APOD: Ganymede's Shadow (2017 Mar 25)

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APOD: Ganymede's Shadow (2017 Mar 25)

Post by APOD Robot » Sat Mar 25, 2017 4:09 am

Image Ganymede's Shadow

Explanation: Approaching opposition early next month, Jupiter is offering some of its best telescopic views from planet Earth. On March 17, this impressively sharp image of the solar system's ruling gas giant was taken from a remote observatory in Chile. Bounded by planet girdling winds, familiar dark belts and light zones span the giant planet spotted with rotating oval storms. The solar system's largest moon Ganymede is above and left in the frame, its shadow seen in transit across the northern Jovian cloud tops. Ganymede itself is seen in remarkable detail along with bright surface features on fellow Galilean moon Io, right of Jupiter's looming disk.

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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by starsurfer » Sat Mar 25, 2017 8:36 am

Damien Peach is a legend, Emily Lakdawalla would be proud! :)

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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by heehaw » Sat Mar 25, 2017 9:49 am

Impressively sharp indeed! A wonderful portrait of Jupiter. When I was an MA student at Toronto, I was one day at the David Dunlap Observatory just after a thunderstorm, and observed Jupiter not from the great 74" telescope but from one of the smaller telescopes atop the administration building. I could not believe my eyes! I had NEVER seen Jupiter like that before! The detail visible was unbelievable! I had, however, seen drawings of Jupiter before giving such detail. This experience made me aware that on rare occasions, local atmospheric conditions can be truly exceptional for observing. A valuable lesson for me; I was truly astonished at what I saw!

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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by ta152h0 » Sat Mar 25, 2017 2:40 pm

Would that be an annular eclipse from the " surface " ?
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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by Chris Peterson » Sat Mar 25, 2017 2:47 pm

ta152h0 wrote:Would that be an annular eclipse from the " surface " ?
No, a total eclipse, with Ganymede subtending a larger angle than the Sun (so no ring of fire).
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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by sunson » Sat Mar 25, 2017 2:48 pm

Given the curvature of the surface, I would think the shadow should look oval !!! Not round, unless you are right on the line of sight from the sun.

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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by Chris Peterson » Sat Mar 25, 2017 3:01 pm

sunson wrote:Given the curvature of the surface, I would think the shadow should look oval !!! Not round, unless you are right on the line of sight from the sun.
It would if you were right above it. But from our viewpoint, the shadow is also foreshortened. We see most transit shadows as appearing quite round, unless they fall very close to the edge.
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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by ta152h0 » Sat Mar 25, 2017 3:04 pm

not really knowing how big the sun looks from the surface of Jupiter, can I translate the answer given as to the Sun iappearing behind ganimede for a time period ?
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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by Chris Peterson » Sat Mar 25, 2017 3:49 pm

ta152h0 wrote:not really knowing how big the sun looks from the surface of Jupiter, can I translate the answer given as to the Sun iappearing behind ganimede for a time period ?
The Sun is about 6 arcmin across from Jupiter (one-fifth the size it appears from Earth). Ganymede is about 18 arcmin across from Jupiter's cloud tops. The transit time from a given location is substantially determined by Jupiter's rotation speed, as Ganymede takes seven days to orbit Jupiter, and Jupiter completes one full rotation in only ten hours.
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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by rleever@pacbell.net » Sat Mar 25, 2017 4:11 pm

Do the moons of Jupiter revolve in the same plane and is that plane at right angles to the poles?

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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by Chris Peterson » Sat Mar 25, 2017 4:37 pm

rleever@pacbell.net wrote:Do the moons of Jupiter revolve in the same plane and is that plane at right angles to the poles?
The inner and Galilean moons all orbit within a degree of Jupiter's equatorial plane. The outer moons orbit well off the equatorial plane, and group into families with different inclinations, presumably based on their collision history. A complete list of orbital inclinations can be found here.
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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by Coil_Smoke » Sat Mar 25, 2017 4:51 pm

Wow, Ground based optical imaging continues to astound :shock: . The above discussion is pretty great too...

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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by NGC3314 » Sat Mar 25, 2017 8:53 pm

sunson wrote:Given the curvature of the surface, I would think the shadow should look oval !!! Not round, unless you are right on the line of sight from the sun.
There's a good illustration of this from a very different viewpoint in this sequence of JunoCam images of Ganymede's shadow.

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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by FLPhotoCatcher » Sat Mar 25, 2017 11:10 pm

I'm sure others have noticed the black rectangles near the two moons... but what are they? Was Jupiter too bright compared to the moons to get them all in one exposure?

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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by Chris Peterson » Sun Mar 26, 2017 1:07 am

FLPhotoCatcher wrote:I'm sure others have noticed the black rectangles near the two moons... but what are they? Was Jupiter too bright compared to the moons to get them all in one exposure?
Actually, the albedos of Jupiter and Ganymede aren't all that different, so both require similar exposures.
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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by Ann » Sun Mar 26, 2017 5:17 am

I'm late to the party, but this is indeed a great picture, and Damien Peach has posted several other superb Jupiter images here at Starship Asterisk*.

And this is an enjoyable and informative thread, too!

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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by LirrelJohn » Mon Mar 27, 2017 7:26 am

Hi.
As you work at an observatory, could I as what "p" is, please? Top left side, I'm guessing that "N" is north but I can't decide between many possibilities for the orthogonal "P" direction.
"Positive"? "Prograde"? "Pandas-this-way"?
Thanks.

