APOD: Watch Jupiter Rotate (2010 Jan 24)

[APOD Robot]

Watch Jupiter Rotate

Explanation: What would it be like to coast by Jupiter and watch it rotate? This was just the experience of the New Horizons spacecraft as it approached and flew by Jupiter in 2007. Clicking on the image will bring up a movie of what the robotic spacecraft saw. Visible above in the extensive atmosphere of the Solar System's largest planet are bands and belts of light and dark clouds, as well as giant rotating storm systems seen as ovals. Other movies compiled by New Horizons and other passing spacecraft have captured the clouds swirling and moving relative to themselves. Jupiter has a diameter of about eleven times that of our Earth, and rotates once in about 10 hours. The robotic New Horizons spacecraft, launched four years ago last week, continues to speed toward the outer Solar System and has recently passed the halfway point between Earth and Pluto. New Horizons will reach Pluto in 2015.

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[pacfandave]

The video shows one complete rotation of Jupiter. The camera is static. You would think that, since the spacecraft is flying by the planet, the camera would be moving, yet the planet's image remains stable. Was ist loss?

[Chris Peterson]

The video shows one complete rotation of Jupiter. The camera is static. You would think that, since the spacecraft is flying by the planet, the camera would be moving, yet the planet's image remains stable. Was ist loss?

I assume the image was taken while the spacecraft was still some distance from Jupiter. The linked stills suggest the image was collected from a distance of about 29 million km, meaning that the planet would have only grown in size by about 3% over a ten hour exposure, and would not have significantly shifted in the frame.

Regardless of the amount of actual shift, however, I expect the imaging team rescaled and repositioned each frame so that the planet appeared completely stationary. By doing so, they limited the animation to just the rotation. If you've seen other animations of planets during flybys, you know that (while interesting in their own right) combinations of shifting and scaling can make it harder to see just the rotation effects.

[pacfandave]

Thanks for your explanation. I'd prefer they'd simply present the actual footage instead of tinkering with it.

[skyhound]

What really gets me is that when they re-assembled the images they forced Jupiter to be round, which it definitely isn't. When you add in the effect of the slight phase this gives Jupiter the appearance of an egg standing on end. Making Jupiter round is a common mistake. I see it in computer generated images of Jupiter (and Saturn too) a lot. What I don't get is 1) why nobody but me seems to notice (it stands out like the proverbial sore thumb to my eye) and 2) why this happens when it is trivial to make it the correct shape.

[makc]

wiki sais: Equatorial radius 11.209 Earths Polar radius 10.517 Earths - do you really expect 11.2:10.5 to give it "appearance of an egg standing on end"?

[skyhound]

wiki sais: Equatorial radius 11.209 Earths Polar radius 10.517 Earths - do you really expect 11.2:10.5 to give it "appearance of an egg standing on end"?

Just look at the rotation movie we are discussing. As I said, the reason it looks elongated in the vertical direction is because of the slight phase, which puts a little of the left side of the image in darkness, in combination with their forcing Jupiter to be round. Have a look at any image of Jupiter and compare it to the movie. The geometric flattening of Jupiter is clearly visible, at least to me, in true images of Jupiter. Again, what I don't understand is why it isn't as obvious to others... Look at this page:

http://www.anthonares.net/2005/11/publi ... ating.html

Compare the actual image (left) with the spherical model on the right. Thanks for fixing my account.

[geckzilla]

It does look like they've put Jupiter on a diet. It may have been unintentional. Who knows? I wouldn't assume that the person who does video editing for NASA knows Jupiter isn't a perfect sphere or that they consulted someone who should. It does look like a pretty elementary mistake, though.

[Chris Peterson]

What really gets me is that when they re-assembled the images they forced Jupiter to be round, which it definitely isn't.

I don't believe that the aspect ratio of the image has been altered significantly. I can lay an ellipse on top of the image which is 6% wider than it is high, and it follows the well delineated edges of the planet. You are seeing an illusion because of the phase angle, which hides the left, and softens the top, and bottom quadrants. The planet is oblate in this video.

[bystander]

Thanks for fixing my account.

You are quite welcome. Welcome aboard. Am I correct in assuming you are The Skyhound?

[geckzilla]

What really gets me is that when they re-assembled the images they forced Jupiter to be round, which it definitely isn't.

