Celestia Forums

Howdy,

Here is a small patch for celestia 1.5.1 that adds two sets of galactic coordinates for stars to the verbose info text. I've used it for a lecture, and I post it here in the hope someone else might find it useful.

The first set of coordinates are the usual heliocentric galactic coordinates, {l,b}, galactic longitude and latitude plus the distance of the star to the center of the milky way in kpc.
The second set of coordinates is galactocentric cylindrical coordinate system, {rho, phi, z}. This can be thought of as cylinder with the z = 0 plane being the galactic plane, and the center being the galactic center. Rho is then the distance in the plane, and z is the height above the galactic plane. Phi is the opening angle when viewed from the center of the galaxy. For example, sol and deneb are about 7 degrees seperated when viewed from the center.

Patch against 1.5.1 source with

Kind regards,

This is interesting. I was asking for something similar since a pretty long time now. Actually, I'm asking for some "coordinates system democracy" in Celestia (horizontal, equatorial, ecliptic, galactic, etc). I didn't tested the patch, since I'm on Celestia 1.6.0.

Chris ?

Chris, you might very well be able to apply it to 1.6.0 as well. I have not seen this version, but the added code is in a relatively static place that probably hasn't changed.

What do you mean with coordinate democracy?

Regards,

hendrik beijeman wrote:What do you mean with coordinate democracy?

Currently, there is no coordinates democracy in Celestia : the equatorial coordinates are favored. We should be able to see the coordinates of any object for any coordinates system on the hood. Maybe a preference option i the preferences dialog box ?

Cham wrote:

hendrik beijeman wrote:What do you mean with coordinate democracy?

Currently, there is no coordinates democracy in Celestia : the equatorial coordinates are favored. We should be able to see the coordinates of any object for any coordinates system on the hood. Maybe a preference option i the preferences dialog box ?

There is absolutely NO sensible reason for "coordinate democracy". Coordinates should be chosen to match the astrophysical/mathematical situation under consideration, including symmetries if there are any! Displaying all these coordinates at once would only clutter the display.

Hence for a problem/task that "lives" on a sphere/ ellipsoid/spheroid/cylinder/... we better choose spherical polar coordinates and ellipsoidal/spheroidal/cylindrical coordinates, respectively.

In astrophysics we should focus on galactic coordinates, for example, if we want to study bodies that are part of that galaxy. Analogously with ecliptical coordinates etc.

Fridger

t00fri wrote:There is absolutely NO sensible reason for "coordinate democracy".
In astrophysics we should focus on galactic coordinates, for example, if we want to study bodies that are part of that galaxy. Analogously with ecliptical coordinates etc.

Yes there is. In many cases, while you get the galactic coordinates of some object, you may want to know the equatorial coordinates, or even the horizontal coordinates of the same object, as seen from your home. Conversion between coordinates should be available in Celestia.

Of course, I'm not asking to put all the coordinates systems on the hud at the same time (it would be a mess!). However, I'm asking of a way to display the coordinates of our choice in the upper-left corner. Maybe a check box in the preferences window, or better : a keyboard shortcut to toggle them ON/OFF. Also, Celestia should give a UI feedback in the lower-right corner, that a given coordinate grid is activated, with proper color.

Maybe this will not be usefull to some users (as YOU), but it would surely be usefull to others (like myself). It's also usefull in education too.

Cham wrote:

t00fri wrote:There is absolutely NO sensible reason for "coordinate democracy".
In astrophysics we should focus on galactic coordinates, for example, if we want to study bodies that are part of that galaxy. Analogously with ecliptical coordinates etc.

Yes there is. In many cases, while you get the galactic coordinates of some object, you may want to know the equatorial coordinates,...

This argument of yours is merely a repetition that you want "coordinate democracy".

Instead, we should collect in the code a listing when certain coordinate systems are adequate and display them accordingly on the canvas. For students, one cannot start early enough to teach them when and WHY a particular coordinate system should be chosen! Allowing for ALL of them at any time needs another 5-6 keys or menu entries etc.

Moreover, any coordinate system is in a sense "unphysical", since physics is coordinate independent... Coordinates are merely "convenience tools" that one uses for measuring distances in a given space (mathematically speaking). Putting too much weight on coordinates means ignoring what is really essential. As a reminder: Einstein's general relativity can be derived from the assumption of "general coordinate invariance"...

Sorry, I am NOT at all convinced...

Fridger

Hendrik,

Thank you for the patch! It's too late to include in the Celestia 1.6.0 release, but the calculations for heliocentric galactic coordinates and galactocentric cylindrical coordinates will be useful in the subsequent version.

As for 'coordinate democracy'... While I favor giving the user some control over what coordinate are shown, I agree with Fridger that complete flexibility isn't necessarily ideal. We don't want to confront the user with a huge number of options for coordinate display when most of those options will be useless. Some intelligent filtering based on object type would be more helpful. Perhaps we should start a thread listing the different coordinate systems and object types and see how many combinations are actually useful.

--Chris

I patched Celestia with the previous patch. It's working very well, and I think this modification is very interesting. It needs some modifications, though. The degrees unit should be shown. Also, it may be better to use the greek symbols instead of "rho" and "phi" (?). The kpc unit isn't consistent with the one shown for the Distance, and the presentation of galactic coordinates isn't consistent with the usual Distance, RA and Dec presentation.



I suggest to add a new "Ultraverbose" mode, to show the complete coordinates sets, in a consistent way (?). The galactic coordinates should also work for any object, not just stars.

This seems too abstract to me. Imy's global positioning overview idea could be use to show that in a graphical way, with grid, positions, coordinates lines etc... Perso I would find this more interesting...

Cham wrote:I patched Celestia with the previous patch. It's working very well, and I think this modification is very interesting. It needs some modifications, though. The degrees unit should be shown. Also, it may be better to use the greek symbols instead of "rho" and "phi" (?). The kpc unit isn't consistent with the one shown for the Distance, and the presentation of galactic coordinates isn't consistent with the usual Distance, RA and Dec presentation.



I suggest to add a new "Ultraverbose" mode, to show the complete coordinates sets, in a consistent way (?). The galactic coordinates should also work for any object, not just stars.

I think this is way too much printout and moreover, there are too few digits for serious positioning work! So what is it good for beyond the respective graphical grid display that is MUCH better suited for more qualitative / educational purposes than these many lines of numbers with lacking accuracy!

Moreover, there are excellent coordinate converters in the net that have been tested and are very accurate. Personally, I used these, if I had to position bodies accordingly.


Fridger