star distance calculation error (solved)

[phoenix]

hi,

I've noticed that celestia's shown star-distance in the upper left corner is somehow miscalculated.

lets take hip 32349 and hip 99240 for example.
celestia shows a rounded distance of 22.446 ly

using startextdump I extracted the XYZ values of those stars:

32349 -1.612565 -5.482873 -6.427964
99240 4.284276 -14.01134 13.49721

now calculating the distance like celestia does:

sqrt(($x1 - $x2) * ($x1 - $x2) + ($y1 - $y2) * ($y1 - $y2) + ($z1 - $z2) * ($z1 - $z2));

that gives me a excat distance of 22.4615235921 (rounded to 22.462)

I'm getting the same margin of error between 0.02 and 0.05 ly for any random stars-combination.
So, where is the error? wrong values in stars.txt or some miscalculation in celestia?

[ajtribick]

Is this to do with the solar system not being at the co-ordinate origin?

Try putting this .stc file in your extras directory (will break some cel URLs)

Code: Select all

0 "Sol:Sun"
{
   RA 0
   Dec 0
   Distance 0
   SpectralType "G2V"
   AbsMag 4.83

   RotationPeriod      609.12  # 25.38 days
   Obliquity             7.25  # correct orientation relative to ecliptic
   EquatorAscendingNode 75.77  #
   RotationOffset       23.00  # standard meridian
}

Does the problem remain?

[Cham]

It is time that we put back our sun on its right place. Who cares about breaking some old cel URL ? Please, put back our star at its place for accuracy !

[Malenfant]

It is time that we put back our sun on its right place. Who cares about breaking some old cel URL ? Please, put back our star at its place for accuracy !

Yeah, I agree here - not having the sun at 0,0,0 is ridiculous. If there's a new Celestia 1.5.0 coming out, this should be changed for that.

[ajtribick]

From what I can tell, the speed of light is being fixed in 1.5.0, which would break the cel URLs anyway.

So we might as well put Sol at the centre. Or maybe the Sol-Jupiter barycentre?

[Cham]

And if really some of those broken cel URL are still alive, their creator will update them.

So PLEASE Chris, update Sol's position ! In the name of accuracy !!

[chris]

I intend to make the origin the solar system barycenter, not the sun itself. The International Celestial Reference System uses the solar system barycenter as it's origin. With a new feature in Celestia 1.5.0, it will be possible to set the origin of a Sun-orbiting body to either the solar system barycenter or the Sun. It will be interesting to watch the Sun's movement around the barycenter.

--Chris

[Cham]

Will sun's movement be perceptible ? I always believed it was very small in amplitude.

[chris]

Will sun's movement be perceptible ? I always believed it was very small in amplitude.

The solar system barycenter lies just outside of the Sun, so the movement will be easily to see if you pick a coordinate system that's fixed with respect to the barycenter.

--Chris

[Cham]

Then this would be a good feature, to be able to see this.

[PlutonianEmpire]

From what I can tell, the speed of light is being fixed in 1.5.0, which would break the cel URLs anyway.

So we might as well put Sol at the centre. Or maybe the Sol-Jupiter barycentre?

The speed of light wasn't accurate in celestia before?

[phoenix]

Is this to do with the solar system not being at the co-ordinate origin?

Try putting this .stc file in your extras directory (will break some cel URLs)

Code: Select all

0 "Sol:Sun"
{
   RA 0
   Dec 0
   Distance 0
   SpectralType "G2V"
   AbsMag 4.83

   RotationPeriod      609.12  # 25.38 days
   Obliquity             7.25  # correct orientation relative to ecliptic
   EquatorAscendingNode 75.77  #
   RotationOffset       23.00  # standard meridian
}

Does the problem remain?

no the distance is still off by about 0,016 ly
moving the solar system to 0,0,0 is about 206.8 au from its original position.
but if only the coordinate-system is shiftet, the relative distances between the stars would remain intact.
something else is wrong here, maybe the difference between the star and the observer is taken into account but not calculated right towards the other star.

[rra]

maybe a little off-topic to this discussion,
but how do you get Celestia to show the distance between
stars in the top-left corner ??

Ren?©

[selden]

GoTo one of them
and Select the other.

[Malenfant]

I intend to make the origin the solar system barycenter, not the sun itself. The International Celestial Reference System uses the solar system barycenter as it's origin. With a new feature in Celestia 1.5.0, it will be possible to set the origin of a Sun-orbiting body to either the solar system barycenter or the Sun. It will be interesting to watch the Sun's movement around the barycenter.

--Chris

That'll work too

[TriKri]

How many at this forum is working with the code of celestia? Are you all programmers?

[Cham]

no the distance is still off by about 0,016 ly
moving the solar system to 0,0,0 is about 206.8 au from its original position.
but if only the coordinate-system is shiftet, the relative distances between the stars would remain intact.
something else is wrong here, maybe the difference between the star and the observer is taken into account but not calculated right towards the other star.

No, it isn't about the coordinate system. The coordinate system is ok, it is only the position of Sol which isn't correct. So changing that position will change the relative distance. It's not enough to explain your discrepancy, however.

[rra]

No, it isn't about the coordinate system. The coordinate system is ok, it is only the position of Sol which isn't correct. So changing that position will change the relative distance. It's not enough to explain your discrepancy, however.

What do you mean Cham ,
how can the distance between 2 bodies (stars) be different if the observer-coordinate changes ?

Ren?©

[Cham]

No, it isn't about the coordinate system. The coordinate system is ok, it is only the position of Sol which isn't correct. So changing that position will change the relative distance. It's not enough to explain your discrepancy, however.

What do you mean Cham ,
how can the distance between 2 bodies (stars) be different if the observer-coordinate changes ?

Ren?©

The problem is with Sol's position, and NOT the coordinate system. Actually, Sol isn't placed at (0, 0, 0). It should be there ! That's all.

[rra]

The problem is with Sol's position, and NOT the coordinate system. Actually, Sol isn't placed at (0, 0, 0). It should be there ! That's all.

I know that, but what has that got to do with the fact that the distance between the stars is not calculated correctly. I shouldn't matter at all.


Ren?©