Hi,
just take a look at this drawing here. Pay attention to the red circle, where the problem is.

[img=http://img15.imageshack.us/img15/3239/earth1g.png]

I have to connect the two ellipses with the oblique line in 2 unique points of interception.
The oblique ellipse and the oblique live must have an angle of 23.5 degrees, but I have a problem with prospective or something because the two ellipses connect themselves in 2 points where the oblique line is not passing.

You probably just have to take a look at the drawning (pay attention to the red circle, where the problem is), because it's easier to look at it then saying it in words.

I actually was the Guest above.
Here is the link for the image: http://img15.imageshack.us/my.php?image=earth1g.png

And here you can find the SVG file: http://rapidshare.com/files/218858132/earth1.svg.html

OK I'm not sure what you are asking here. Did you do the math given the angle between the ellipses and get a result they they should intercept at a 23.5 angle? Did you make them using a plotting software so you know the equation for the ellipse? I don't know for sure if inkscape create a real ellipse or does and an approximation when you make them in inkscape. If you are plotting function I don't think inkscape is the best place to do that. If you can fudge it just fatten up the ellipse so they intercept where you want them too. It's going to be a iterate process fatten one then fatten the other a bit.

EarlyBlake wrote:OK I'm not sure what you are asking here. Did you do the math given the angle between the ellipses and get a result they they should intercept at a 23.5 angle? Did you make them using a plotting software so you know the equation for the ellipse? I don't know for sure if inkscape create a real ellipse or does and an approximation when you make them in inkscape. If you are plotting function I don't think inkscape is the best place to do that. If you can fudge it just fatten up the ellipse so they intercept where you want them too. It's going to be a iterate process fatten one then fatten the other a bit.

Thanks, I think so. I think that Inkscape makes just an approssimation of the ellipses, so I won't ever be able to obtain a point where both the two ellipses that I have drawn and the oblique line are intercepting each other!

I think the problem is you have rotated the ellipse towards you but not the earth as the poles are at the top and bottom.Both need to be rotated together.

Actually I want to do something like this, only that bigger and more detailed.

http://www.realityreviewed.com/Celestial%20Sphere.png

And also I want to have dashes in the rear parts of the ellipses.

Does anybody know how it could be done properly?

I'm pretty sure Inkscape's ellipses would be mathematically correct.

So, I took your example image. I positioned an ellipse that matched the equator. I then rotated it so it matched the 25.5 ellipse. Lo and behold, the 23.5 ellipse is wider.

I would guess that with a correct 2d projection of a 3d celestial sphere this would be the case, but I'm not going to bother working it out. It's clear that it's width varies according the rotation around the (earth's) axis.

The assumption that the two ellipses are the same width is the cause of the initial error.

yeah, only if the points of intersection aligned with the axis would the ellipses be identical.