Selections and objects confusion

[Stan]

Have patience with a geriatric space traveller but can someone tell me the subtle difference between an 'object' and a 'selection'. Being new to Celestia (sheer paradise even at my level of competence) I am beginning to get the hang of the commands and sense a subtle difference. I suppose it couldn't be that one can locate oneself at a 'selection' and goto (view in the direction of) an object (or vice versa). It would be nice to know what I am doing, even if, most of the time, I'm succeeding!

Stan

[selden]

Stan,

Since you didn't provide any examples, it isn't entirely clear what differences you've encountered. I'll try to make a couple of guesses.

If you type just the name of a planetary object that's defined in an SSC file (a text file), it has to be unique and within 0.1 light year of your viewpoint. Otherwise you have to specify its full "path name", including the star it's orbiting.

If you type the name of a star, you can use one of a number of synonyms for it, but it has to be one of the identifiers defined in either stars.dat (a binary file) or an STC file (a text file).

If you type the name of a galaxy (or globular cluster) it has to be one of those defined in galaxies.dat (a text file). I don't think the code that uses galaxies.dat supports synonyms yet.

At the moment, no other types of objects are supported in Celestia. In particular, gaseous nebulae cannot (yet) be defined.

I suspect part of the confusion is caused by Celestia itself not being internally consistant. At the moment, it has several different categories of "objects": Thise defined by SSC files (planets, moons, spacecraft, asteroids and comets), those defined in the file "stars.dat", which now can be augmented by STC files (stars), and those defined in "galaxies.dat" (galaxies and more recently globular clusters).

I didn't write any of the code, and only encountered Celestia recently myself. I could be mistaken, but my impression is that these differences are caused by two effects: the numeric resolution available in graphics hardware (which makes it very difficult to represent planets and galaxies in the same display) and that STC files are relatively recent additions.

Does this help at all?

[Stan]

Many thanks. I've been playing with Celestia for about a couple of hours and I had enjoyed viewing Saturn from Mimas or Enceladus or Jupiter from Io and so on and on and on . . . . I had gone through the various ways of following the relationship and found how I could keep myself with my back to the Sun so that I could avoid the dark-side with the major planet (relatively) orbiting me or with me orbiting the planet with alternating night and day. I hadn't got into thinking about the programming implications (except to jealously admire the competence of it all) and was was simply puzzled by the definition 'object' and 'selection' since one could go to either and each seemed to respond to the orientational commands.

Your comments are interesting food for thought though.

I suppose I could 'park' myself at any coordinate-defined location in space, chobble my sandwiches and watch the world go by . . or could I. Must have a go.

Incidentally, having read in this forum about eclipse (not total but simply shadows?) phenomena, it is interesting that, in my travels, I am temporarily put in the dark as Jupiter (or whatever) passes behind me and its satellite. All very clever stuff eh?

Thanks for your response. I guess I've a long way to go - intellectually that is!

Stan

[selden]

Stan,

OK, I think I understand where you're coming from. It's partly English usage as well as noticing some of Celestia's features.

When an object is "selected" it gets listed at the upper left of the display. It also will be the subject of subsequent position or viewpoint changing commands, but that's all. Selecting an object does not by itself change the object that's being followed or watched.

To see what object the viewpoint motion commands currently reference, look at the display at the lower right. The bottom line shows the size of the current field of view. The next line up shows the object that your position is related to. And the 3rd line up shows what object you're "Track"ing -- that you've forced Celestia to watch.

You have to type the F ("Follow") command (or O or a similar command) to cause the object currently selected to become the object the mouse commands reference.

At least one command refers to the "selected" object but don't change the object being "followed". In paticular, the "Track" command doesn't change your position, but does change what you're watching. One of the fun things one can do in Celestia is to "follow" a spacecraft, like Galileo, and "track" the next object it'll be encountering.

Does this help?

[Guest]

Thanks Sheldon; that second note explains all. I couldn't understand the apparently ambiguous use of object and selection when they seemed both to be the same. I hadn't got (gotten?) as far as reading what was accumulating on the screen - I was so caught up in the beautiful images there.

It's the old story I guess; when all fails read the instructions
I've also noticed a glitch with shadows - will explain that on the bugs section.
Thanks again for the help - you make me feel like some of my brain cells have not yet atrophied!
Stan