Jupiter Moon Movie (APOD 29 Mar 2007)

[Andy Wade]

The descriptive text says there are three moons and two red spots,
But on the bottom left corner of the picture just showing as a dark spot on the limb of Jupiter, there is a fourth moon visible right at the start of the movie. Anyone know which moon it is?

http://antwrp.gsfc.nasa.gov/apod/ap070329.html

[ebow]

I was wondering about that dark spot as well. According to Sky & Telescopes Jupiter's Moons JavaScript utility (I couldn't find a simple chart like the ones that [used to?] appear in magazines) that spot was not Callisto, the only Galilean moon not accounted for (too far outside this frame). So it may just be a dark spot on the planet, a mole or beauty mark, if you will.

But... why does the "movie" alternate between forward and reverse motion? And why is south up?? Is it an Australian astronomy thing?

Edit: I know the telescope probably showed the scene south-up, but I would think it'd be convention to invert the image for production/publication/posting.

[Andy Wade]

I was wondering about that dark spot as well. According to Sky & Telescopes Jupiter's Moons JavaScript utility (I couldn't find a simple chart like the ones that [used to?] appear in magazines) that spot was not Callisto, the only Galilean moon not accounted for (too far outside this frame). So it may just be a dark spot on the planet, a mole or beauty mark, if you will.

But... why does the "movie" alternate between forward and reverse motion? And why is south up?? Is it an Australian astronomy thing?

Edit: I know the telescope probably showed the scene south-up, but I would think it'd be convention to invert the image for production/publication/posting.

Actually I think I've twigged what it is.
It's the shadow from the lowest moon in the picture.

It is a bit strange Jupiter being upside down, I wondered about that too.

[stonse]

This movie of Jupiter looks a little odd:
1) The moons are not orbiting in great circles aroud the planet; bodies in orbit should not do that when they go around a spherical mass. (Around a cylindrical mass dressed up to look like a sphere, maybe.) They are orbiting in lesser circles at higher latitudes.
2) This might just be a coincidence , but the moons and the planet's surface seem to have the same period.

After looking at it for quite a while, it does not seem to be an optical illiusion or a perspective problem either. The observation is from earth, i.e. ~infinity.
??
Shaheen Tonse.

[BMAONE23]

That "Dark spot" just entering to the bottom left of the image IS in fact the Shadow of Ganymede as this moon is moving into the front of Jupiter and causing an eclipse.

[NoelC]

The animation runs forward and backward. You're not seeing a full moon nor planet rotation, but rather just frames from a short period of time run back and forth. In some opinions that's easier on the eyes than just stopping and restarting the animation from the beginning.

-Noel

[ebow]

This movie of Jupiter looks a little odd:
1) The moons are not orbiting in great circles aroud the planet; bodies in orbit should not do that when they go around a spherical mass. (Around a cylindrical mass dressed up to look like a sphere, maybe.) They are orbiting in lesser circles at higher latitudes.

I'm not sure what you mean by "great circles" there. I only know that term in its usage describing a line on (or very near) the surface of a spherical object, as described in this Wikipedia article.

2) This might just be a coincidence , but the moons and the planet's surface seem to have the same period.

After looking at it for quite a while, it does not seem to be an optical illiusion or a perspective problem either. The observation is from earth, i.e. ~infinity.

I think I noticed the same thing as well. Ganymede and Europa, especially Europa, appear to remain over exactly the same spot on the planet's cloud-tops, as if they were mounted on poles reaching from the planet upward. IIRC, they are tidally locked, but that just means one side always faces Jupiter. I'm going to chalk it up to illusion / coincidence for now...

[Andy Wade]

This movie of Jupiter looks a little odd:
1) The moons are not orbiting in great circles aroud the planet; bodies in orbit should not do that when they go around a spherical mass. (Around a cylindrical mass dressed up to look like a sphere, maybe.) They are orbiting in lesser circles at higher latitudes.

I'm not sure what you mean by "great circles" there. I only know that term in its usage describing a line on (or very near) the surface of a spherical object, as described in this Wikipedia article.

2) This might just be a coincidence , but the moons and the planet's surface seem to have the same period.

After looking at it for quite a while, it does not seem to be an optical illiusion or a perspective problem either. The observation is from earth, i.e. ~infinity.

I think I noticed the same thing as well. Ganymede and Europa, especially Europa, appear to remain over exactly the same spot on the planet's cloud-tops, as if they were mounted on poles reaching from the planet upward. IIRC, they are tidally locked, but that just means one side always faces Jupiter. I'm going to chalk it up to illusion / coincidence for now...

An example of a 'Great Circle' is the Equator. This circumnavigates the globe at it's greatest dimension.
An example of a lesser or small circle would be the Arctic Circle, or the Tropic of Capricorn. They also circumnavigate the globe but at a higher or lower latitude than the equator, so they are smaller circles.
Just another snippet of info - 'Great Circle Sailing' is where you draw part of a great circle on a chart and sail along it in a curve, because this equates to a straight line over the surface of a globe and a straight line is of course the shortest distance between two points..

[ebow]

An example of a 'Great Circle' is the Equator. This circumnavigates the globe at it's greatest dimension.
An example of a lesser or small circle would be the Arctic Circle, or the Tropic of Capricorn. They also circumnavigate the globe but at a higher or lower latitude than the equator, so they are smaller circles.
Just another snippet of info - 'Great Circle Sailing' is where you draw part of a great circle on a chart and sail along it in a curve, because this equates to a straight line over the surface of a globe and a straight line is of course the shortest distance between two points..

I knew all that, but it helped me understand what Stonse meant: that projections of the moons' orbits onto the surface seem to be lesser circles. That was probably obvious to others. Anyway, I see what you mean. I would say that it has to be an illusion caused by perspective.

[Andy Wade]

An example of a 'Great Circle' is the Equator. This circumnavigates the globe at it's greatest dimension.
An example of a lesser or small circle would be the Arctic Circle, or the Tropic of Capricorn. They also circumnavigate the globe but at a higher or lower latitude thaon't know. the equator, so they are smaller circles.
Just another snippet of info - 'Great Circle Sailing' is where you draw part of a great circle on a chart and sail along it in a curve, because this equates to a straight line over the surface of a globe and a straight line is of course the shortest distance between two points..

I knew all that, but it helped me understand what Stonse meant: that projections of the moons' orbits onto the surface seem to be lesser circles. That was probably obvious to others. Anyway, I see what you mean. I would say that it has to be an illusion caused by perspective.

Yes, I guess you'd expect all the moons to orbit somewhere above the equator. Like Saturn's ring system.
Whether this is always true in practice, I don't know.

[iamlucky13]

I didn't catch it until you pointed it out, but you're right, the moons should either orbit over the equator or oscillate back and forth. A lesser circle is not a stable orbit because there is a component of gravitational force towards the center. Regardless of the inclination, an orbiting body always follows the circumference, not a lesser chord.

The clock on the animation shows it lasting about 2 hours. Jupiter's day is 10 hours. I'd really think we could see something noticeable in that amount of time.

Perhaps Jupiter's tilt is disguising the vertical movement. For that matter, when I've looked at Jupiter's moons through a telescope, I don't recall them seeming that close to Jupiter. Maybe there's another perspective issue here disguising how fast they're circling Jupiter.

According to Wikipedia, the closest moon is Io, which has an orbit about 6 times Jupiter's diameter and a period of 1.77 days.

[makc]

In some opinions that's easier on the eyes than just stopping and restarting the animation from the beginning.

...including the author.