equatorial vs planetographic frames

All about writing scripts for Celestia in Lua and the .cel system
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cpotting
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equatorial vs planetographic frames

Post #1by cpotting » 21.09.2004, 15:08

Hello everyone,

I am hoping that someone can help me understand the difference between equatorial and planetographic frames of reference.

I was under the impression that the equatorial frame of reference would have an X-Z plane that runs through the equator of the reference object (hence "equatorial") and that the Y axis would then run through the object's poles of rotation. I never understood what the planetographic frame of reference was supposed to be.

With that in mind, I was surprised when

Code: Select all

earth_frame_equatorial = celestia:newframe("equatorial", earth)
above_pole_position = celestia:newposition(0, 100000 / KM_PER_MLY, 0)
earth_above_pole_position_universal = earth_frame_equatorial:from(above_pole_position)
did not create a position over earth's north pole, but rather over the arctic circle - appearently aligned with the earth's orbital plane. I was further suprised when

Code: Select all

earth_frame_planetographic = celestia:newframe("planetographic", earth)
above_pole_position = celestia:newposition(0, 100000 / KM_PER_MLY, 0)
earth_above_pole_position_universal = earth_frame_planetographic:from(above_pole_position)

did create a position over the north pole. Now I don't know what the equatorial frame of reference is supposed to be.

Any help would be appreciated.
Clive Pottinger
Victoria, BC Canada

Harry
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Re: equatorial vs planetographic frames

Post #2by Harry » 21.09.2004, 15:42

cpotting wrote:I am hoping that someone can help me understand the difference between equatorial and planetographic frames of reference.

Apparently there is no special code in frame.cpp to handle "Equatorial", which makes it the same as the "Ecliptical" frame. Not sure about "observer" (a.k.a. as ObserverLocal) either. Maybe Chris can explain what their purpose is...

Harald

chris
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Post #3by chris » 21.09.2004, 17:47

Oops. Looks like an omission in the scripting system that needs to be corrected. Then, your code should work as you expect it to.

--Chris

Topic author
cpotting
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Post #4by cpotting » 22.09.2004, 15:35

chris wrote:Oops. Looks like an omission in the scripting system that needs to be corrected. Then, your code should work as you expect it to.

--Chris


Well, I am happy to find out that my interpretation of "equatorial" was correct... but that still leaves me wondering what "planetographic" is supposed to be.
Clive Pottinger
Victoria, BC Canada

chris
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Post #5by chris » 22.09.2004, 18:41

cpotting wrote:
chris wrote:Oops. Looks like an omission in the scripting system that needs to be corrected. Then, your code should work as you expect it to.

--Chris

Well, I am happy to find out that my interpretation of "equatorial" was correct... but that still leaves me wondering what "planetographic" is supposed to be.


The planetographic coordinate system accounts for planet rotation, and equatorial does not. An object at a constant position in the planetographic system will remain fixed with respect to the surface of the planet. Perhaps I could have used better terminology.

--Chris

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cpotting
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Age: 63
With us: 20 years 8 months
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Post #6by cpotting » 22.09.2004, 20:36

chris wrote:The planetographic coordinate system accounts for planet rotation, and equatorial does not. An object at a constant position in the planetographic system will remain fixed with respect to the surface of the planet. Perhaps I could have used better terminology.

--Chris



No, I think the terminology is fine - now that I know what it is. I can't think of a term that would work better. Thanks guys, this helps a lot.
Clive Pottinger
Victoria, BC Canada


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