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Posted: **Wed Apr 27, 2022 4:08 am**

Moon Shadow on Jupiter

Explanation: What is that large dark spot on Jupiter? It's the shadow of Ganymede, Jupiter's largest moon. When Jupiter's moons cross between the Jovian giant and the Sun, they created shadows just like when the Earth's moon crosses between the Earth and the Sun. Also like on Earth, if you were in a dark shadow on Jupiter, you would see a moon completely eclipse the Sun. Unlike on Earth, moon shadows occur most days on Jupiter -- what's more unusual is that a spacecraft was close enough to record one with a high-resolution image. That spacecraft, Juno, was passing so close to Jupiter in late February that nearby clouds and the dark eclipse shadow appear relatively large. Juno has made many discoveries about our Solar System's largest planet, including, recently, rapidly expanding circular auroras.

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Posted: **Wed Apr 27, 2022 4:50 am**

APOD Robot wrote: ↑Wed Apr 27, 2022 4:08 am Moon Shadow on Jupiter

Explanation: What is that large dark spot on Jupiter? It's the shadow of Io, one of Jupiter's largest moons.
...
Unlike on Earth, moon shadows occur most days on Jupiter -- what's more unusual is that a spacecraft was close enough to record one with a high-resolution image.

There's a mix-up somewhere. "high-resolution" link says the moon is Ganymede.

Posted: **Wed Apr 27, 2022 6:11 am**

alter-ego wrote: ↑Wed Apr 27, 2022 4:50 am the moon is Ganymede.

Yes, my bad. Thank you. I have now changed the identification of the moon to Ganymede on the main NASA APOD pages. I apologize for the mistake.
- RJN

Posted: **Wed Apr 27, 2022 7:12 am**

There is something strange with this picture. The diameter of the moon's shadow here appears to by about one fifth of the equatorial diameter of the planet, but the actual diameters are 5200 km and 143000 km respectively, a ratio or 1 to 27.5. How can this be?
(Is perhaps the neat circular shape of Jupiter in this picture an artifacts, a cropping or vignette of its full view?)

Posted: **Wed Apr 27, 2022 11:37 am**

You are right, too big that Ganymed's shadow for the planet.

Posted: **Wed Apr 27, 2022 12:25 pm**

Ganymede shadow on Jupiter!

Ganymede! Jupiter's largest moon!

Kitty keeping head warm!

Posted: **Wed Apr 27, 2022 12:56 pm**

Roberto Molteni wrote: ↑Wed Apr 27, 2022 7:12 am (... Is perhaps the neat circular shape of Jupiter in this picture an artifacts, a cropping or vignette of its full view?)

Apparently, the picture is only showing a portion of Jupiter. Picture below is from a link in the APOD description.
https://www.missionjuno.swri.edu/news/j ... on-jupiter

: Jupiter_Ganymede_20220427.jpg (19.68 KiB) Viewed 8145 times

Posted: **Wed Apr 27, 2022 4:02 pm**

I wonder what effect the shadow has on the clouds and weather on Jupiter.

On Earth a total solar eclipse means the temperature drops noticeably for a few minutes, but we are a lot closer to the sun, so maybe this effect would not be so strong on Jupiter.

Any suggestions?

Posted: **Wed Apr 27, 2022 4:10 pm**

Roberto Molteni wrote: ↑Wed Apr 27, 2022 7:12 am There is something strange with this picture. The diameter of the moon's shadow here appears to by about one fifth of the equatorial diameter of the planet, but the actual diameters are 5200 km and 143000 km respectively, a ratio or 1 to 27.5. How can this be?
(Is perhaps the neat circular shape of Jupiter in this picture an artifacts, a cropping or vignette of its full view?)

This picture demonstrates a common misunderstanding around Juno images. They are taken at far too low an altitude to show more than a fraction of Jupiter. Just like we can't see much of the Earth in images made from the ISS. Or an airplane. (Imagine what you'd get with a wide angle lens on a downward pointing camera mounted under an airplane. It would look like you were seeing the globe of the Earth, but you'd only be seeing to a horizon a few hundred miles away. And here, we're only seeing to a horizon a few thousand miles away.)