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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by NGC3314 » Mon Mar 27, 2017 1:24 pm

LirrelJohn wrote:Hi.
As you work at an observatory, could I as what "p" is, please? Top left side, I'm guessing that "N" is north but I can't decide between many possibilities for the orthogonal "P" direction.
"Positive"? "Prograde"? "Pandas-this-way"?
P=preceding, the opposite of F=following. This convention in observational astronomy (going back at least to the 19th century) avoids confusion in using east/west, since the usual astronomical definition of those looks backwards because we're looking up rathe than down as for a terrestrial map. "Preceding" is the part of the scene that goes by first while the Earth rotates (so west by the astronomical map definition). So if north is up, preceding to the right matches the appearance in the sky.

East and west flipped on Moon maps during the 1950s, from the astronomical convention to the one matching Earth maps, when it became clear that people would shortly be needing to deal with lunar landscapes in the same way.

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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by Visual_Astronomer » Mon Mar 27, 2017 7:11 pm

Chris Peterson wrote:
sunson wrote:Given the curvature of the surface, I would think the shadow should look oval !!! Not round, unless you are right on the line of sight from the sun.
It would if you were right above it. But from our viewpoint, the shadow is also foreshortened. We see most transit shadows as appearing quite round, unless they fall very close to the edge.
I've watched shadow transits from start to finish, and yes, the shadow is appears oval when it is at the extreme edge. Otherwise, it looks pretty round.

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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by LirrelJohn » Tue Mar 28, 2017 12:22 am

NGC3314 wrote:
LirrelJohn wrote:Hi.
As you work at an observatory, could I as what "p" is, please? Top left side, I'm guessing that "N" is north but I can't decide between many possibilities for the orthogonal "P" direction.
"Positive"? "Prograde"? "Pandas-this-way"?
P=preceding, the opposite of F=following. ...
Thank you, Professor, that was useful.
I didn't really imagine it referred to the preferential direction for finding pandas but in Science one can never really tell. Any bunch that has colour, charm, strangeness and quarks is capable of just about any terminological humour.

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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by LirrelJohn » Tue Mar 28, 2017 12:24 am

FLPhotoCatcher wrote:I'm sure others have noticed the black rectangles near the two moons... but what are they? Was Jupiter too bright compared to the moons to get them all in one exposure?
Black rectangles? Where? I see none.




I'm manfully avoiding the humorous retort of "Monoliths".

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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by Chris Peterson » Tue Mar 28, 2017 12:28 am

LirrelJohn wrote:
FLPhotoCatcher wrote:I'm sure others have noticed the black rectangles near the two moons... but what are they? Was Jupiter too bright compared to the moons to get them all in one exposure?
Black rectangles? Where? I see none.
There are large black rectangles between each moon and Jupiter- darker than the background. If you're not seeing them, you need to adjust your monitor- your black point is set too high, and you're missing a lot, especially in astronomical images.
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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by LirrelJohn » Tue Mar 28, 2017 12:43 am

Chris Peterson wrote:
LirrelJohn wrote:
FLPhotoCatcher wrote:I'm sure others have noticed the black rectangles near the two moons... but what are they? Was Jupiter too bright compared to the moons to get them all in one exposure?
Black rectangles? Where? I see none.
There are large black rectangles between each moon and Jupiter- darker than the background. If you're not seeing them, you need to adjust your monitor- your black point is set too high, and you're missing a lot, especially in astronomical images.
Okay, thank you, sir, I'll take your word for it as I was rather expecting something like this. I am reluctant to fiddle with the monitor or the OS's colour, gamma, brightness and other settings as it takes me quite a while to get each new setup set up so it doesn't hurt my eyes. New kit is, to me, garish and painful.
I'm happy to accept that it's my fault.
Sorry to have bothered you and thank you for the help.




So, definitely not Swiss Army Monoliths, then?

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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by LirrelJohn » Tue Mar 28, 2017 12:50 am

Chris Peterson wrote:
LirrelJohn wrote:
FLPhotoCatcher wrote:I'm sure others have noticed the black rectangles near the two moons... but what are they? Was Jupiter too bright compared to the moons to get them all in one exposure?
Black rectangles? Where? I see none.
There are large black rectangles between each moon and Jupiter- darker than the background. If you're not seeing them, you need to adjust your monitor- your black point is set too high, and you're missing a lot, especially in astronomical images.
Okay, I saved the best resolution image and opened it in IrfanView. Auto-colouring it shows the black rectangles around the moons.
It also shows JPEG-ish coronal-flaring artefacts around the planet. All other considerations aside, I prefer the view I normally get.
Thank you.

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Re: APOD: Ganymede's Shadow (2017 Mar 25)

Post by Chris Peterson » Tue Mar 28, 2017 12:59 am

LirrelJohn wrote:
Chris Peterson wrote:
LirrelJohn wrote: Black rectangles? Where? I see none.
There are large black rectangles between each moon and Jupiter- darker than the background. If you're not seeing them, you need to adjust your monitor- your black point is set too high, and you're missing a lot, especially in astronomical images.
Okay, I saved the best resolution image and opened it in IrfanView. Auto-colouring it shows the black rectangles around the moons.
It also shows JPEG-ish coronal-flaring artefacts around the planet. All other considerations aside, I prefer the view I normally get.
Thank you.
Just keep in mind that astronomical images are normally processed so that the darkest areas are somewhat above totally black, in order to allow subtle detail in the dark areas to be visible. While this image may appear better with a high black point, you will be losing something in some images.
Chris

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