I don't believe that the aspect ratio of the image has been altered significantly. I can lay an ellipse on top of the image which is 6% wider than it is high, and it follows the well delineated edges of the planet. You are seeing an illusion because of the phase angle, which hides the left, and softens the top, and bottom quadrants. The planet is oblate in this video.

Are you totally sure? I did the same thing except I tried with both a "perfect" (I'm not sure vector-based circles are actually 100% perfect) and an oblate circle and got different results. I could easily be fooling myself but the "perfect" circle seems to fit better. The poles seem a little off center for the oblate one but it's hard to tell.
http://www.geckzilla.com/apod/is_jupiter_squished.jpg

[skyhound]

I did the same thing except I tried with both a "perfect" (I'm not sure vector-based circles are actually 100% perfect) and an oblate circle and got different results. I could easily be fooling myself but the "perfect" circle seems to fit better. The poles seem a little off center for the oblate one but it's hard to tell.
http://www.geckzilla.com/apod/is_jupiter_squished.jpg

I tried this as well by making a screen capture and placing a circle and then an ellipse over the image and got an even stronger correlation with the circle than you did. Unfortunately It depends on the size of the circle you choose so I'd have to say it is inconclusive. Given that this is a spacecraft image the phase angle is larger than we usually see on earth, so Chris may be right that it is an optical illusion. But sadly there are many other examples of computer-generated images of Jupiter that are round.

I wonder if there is an issue with the aspect ratio delivered by YouTube on different computers?

[skyhound]

Thanks for fixing my account.

You are quite welcome. Welcome aboard. Am I correct in assuming you are The Skyhound?

Yes, that's me.

[geckzilla]

As far as I can tell, here are the original Jupiter rotation images:
http://pluto.jhuapl.edu/soc/index.php?page=15
http://pluto.jhuapl.edu/soc/index.php?page=16
http://pluto.jhuapl.edu/soc/index.php?page=18

Clearly, whoever made the animation has manipulated the shape of Jupiter.

[Chris Peterson]

I tried this as well by making a screen capture and placing a circle and then an ellipse over the image and got an even stronger correlation with the circle than you did. Unfortunately It depends on the size of the circle you choose so I'd have to say it is inconclusive.

I agree. It depends very much on where you assume the invisible edges actually are. However, looking at other images posted on the JHUAPL site, made around the same time, it does look like the animated sequence has had its aspect ratio altered. I don't know why, since the still images themselves are not altered, and other images associated with press releases are unaltered as well. It is clear that they normally leave the aspect ratio correct in their published images.

I might speculate that the alteration was inadvertent- associated with with the video software used to construct the image. Video software tends to try very hard to force things into a 3:4 (or sometimes 9:16) ratio, which causes all sorts of problems when the source material doesn't start with that aspect ratio. In this case, the images had to be individually rotated and aligned, then appropriately cropped and possibly resized. Plenty of places for things to go wrong.

[geckzilla]

I do wonder if the LORRI actually did a video capture or if they took the individual images and created a map of Jupiter and then applied it to a sphere and then animated that sphere. I'm being really speculative though and am probably wrong.

[Chris Peterson]

I do wonder if the LORRI actually did a video capture or if they took the individual images and created a map of Jupiter and then applied it to a sphere and then animated that sphere. I'm being really speculative though and am probably wrong.

This looks like it was constructed simply by animating frames. The synthesized images have a different look.

[geckzilla]

Yeah, I'm just annoyed because none of the frames from the website seemed to match up. And not a single moon ever gets near or transits. The frame rate seems smoother than the raw images and the raw images weren't taken at perfectly regular intervals either. And the clouds seem static, though the movement of the individual bands might not be apparent from one rotation. Oh well, way too much free time on my hands again.

[lambertd]

I propose to prepare a practical exercise about the Period of rotation and the speed of Jupiter with your movie "Jupiter rotation from new
horizons". But I need to know the time between each picture of the movie.
Could you help me?

[geckzilla]

None of that information was provided as far as I know, lambert. Which is why all of the silly speculation is contained in this thread. However, there are some shots of Jupiter with very specific time stamps which would probably work for your exercise right here:
http://pluto.jhuapl.edu/soc/index.php?page=15
http://pluto.jhuapl.edu/soc/index.php?page=16
http://pluto.jhuapl.edu/soc/index.php?page=18