Celestia Forums

I have a .stc file with the following data:



All my stars are showing up with the correct data, except for their positions, which are showing up in the constellation Cetus, about 1900 light-years from where they are supposed to be. Does anyone know why this might be happening? I may be mistaken, but the Declination may be showing up accurately as well, just not the right ascension. Trying to find a star map to confirm that but I'm not seeing anything useful to me.

I have some more to add in but obviously that's useless if all four of these are broken anyway...

Feel free to contact me via any messenger if you like. Thanks in advance to whoever can point out my idiotic noobish error, whatever it my be.

The RA values are in the wrong units.

RA in STC files must be specified in degrees, not Hours.
(However, RA in DSC files, e.g. for the surrounding Nebula, must be specified in Hours, not degrees. *sigh*)

Ahh excellent, thank you.

Yes, I had been working with .dsc files as well and those were in the right place. Can you place stars in .dsc files?

Addendum: 3 of the 4 Trapezium stars are in Celestia already: their positions were measured by the Hipparcos satellite. Grant Hutchison provided the location of the 4th as an Addon at http://www.lepp.cornell.edu/~seb/celestia/hutchison/trapezium.html

To be properly handled as stars, they do have to be defined in STC files.

My problem is that I'm using the 2 million stars database which apparently has the distances to those stars wrong and also breaks them so you cannot target them except for Theta 1 Orionis C. Since I need to have the Trap stars and the Orion Nebula (an add-on I acquired specifies its distance as 1477 LY) I figured it would just be simpler to add the things myself.

I haven't looked into the error bars of Hipparcos at that distance, how much margin of error would you guess the distances to have?

Addendum: okay, thanks.

Is there any way to actually get into the data of stars.dat? That would be so useful to me...

Yup. That's a known problem in the expanded database.
(See the Web page I mentioned above.)

Hipparcos' errors aren't simply a function of the stars' distances, although that's one of the factors. The best way to find the errors of individual measurements is to query the Hipparcos database. (But it seems to be offline right now: http://astro.estec.esa.nl/Hipparcos/site-guide.html) Or you can download their entire database from http://cdsweb.u-strasbg.fr/cgi-bin/Cat?I/239

The sourcecode used for stars.dat (stars.txt) for the next version of Celestia (v1.4.0) is available on SourceForge at http://cvs.sourceforge.net/viewcvs.py/c ... 3&view=log

Unfortunately, the older version used by v1.3.2 and earlier, isn't readily available.

I've been using this link for the Hipparcos query.

http://archive.ast.cam.ac.uk/hipp/hipparcos.html

Nice resource. I'm kind of fuzzy on the error definitions for parallax being greater than the parallax itself, and also on why a parallax would be negative... hm.

I got my thing working; it's the first step in a pretty large project I have in mind... probably will end up being about the size of the Orion's Arm add-on you have around here once I figure out the quirks of PNGs...

I'm kind of fuzzy on the error definitions for parallax being greater than the parallax itself, and also on why a parallax would be negative... hm.

Suppose the design of the measurement method can only produce values reliably to the nearest (for example) 0.1 arc seconds, but the numbers it provides are 0.001 arc seconds. When you try to measure the position of something so far away that its parallax should be 0.01 arc seconds (10 times smaller than the error), the errors in the measurements will be larger than the measured value.

Also, every time you try to measure the object's position, you'll get a slightly different value. You can't control what the erroneous offset will be. For some objects, that offset will just happen to be in the opposite direction from what the parallax shift ought to be. Since the errors are larger than the parallax, this results in a "negative parallax".

Does this help?

Yes, that explains it. Thank you. Thus the errors may be so significant that the parallax measurement may be totally useless, I'm guessing?

Yup.

It's been a long time since I've actually used astro calculations to, say, find distances based on luminosity and estimations of extinction, but I am aware that I've come up with some answers that Hipparcos didn't exactly agree with... My calculations for Tarazed, for example, ranged from 200 to 300 ly depending on which end of the luminosity spectrum that particular K3II fell on; Celestia (presumably from Hipparcos) says 460. WTF?

I imagine my error range for the Trap stars would be even worse.

By the way, I don't suppose anyone might have come up with a tool to create an add-on to splatter a few thousand random/fictious stars of various types around a cube of a defined RA/decl/distance?

They're still arguing over the discrepancy between the luminosity distance and Hipparcos distance to the Pleiades.

We'll just have to wait until the GAIA satellite flies in 2011.

Hah.

Seems strange that if you have multiple data points with both methods that you get results that are very far off the mark... maybe if there's a cloud of gunk in the way that would skew the luminosity mark, but seems that's so obvious that it can't be that easy. I heard of the discrepancy but I'll have to look around on that one, haven't heard the details.

The Pleiades are important in a number of religions; there's a lot of weird shit hypothesized about them in Jewish literature, among others...

StarSeeker wrote:there's a lot of weird shit hypothesized about them in Jewish literature, among others...

Nice of you to be so respectful.

Tanketai wrote:

StarSeeker wrote:there's a lot of weird shit hypothesized about them in Jewish literature, among others...

Nice of you to be so respectful.

I agree Tanketai.

Some people believe this 'Wierd Shit', Star Seeker.

Including myself, yes; no disrespect intended. Please forgive my wording.

"Weird shit" not not imply "false" in my lexicon. I'm just still trying to puzzle it all out.

That's all good.