Posted: **Wed Apr 27, 2022 4:20 pm**

Cool picture I've never seen before. This must be the month for unusual planet shadows. Here is a shadow from the solar system's largest moon. Not to be outdone one of the smallest moons is captured just this month eclipsing the Sun. Phobos. (Although this is the reverse of a shadow on the Sun.)

accuweather.com/en/space-news/nasa-rover-captures-the-best-ever-video-of-martian-solar-eclipse/1177894

(I wanted to avoid hot linking, so add the usual https: //www in the front.)

Posted: **Wed Apr 27, 2022 8:48 pm**

Chris Peterson wrote: ↑Wed Apr 27, 2022 4:10 pm
Roberto Molteni wrote: ↑Wed Apr 27, 2022 7:12 am There is something strange with this picture. The diameter of the moon's shadow here appears to by about one fifth of the equatorial diameter of the planet, but the actual diameters are 5200 km and 143000 km respectively, a ratio or 1 to 27.5. How can this be?
(Is perhaps the neat circular shape of Jupiter in this picture an artifacts, a cropping or vignette of its full view?)
This picture demonstrates a common misunderstanding around Juno images. They are taken at far too low an altitude to show more than a fraction of Jupiter. Just like we can't see much of the Earth in images made from the ISS. Or an airplane. (Imagine what you'd get with a wide angle lens on a downward pointing camera mounted under an airplane. It would look like you were seeing the globe of the Earth, but you'd only be seeing to a horizon a few hundred miles away. And here, we're only seeing to a horizon a few thousand miles away.)

Yup. This confused the heck out of me too when I saw a similarly close-up image in another APOD months ago. This shot was taken from almost exactly one Jupiter radius away, and if you diagram it out, and draw tangents from a point one diameter away from a circle, it's easy to see the smaller disc you end up seeing:

: Close-up perspective view shows less than the diameter!

Posted: **Wed Apr 27, 2022 8:58 pm**

From the "moon shadows" link:

[ EDIT: hmm, the lyrics in the video don't match the "official" lyrics below. Guess he improvised here! ]

Click to play embedded YouTube video.

Moonshadow (Olympic Studio Demo / 1970)
Cat Stevens

Yes, I'm being followed by a moonshadow
Moonshadow, moonshadow

And if I ever lose my eyes
Oh, if, I lose my eyes
I won't have to cry

And if I ever lose my hands
Oh, if I lose my hands
I won't have to fight

Yes, I'm being followed by a moonshadow
Moonshadow, moonshadow
Leaping and heaping a moonshadow
Moonshadow, moonshadow

And if I ever lose my mouth
Oh, if, I lose my mouth
I won't have to talk

And if I ever lose my legs
Oh, if, I lose my legs
I won't have to walk

Did it take long to find me?
I ask the faithful light
And did it take long to find me?
And, is it gonna stay the night?

I'm being followed by a moonshadow
Moonshadow, moonshadow
Raising and chasing a moonshadow
Moonshadow, moonshadow

And if I ever lose my mind
Oh, if, I lose my mind
I won't have to think
No more

Moonshadow, moonshadow
Raising and chasing a moonshadow
Moonshadow, moonshadow
Moonshadow, moonshadow

[Songwriters: Islam Yusuf. For non-commercial use only

Posted: **Wed Apr 27, 2022 10:58 pm**

I saw this movie. In a few moments millions of black rectangular objects will cover Jupiter causing it to implode and become a second star in our solar system.

Posted: **Thu Apr 28, 2022 12:36 am**

johnnydeep wrote: ↑Wed Apr 27, 2022 8:58 pm From the "moon shadows" link:

[ EDIT: hmm, the lyrics in the video don't match the "official" lyrics below. Guess he improvised here! ]

The lyrics I found do:

Moonshadow
Song by Cat Stevens (aka Yusuf Islam)

Yes, I'm being followed by a moonshadow
Moonshadow, moonshadow
Leaping and hopping on a moonshadow
Moonshadow, moonshadow

And if I ever lose my hands
Lose my plow, lose my land
Oh, if I ever lose my hands
Oh, if, I won't have to work no more

And if I ever lose my eyes
If my colours all run dry
Yes, if I ever lose my eyes
Oh, if, I won't have to cry no more

Yes, I'm being followed by a moonshadow
Moonshadow, moonshadow
Leaping and hopping on a moonshadow
Moonshadow, moonshadow

And if I ever lose my legs
I won't moan, and I won't beg
Oh, if I ever lose my legs
Oh, if, I won't have to walk no more

And if I ever lose my mouth
All my teeth, north and south
Yes, if I ever lose my mouth
Oh, if, I won't have to talk

Did it take long to find me?
I ask the faithful light
Oh, did it take long to find me?
And, are you gonna stay the night?

