Question regarding gotoloc script command

[selden]

Don,

Just a quick interjection:

The equatorial coordinate system measures positions relative to the plane of the Earth's equator.

The ecliptical coordinate system measures positions relative to the plane of the ecliptic -- i.e. the plane of the Earth's orbit.

The equatorial coordinate system is rotated about 23.45 degrees (the Earth's tilt) relative to the ecliptic coordinate system.

[don]

Thanks Selden. I knew what Equatorial was but was not sure about Ecliptical (without doing an on-line search). And I wouldn't think that Follow is Observer, but I don't have a clue what Observer is either, as I've not tested it explicitly (yet).

Care to take a crack at filling in the missing Celestia "modes" to the setframe CoordSys values I listed?

PS. Yes, you described *what* the Observer CoordSys is in the "What is 'up' in goto script command?" thread. But does it relate to a mode that Celestia can be put into via the keyboard, like "Chase"?

[selden]

Don,

The "observer" coordinate system is the one which is "tied to the observer": +Y always points up, +X to the left and +Z is in front of you. I say "you" because Celestia's "observer" corresponds to the viewpoint it shows on the screen.

Whenever Celestia draws an object, it has to calculate the object's position and orientation in the observer's coordinate system in order to start to figure out where to draw it on the screen.

Trying to rephrase what Chris wrote, the various movement commands tell Celestia to calculate the observer's position and orientation within a coordinate system which is appropriate to the specific type of movement that has been selected.

Celestia transforms from whatever coordinate system the script author has chosen into that movement-specific coordinate system. From what Chris wrote, the movement-specific coordinate systems differ in their orientations with respect to the currently selected object, but the center of that selected object is always positioned at their origins. (The "origin" of a coordinate sytem is defined to be the position which has [x,y,z] values of [0,0,0].)

Celestia then has to transform the location and orientation of the selected object back into the observer's coordinate system.

Does this help at all?

[don]

Does this help at all?

Hi Selden,

Ummmmm, no, it doesn't, because you and Chris already explained these things, thank you <smile>.

Chris's earlier message stated, "Each of Celestia's modes--follow, sync orbit, lock, chase, and free [universal]--together with one or two objects defines a coordinate system". And I now know that the user can set the Coordinate System by pressing a key in the user interface (Chase, Lock, Follow, Sync Orbit, or Universal), or by using the setframe script command. But, there is no "observer" coordinate system keystroke that I've seen, even though this coordsys can be set via the setframe script command.

In another topic, Chris wrote, "The coordinate system names accepted by setframe map to Celestia's modes as follows:

equatorial - follow
geographic - synchronous
lock - lock
chase - chase
universal - no mode selected (e.g. you just pressed ESC)"

However, "equatorial" is not one of the setframe Coordinate System options, and he didn't list "observer". Thus my follow-up questions.

In my last message, I asked: "Care to take a crack at filling in the missing Celestia "modes" to the setframe CoordSys values I listed?", which was referring to the following:

Code: Select all

   CoordSys        Celestia Mode / Astronomical Term
--------------  ---------------------------------------------------
chase ........  Chase -- any astronomical term for this?
ecliptical ...  Follow?
geographic ...  Sync Orbit
lock .........  Lock -- any astronomical term for this?
observer .....  Celestia mode? -- any astronomical term for this?
universal ....  None -- no mode selected

The four questions this list puts forth are as follows:

1) Is there an Astronomical term for Celestia's "Chase" mode?

2) What Coordinate System does Celestia's "Follow" relate to?

3) Is there an Astronomical term for Celestia's "Lock" mode?

4) What Celestia mode does "Observer" relate to? Since this is a Coordinate System that can be selected by a script writer, what is the corresponding keystroke in the user interface?

I hope this explains my questions more clearly.

[chris]

I also call universal the absolute coordinate system.

There's no astronomical term for chase or lock, though I could perhaps come up with a more precise term for lock.

Geographic isn't the best term for sync orbit--planetographic would be more appropriate, as the geo- prefix is Earth specific.

--Chris

[don]

Hi Chris,

I also call universal the absolute coordinate system.

Yep, Free, Absolute, and Universal. Got any more names for it <grin>?

There's no astronomical term for chase or lock, though I could perhaps come up with a more precise term for lock.

Geographic isn't the best term for sync orbit--planetographic would be more appropriate, as the geo- prefix is Earth specific.

--Chris

The CoordSys items on the left side of the above list came directly from the Celestia setframe command, including geographic and observer <smile>. That's why I was asking what Observer is related to in the Celestia UI.

I agree about geo (so does Webster) but it's in the setframe command syntax already.

Thank you for your help Chris.