Celestia Forums

Now that the orbits of the moons of Jupiter and Saturn are well defined in Celestia, maybe it's time to tackle the moons of some of the more distant planets...

Unfortunately, it appears that the orbits of the Uranian moons may be off by 180 degrees.

Dated photographs of Uranus and its moons really seem to be scarce on the public parts of the Web.

Hubble took a picture of Uranus on August 14, 1994, revealing its rings and the positions of quite a few of its moons. See http://observe.arc.nasa.gov/nasa/gallery/image_gallery/solar_system/solar_uranus2.html and also http://hubblesite.org/newscenter/archive/1994/50/image/a. The picture shows Miranda at about the 4 o'clock position and Ariel at 10 o'clock.

(The only other picture I've found doesn't include Miranda, Ariel or Oberon, the three moons that Celestia tries to have accurate orbits for. It's at http://hubblesite.org/newscenter/archive/1997/36/image/a.)

Setting that date in Celestia, below is the best correspondance that I can get using a telescopic view of Uranus from the near vicinity of the Earth. The orbital periods of both Miranda and Ariel are significantly more than a day. This is the only view I could manage that shows them on opposite sides of the planet on the 14th. Ariel is at the 4 o'clock position and Miranda at 10 o'clock. Rotating the image 180 degrees would put the moons in the right positions, but then the illuminated pole of Uranus would be pointing toward the right instead of toward the left to agree with Hubble's view.


(this links to a higher resolution image)

selden wrote:Dated photographs of Uranus and its moons really seem to be scarce on the public parts of the Web.

The best way to check is to use heliocentric positions from Horizons to create a .xyz file for the different moons.

(The only other picture I've found doesn't include Miranda, Ariel or Oberon, the three moons that Celestia tries to have accurate orbits for. It's at http://hubblesite.org/newscenter/archive/1997/36/image/a.)

Celestia also has accurate orbits for Titania and Umbriel.

Anyhow, it's worth checking the orbits of the Uranian satellites . . . I compared them against JPL's solar system simulator when I wrote the custom orbits, but with the changes made to solarsys.ssc recently, it's quite possible an extra 180 degree rotation was introduced.

--Chris

chris wrote:The best way to check is to use heliocentric positions from Horizons to create a .xyz file for the different moons.

You're right of course. But I'm in a visual mood this evening
And it's given me an excuse to see what's on the Voyager CDs I've had in storage for so long. I just hope I can find software that can display the images.

I'll try Horizons xyz files in a little while.

chris wrote:Celestia also has accurate orbits for Titania and Umbriel.

Yup. Sorry. Obviously I was scrolling too fast and managed to skim right past them.

grr. I'm annoyed that a quick search did not turn up a working version of imdisp or descendant.

So I got the xyz orbits of the Uranian moons defined in Celestia.
I am sad to report that indeed they're all 180 degrees away from where they're supposed to be on that day in 1994. They do travel in the right directions, always staying on the opposite side of the planet.


(As usual, this links to a high-resolution image)

The moons labeled with just their names are at the positions calculated by Celestia. Those with the suffix -94-xyz are at the locations calculated by Horizons. The xyz and ssc files that I used are available at http://www.lns.cornell.edu/~seb/celestia/spacecraft.html#3.4.10

selden wrote:Unfortunately, it appears that the orbits of the Uranian moons may be off by 180 degrees.

Hi Seldon, are the pics on this ESO site any help ? :-
http://www.eso.org/outreach/press-rel/p ... 31-02.html

I have not studied your prob/this thread in detail (so not sure how relevant this will be) but remembered your post when I was looking on APOD and found the above link via the APOD Jan15
http://antwrp.gsfc.nasa.gov/apod/ap030115.html

Malcom,

Thanks! That does provide another visual example to compare to Celestia.

Well, I loged back in to say that I'm still only learning to drive Celestia, so this may not be of any help, (it may not even be valid!), but I have to advance my time to 2100ut to get Miranda into position and get a reasonable match with that ESO picture.

But now that I am back I have only just seen Grant's post about Miranda on Jan 15 in which he also pointed out the ESO site !
Sorry about the duplication of links folks,,
ho-hum so much to read and do and so little time to do it all :( !

Malcom,

Celestia does not (yet) implement light travel time delay.
Uranus was 20AU away on November 19th 2002.
Light takes about 8.3 minutes to travel 1AU, so that's a delay of 166 minutes, or 2h46min.
The picture was taken at 03:00 UT so that should correspond to 00:14 UT at Uranus or as Celestia shows it.

And add on to that the fact that Celestia shows the moons off by 180 degrees, which explains why you had to advance the clock to get a reasonable visual match.

(I'll generate an image using xyz trajectories from Horizons as soon as I've caught up with reading Forum posts.)

Here's the image generated by Celetia of Uranus on Nov. 19, 2002 at 00:16 UT.

Moons labelled with just their names are at positions calculated by Celestia. the -02-xyz suffix indicates the heliocentric xyz positions calculated by Horizons. Again, they're 180 degrees away.


(as usual, this is a link to a high resolution picture]

The xyz positions seem to produce a reasonable approximation to the ESO image, although I didn't try to rotate it to the same orientation.

Thanks Selden, excellent explanation, I had not thought of the speed of light issue !
Thinks,, : and for Celestia's next trick --- to simulate the speed of gravity also !! I have just been reading of some (controvercial?) measurements involving a recent close pass of Jupiter and a quasar which supposedly confirmed speed of light applies to gravity within err 20%(?)

The xyz positions seem to produce a reasonable approximation to the ESO image, although I didn't try to rotate it to the same orientation.

I have just done a quick edit/overlay/rotate of a line through your xyz positions and placed them on the ESO image, and yes I agree, good fit !
( would have been a perfect fit if I had been better with my drawing-tool skills )
my humble efforts can be seen here http://www.ptarmigan.fsnet.co.uk/ufitt.jpg
in which the green lines are from your Celestia pic