Celestia Forums

I think that a good add-on for Celestia would be some means odetermining the angular separation between two stars; i. e. something in the line of, clicking on one star, then hitting some unused "F" key, then clicling on another star. The result would display in a small window the angular separation between the two points ( preferably in decimal degrees; it would be easier than dgrees and minutes)

Such an add-on would be very useful in working out problems in astrogation. How many times have we found ourselves lost in interstellar space and manged to find our way back to Sol WITHOUT cheating (cliicking on "Select Sol")? Sound like a good ?[color=green]

But this you can easily do already without any further add-on.

Select one object (star) and then land on it. Click on another one and the distance to that selected object from the first star will be displayed in the top left corner under 'Distance'.

Please take away the 'Sticky' label. We tend to be very conservative using those for good reasons!

Bye Fridger

Fridger:
No, I am speaking in terms of ANGULAR separation (degrees), not distances in light years.

PERTINAX wrote:Fridger:
No, I am speaking in terms of ANGULAR separation (degrees), not distances in light years.

PERTINAX,

OK, is see. This feature would certainly be part of a "readout" package for data of astronomical interest that is since long on our todo list. Like cursor readout in various standard coordinate systems together with corresponding grid displays etc.

Another quick possibility is to write a small Lua script, analogously to the existing lat-long readout script by Harry. This should be quite easy to do.

Bye Fridger

fridger:

Yeah! That's the ticket! Easy for you, maybe, tho. When it comes to actually BUILDING the starship (welding and machining), I'm your man; YOU can write the code

One place this would be useful: center your cursor on earth or.. oh, anyplace, and go to it. Then lean on the "end" key until you reach the "edge" of the universe, when the distance indicator goes all wonky. Then lean on the "home' key until the distance falls to more or less zero. Where are you? Due to some quirk in the program , you CAN end up many light years from where you started. How to get home?
Well, your more prominant stars are labeled. In actual practice, you would have aboard your ship a database of major stars (preferably ones with high absolute magnitude, like Deneb, rho Leonis, Rigel, etc.,) showing their spectra. You would be able to computer match spectra(as unique as a fingerprint, except for some variables) with an observed star, the way the FBI matches fingerprints. Since your database also shows the distances between these various intrinsicly bright stars (Deneb remains a naked eye star out to 25,000 light years! Toasty!), there are your baselines. Measure the angles between 3 of them and you can figure out where the hell you're at, because you can't stop and ask for directions.

If nothing else, this would be a fun way to teach plane trignometry!

BTW,, you will notice I undid the "sticky" thing. Truth to tell, I hadn't a clue as to what that was.

PERTINAX wrote:fridger:

Yeah! That's the ticket! Easy for you, maybe, tho. When it comes to actually BUILDING the starship (welding and machining), I'm your man; YOU can write the code
...
.

Harry is the man to get interested in a LUA task. I know LUA since the basic syntax happens to be ~identical with the MAPLE programming langage that I am most familiar with. Harry has implemented most or at least many Celestia related macros into the LUA script parser.

Bye Fridger

PERTINAX wrote:Fridger:
No, I am speaking in terms of ANGULAR separation (degrees), not distances in light years.

But, similarly, it would be very interesting to be able to click on two stars sequentially and see the distance between them (without actually traveling to one and viewing the other).

Hm, I think this would only work if LUA/celx would provide full access to the objects data. If you ask Harry, perhaps he would incorporate such an interface.

maxim

maxim wrote:Hm, I think this would only work if LUA/celx would provide full access to the objects data. If you ask Harry, perhaps he would incorporate such an interface.

Only a short post (my last attempt to post was lost because shatters.net became unreachable for me):
I am not sure what data you've been thinking of. AFAICS all that's needed is available: positions of the observer and both bodies (stars). The angular separation should be easy to calculate from that.

Harald

Harry wrote:AFAICS all that's needed is available: positions of the observer and both bodies (stars). The angular separation should be easy to calculate from that. Harald

Yes. The STC definition contains RA and DEC. Together with the observers position the AS can be calculated. If this is already accessible in celx, the interface I was talking about already exists, and I just missed it.

maxim

maxim wrote:Yes. The STC definition contains RA and DEC. Together with the observers position the AS can be calculated. If this is already accessible in celx, the interface I was talking about already exists, and I just missed it.

You can't access the RA/DEC values used in the stc-defintion, but you can get the position (in normal 3D coordinates) from any object. In fact the RA/DEC isn't even stored anymore after an object was loaded, it's just used to compute the 3D coordinates.

Harald

Well! glad to see this thing is up and running again!

Anyway, here's what i'm really about: I'. sure you hare familiar with the grid found in the back of most road atlases wher one can find the road distance beteen two cites by finding the intersection point of the two cities along the hozontal and verticle axis. i am compiling, in Excel format, a similar table of 40 prominant stars. where on can look up the distance between any two of them.

now , if i can measure the angular separation between the two stars, i have a basis for insterstellar navigation.
I do not have the first idea about writing code [404-clue not found!] I'm a machinist and welder turned history student, fer Gawd's sake. gimme a break here! I see that Harry has already noticed this thread, and he has been been referred by more than one person as the "goto" guy for this sort of thing.

what i have been going on about has nothing to do with Right Ascension or declination, as such. I only want to know if it is possible to build an add-on to Celestia that will yield a direct read-out (preferably to decimal hundreths of a degree, but i would take tenths and walk away happy) of the angle between two stars from the observer's point of reference, where ever that point of reference happens to be.

Personally, I am a little surprised that no one has done this already. Perhaps they tried and decided it was impossible or was too complicated to be worthewhile.. or something in that line, i am hardly qualified to judge.

Anyway, thank you for attention, any useful suggestions or ideas would be appreciated.

greg

Greg,

I can recommend to use a highly accurate and specialized software for your task: XEphem. It uses state of the art star catalogs and you can just read out the angular distances graphically. This classical 2d emphemeris program is found since a decade in every scientific institution. It is explicitly /earth/ based, while Celestia is not.

XEphem is free of charge for Linux. It also compiles without problems under CYGWIN (Linux) in a Windows XP environment. There is also a native Windows version, that costs a few bucks but also includes a huge set of professional star catalogs and astronomical images on CD.

Here is the Web site. http://www.clearskyinstitute.com/xephem/xephem.html

I have been involved in XEphem development for 11 years (!).

Bye Fridger

Something that is explicitly Earth-based is of no use for what I have in mind. I have tried very hard, as can be seen above, but seem to be unable to make anyone understand this.

I have been under the impression that I was making myself clear- you are at a point in space, any point at all. You want to be able to determine the angle between two selected points. That's all! Why does this have to be so hard?

Well, knowing the actual FOV it's easy to compute the angular distance between two points. The only thing you seem to need, is a GUI function for clicking and storing two points on the screen.

maxim

To clarify Maxim's point:

Orient Celestia's view so that the objects of interest are vertically above one another, perhaps at the left or right edge of the window.

Shrink the window or change the magnification until one object is at the top edge of the window and the other is at the bottom edge.

Read their angular separation at the bottom right of the screen -- it's the FOV value.