Correct orientation of satellites

[Guest]

Does that imply that the current maps would be correctly alighed if the EAN were corrected?

No, the RotationOffset I give is also required to bring the prime meridian into the correct position.
Celestia seems to orientate the planets at J2000.0 with their equators aligned with the ecliptic, and their prime meridians pointing towards 12h RA. The Obliquity setting then tilts their north poles towards 18h, and EquatorAscendingNode screws them around in the plane of the ecliptic. To finally orientate the prime meridian, you need to insert a RotationOffset, too, which requires a bit of fiddly calculation because the position of the prime meridian is given in the books with reference to the ascending node of the planet's equator on the plane of the Earth's equator, not the ecliptic plane.
So the RotationOffset I give also plays an important part.

Grant

[selden]

Grant,

Well, I'm still confused, which is nothing new

I just inserted all of your suggested changes for Mars, and Syrtis Major is sill not quite centered when NASA predicted it would be during the 2001 opposition.

As best I can tell from their Web page at http://mars.jpl.nasa.gov/mgs/msss/camera/images/opposition_6_2001/, Syrtis Major should be centered from the Earth's POV at some time between 8 and 9 UTC on June 13, 2001. Maybe closer to 9. I find that hard to judge.

With your corrections, Celestia seems to show Syrtis Major centered at about 7:32 UTC. Even allowing for light travel time (4 minutes?), that's a little too far off, I think.

sigh.

[granthutchison]

Well, I'm still confused ...

Me too. The NASA images are definitely different from the ones my changes to Celestia generate.
But if you download Sky & Telescope's nice little freeware program Mars Previewer II at http://skyandtelescope.com/resources/software/article_328_1.asp, you'll find my stuff puts Mars pretty much precisely as it should be. (If you lay my grid on to Mars, you can eyeball this better than using albedo features - but remember the grid is marked in east longitude while Mars Previewer works in west longitude.)
I've also searched around and checked Mars Previewer's central meridian predictions against various websites and books, and it matches other published data, so it seems unlikely that this is a fortuitious agreement between some error of mine and some other error in Mars Previewer.
It's a worry, but NASA does seem to be the odd one out on this occasion .

Grant

[selden]

I suspect you're quite right. Methinks NASA may have slipped a digit in their timing -- like maybe miscalculated the offset from their local time zone to GMT.

[chris]

Does that imply that the current maps would be correctly alighed if the EAN were corrected?

No, the RotationOffset I give is also required to bring the prime meridian into the correct position.
Celestia seems to orientate the planets at J2000.0 with their equators aligned with the ecliptic, and their prime meridians pointing towards 12h RA. The Obliquity setting then tilts their north poles towards 18h, and EquatorAscendingNode screws them around in the plane of the ecliptic. To finally orientate the prime meridian, you need to insert a RotationOffset, too, which requires a bit of fiddly calculation because the position of the prime meridian is given in the books with reference to the ascending node of the planet's equator on the plane of the Earth's equator, not the ecliptic plane.
So the RotationOffset I give also plays an important part.

Your explanation of Obliquity, EquatorAscendingNode, and RotationOffset is exactly correct . . . An important thing to note is that in an .ssc file, the obliquity is the angle between the rotation axis of a planet and the ecliptic, whereas many tables of solar system data list the angles between rotation axis and orbital plane. I'm not certain--but it's likely--that my use of 'obliquity' for the angle between rotation axis and ecliptic is incorrect. In any case, the incorrect value for the tilt of Mars is due to my confusion (many months ago ) about the reference plane for the obliquity. In anyone's interested, I have a small C program that I use to convert J2000 RA and declination into Obliquity and EquatorAscendingNode.

There's also a PrecessionRate parameter that gives the rate at which the rotation axis rotates about the normal to the plane of the ecliptic. One other useful parameter is Orientation, which is an axis-angle rotation applied before Obliquity, etc. that doesn't affect the axis of rotation; it's helpful for adjusting spacecraft and irregular satellite models so that they face the right way.

--Chris