I'm being followed by a moonshadow
Moonshadow, moonshadow
Leaping and hopping on a moonshadow
Moonshadow, moonshadow

Moonshadow, moonshadow
Moonshadow, moonshadow

Posted: **Thu Apr 28, 2022 12:47 am**

LWL wrote: ↑Wed Apr 27, 2022 10:58 pm I saw this movie. In a few moments millions of black rectangular objects will cover Jupiter causing it to implode and become a second star in our solar system.

2010: The Year We Make Contact (Odyssey Two)

ALL THESE WORLDS ARE YOURS — EXCEPT EUROPA
ATTEMPT NO LANDING THERE.

Posted: **Thu Apr 28, 2022 2:22 am**

johnnydeep wrote: ↑Wed Apr 27, 2022 8:48 pm
Chris Peterson wrote: ↑Wed Apr 27, 2022 4:10 pm
Roberto Molteni wrote: ↑Wed Apr 27, 2022 7:12 am There is something strange with this picture. The diameter of the moon's shadow here appears to by about one fifth of the equatorial diameter of the planet, but the actual diameters are 5200 km and 143000 km respectively, a ratio or 1 to 27.5. How can this be?
(Is perhaps the neat circular shape of Jupiter in this picture an artifacts, a cropping or vignette of its full view?)
This picture demonstrates a common misunderstanding around Juno images. They are taken at far too low an altitude to show more than a fraction of Jupiter. Just like we can't see much of the Earth in images made from the ISS. Or an airplane. (Imagine what you'd get with a wide angle lens on a downward pointing camera mounted under an airplane. It would look like you were seeing the globe of the Earth, but you'd only be seeing to a horizon a few hundred miles away. And here, we're only seeing to a horizon a few thousand miles away.)
Yup. This confused the heck out of me too when I saw a similarly close-up image in another APOD months ago. This shot was taken from almost exactly one Jupiter radius away, and if you diagram it out, and draw tangents from a point one diameter away from a circle, it's easy to see the smaller disc you end up seeing:

close up view of Jupiter.JPG

Yes, I see the point.
Yet, even at a distance as close as one radius of Jupiter (77000 km) from the surface, the difference from 1:27.5 to 1:5 is not justified. There must be quite some cropping/vignetting (as also hinted by the very sharp edges of the planet's orb).

Posted: **Thu Apr 28, 2022 2:25 pm**

RobertoMolteni wrote: ↑Thu Apr 28, 2022 2:22 am
johnnydeep wrote: ↑Wed Apr 27, 2022 8:48 pm
Chris Peterson wrote: ↑Wed Apr 27, 2022 4:10 pm

This picture demonstrates a common misunderstanding around Juno images. They are taken at far too low an altitude to show more than a fraction of Jupiter. Just like we can't see much of the Earth in images made from the ISS. Or an airplane. (Imagine what you'd get with a wide angle lens on a downward pointing camera mounted under an airplane. It would look like you were seeing the globe of the Earth, but you'd only be seeing to a horizon a few hundred miles away. And here, we're only seeing to a horizon a few thousand miles away.)
Yup. This confused the heck out of me too when I saw a similarly close-up image in another APOD months ago. This shot was taken from almost exactly one Jupiter radius away, and if you diagram it out, and draw tangents from a point one diameter away from a circle, it's easy to see the smaller disc you end up seeing:

close up view of Jupiter.JPG
Yes, I see the point.
Yet, even at a distance as close as one radius of Jupiter (77000 km) from the surface, the difference from 1:27.5 to 1:5 is not justified. There must be quite some cropping/vignetting (as also hinted by the very sharp edges of the planet's orb).

Yeah, it doesn't quite add up to me either. Even using the inner much darker portion of the shadow only seems to get the ratio to about 1:10. (I think your 1:5 was overestimating the Ganymede shadow width.) But I think the angular perspective makes a difference too: we clearly see an extreme horizontal shortening of the shadow. Is there any vertical lengthening going on?

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