaying these two. I think they are both Mollweide projections centered on (0,0)

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#### anorlunda

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There are also planetarium type programs that allow you to set your lat/lon to any value the time/date to any value and see what the sky looks like, including constellations. I use a free one called stellarium.

"Is there a standard mapping of celestial coordinates to geo-coordinates"

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