http://space.jpl.nasa.gov/cgi-bin/wspace?tbody=599&vbody=399&month=4&day=9&year=2007&hour=14&minute=00&rfov=120&fovmul=-1&bfov=50&porbs=1&brite=1

That is Jupiter as seen from Earth on April 9th, 2007 at 14:00 UTC (I don’t know when the data was recorded by Hubble, but based on the simulator output, it must have been around that time.)

For Algirdas, and other people wondering about the “poodle spinning about about the elephant’s ankles”, remember that the axis of both Earth and Jupiter are inclined with respect to the ecliptic. The JPL simulator output displays a line along Ganymede’s orbit, which makes the effect of the axial tilt quite clear.

But can anyone answer my questions please :

1) Which is the solar system’s largest moon – Ganymede or Titan?
(Various sources keep saying one or the other but still not sure which.)

2) Is the answer to (1) affected by whether or not you count Titan’s atmosphere?

&

3) Does density / mass make a difference here? Ie. is Ganymede larger but Titan more massive or vice-versa?

Answers anybody? Please?

I must admit i wasn’t familiar with what Ganymede looked like before seeing the video, what i mistakenly saw as a specular highlight (as in, light reflected from a light source to the viewer on a shiny surface) is indeed a large whitish spot on the moon that i can now see on other pictures. DUH
Thanks a lot for your explanation, it makes perfect sense, it’s just that, ‘intuitively’ speaking (for someone who really doesn’t know much about astronomy / astrophotography) the video felt fake… a good example of how one can fool himself when he doesn’t know much about something!

I’m not sure what you mean by this highlight that looks fake. I thought it looked pretty much like every other photo of Ganymede I’ve seen.

As for it not changing, let’s do some math. First, remember that Ganymede–just like Earth’s moon–is tidally locked to the planet it orbits. In other words, Ganymede always shows the same face to Jupiter. In other other words, its orbital period of 171.75 hours is also its rotational period. The video was compiled from pictures shot over a 2 hour period. Ganymede appears in about 14 seconds of that 18 second video (I was too lazy to get an exact frame count for the moon), meaning that we’re seeing Ganymede for (14/18)*2hours = 1.6 hours (roughly). Since Ganymede’s rotational period is about 172 hours, what you are seeing in the video, is about 0.9% of Ganymede’s rotation.

Do you really think that you would be able to see a less than 1% rotation? (hint: In the high definition video, Ganymede’s diameter is only 50 pixels.)

this looks totally fake to me. I already got the feeling that something was wrong when looking at the picture in a previous post, but it’s even more obvious in the movie: what’s that specular higlight showing up on Ganymede ??? (it looks like a marble ball…)
when you look at the HD version you can see Jupiter spinning (rather quickly) yet ganymede doesn’t change *AT ALL* except for the translation motion.
To me this look like something made in 5 minutes with 3D studio or Maya, wouldn’t be surprised if it turns out to be an hoax.

Over the years & sources some say one some t’other. Is there any final answer?

But which is bigger – Titan or Ganymede?

I still get different peoples saying each is larger – & does counting the atmosphere for Titan’s size make any difference or not?

Or is one larger in actual diameter but more massive?

In response to T_U_T’s statement that this might be a hoax, this appears to be exactly what the Hubble Site claims it is: approx 2 hours of Jupiter/Ganymede viewing. Since Jupiter’s period of rotation is under 10 hours, we get to see it rotate about 75 degrees. If you track that “little” red storm that starts out on the left side, you can confirm this for yourself.

As for cloud movement, I think you are just being thrown off by videos you have seen in the past, which are over much longer periods, where there is at most 1 exposure every rotation, so that a given storm is seen to stay in one place, and then the spinning of storms and relative motion of the bands becomes apparent.

As for Ganymede’s orbit following Jupiter’s rotation, that is just not the case, since its orbital period is 172 hours, so we are privileged to watch just about 1% of it in this video. But this makes sense when since Ganymede’s average orbital radius is about 7.5 Jupiter diameters, so if this video started 40 hours earlier, we would need a MUCH wider view to see both bodies in the same frame.

Finally, pertaining to Algirdas’ “poodle spinning about elephant’s ankles” comment, this confusion is also caused because we are only seeing 1% of Ganymede’s orbit, and because our view from Earth did not provide an edge-on view of this orbit. The orbit projected to our viewing angle of course ends up being an ellipse (same as all those spiral galaxies that look like ellipses from our vantage point), and if the video showed the moment 86 hours later when Ganymede passes in front of Jupiter, we would see it as a “bird flying around a giraffe’s neck”.

T_U_T, there does not seem to be anything wrong with the video to me. The timescale of the cloud movement is much longer than the rotation speed.

I am SOOO putting that movie up on my blog too!