Brent Thanks. You are right. I found this simple equation xyz = [cos(dec)*cos(ra), cos(dec)*sin(ra), sin(dec)] which works when ra and dec are given in radians I have to do a little translation since the z-axis in my coordinate system is perpendicular to the ecliptic, where in the equatorial system the z-axis is aligned with the earth's axis of rotation. Should be easy enough, if I can just remember vector math (add? subtract? cross product? dot product?) Thanks again Dion ________________________________ From: Brent Watson <brentjwatson@yahoo.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Thursday, February 7, 2013 3:10 PM Subject: Re: [Utah-astronomy] Math problem: vectors for planet axial tilt Dion,   You can use the RA and DEC for pointing your aves.  The stars are far enough away that tere is negligible change in direction.  What will cause errors is precession, but that happens over such a long time period (tens of thousands of years) that it shouldn't make any difference.  I believe you'll be good by just using the RA and DEC.   Brent From: Dion Davidson <diondavidson@yahoo.com> To: "Utah-Astronomy@mailman.xmission.com" <Utah-Astronomy@mailman.xmission.com> Sent: Thursday, February 7, 2013 9:14 AM Subject: [Utah-astronomy] Math problem: vectors for planet axial tilt I'm working on a visual simulation of the solar system for presentation to a homeschool group. I'm using GlowScript (very cool - a lot like VPython). The theme of the presentation is "Why don't things line up?" and I want to highlight how axial rotations and orbital planes don't really line up in our solar system. I can see from nasa fact pages the angle of axial tilt for each planet, which I assume is the angle between the rotation axis and the orbital plane of the planet (as opposed to the ecliptic).  (ex: http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html) Here's my question: what is the vector that represents these axes in 3D? Right now all my axes point in the same (roughtly celestial north) direction, which is less interesting than real-life, I assume. I see a section titled "North Pole of Rotation" which gives an RA and Dec for each planet. Does that mean if I draw a straight line through the axis of each planet it points to that part of the sky? (I don't really believe that because things move, so it seems those coordinates would be specific to this moment in time.) I'm looking for suggestions if anyone knows a good way to define the axes of our solar system in 3D. (I'm using an XYZ coordinate system centered on the center of the sun.) One other question, for those who may be inclined - I'm trying to build my model so that the motions are represented. I'm not building the model to scale so its easier to see, which means I'm not using gravity to model the motions. Do you know a good way to model the motions of the planets (and moons in particular) using only velocity vectors (no acceleration vectors)? There is probably a better way to ask the question I'm asking. I just want to model things moving in a realistic way without having to introduce gravity. Thanks! Dion _______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Send messages to the list to Utah-Astronomy@mailman.xmission.com The Utah-Astronomy mailing list is not affiliated with any astronomy club. To unsubscribe go to: http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Then enter your email address in the space provided and click on "Unsubscribe or edit options". _______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Send messages to the list to Utah-Astronomy@mailman.xmission.com The Utah-Astronomy mailing list is not affiliated with any astronomy club. To unsubscribe go to: http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Then enter your email address in the space provided and click on "Unsubscribe or edit options".