Moon Shadow Over Jupiter

From NASA

Sept. 18, 2020

Moon Shadow Over Jupiter

Jupiter’s volcanically active moon Io casts its shadow on the planet in this dramatic image from NASA’s Juno spacecraft. As with solar eclipses on the Earth, within the dark circle racing across Jupiter’s cloud tops one would witness a full solar eclipse as Io passes in front of the Sun.

Such events occur frequently on Jupiter because it is a large planet with many moons. In addition, unlike most other planets in our solar system, Jupiter’s axis is not highly tilted relative to its orbit, so the Sun never strays far from Jupiter’s equatorial plane (+/- 3 degrees). This means Jupiter’s moons regularly cast their shadows on the planet throughout its year.

Juno’s close proximity to Jupiter provides an exceptional fish-eye view, showing a small fraction near the planet’s equator. The shadow is about 2,200 miles (3,600 kilometers) wide, approximately the same width as Io, but appears much larger relative to Jupiter.

A little larger than Earth’s Moon, Io is perhaps most famous for its many active volcanoes, often caught lofting fountains of ejecta well above its thin atmosphere.

Citizen scientist Kevin M. Gill created this enhanced-color image using data from the spacecraft’s JunoCam imager. The raw image was taken on Sept. 11, 2019 at 8:41 p.m. PDT (11:41 p.m. EDT) as the Juno spacecraft performed its 22nd close flyby of Jupiter. At the time the image was taken, the spacecraft was about 4,885 miles (7,862 kilometers) from the cloud tops at a latitude of 21 degrees.

JunoCam’s raw images are available for the public to peruse and process into image products at: https://missionjuno.swri.edu/junocam/processing.

Image Credit: NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill (CC-BY) 

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Carl Friis-Hansen
September 23, 2020 3:18 am

Great image, but just a question:

Why is the edge of shadow so blurred?
In contrast, the edge of Jupiter is razor sharp.

Alastair Brickell
Reply to  Carl Friis-Hansen
September 23, 2020 3:44 am

Carl Friis-Hansen
September 23, 2020 at 3:18 am

This is due to the refraction of light around the edge of Io. This is the same reason that the shadow of anything is not a razor sharp line. For example the shadow of a building on the ground is not a sharp boundary…it is fuzzy due to refraction. This is why we can not build sundials hundreds of metres high to give us the movement of the shadow second by second…the edge of the gnomon becomes a gradual moving transition from dark to light spread over several metres.

The edge of Jupiter is not a shadow so is very sharp in the image.

Carl Friis-Hansen
Reply to  Alastair Brickell
September 23, 2020 4:38 am

Thanks for the explanation.

I first assumed the distance Jupiter-IO would near null compared to distance Jupiter-Sun, thus the penumbra would be so small that the softness would not appear in the picture.

MarkW
Reply to  Carl Friis-Hansen
September 23, 2020 7:24 am

All that matters is that the sun appear as a disk, rather than a point. The light from one edge of the solar disk takes a slightly different path as it passes Io, compared to light from the opposite edge of the solar disk. Because of this path, the two rays of light strike Jupiter at different points.

Jean Meeus
Reply to  Alastair Brickell
September 23, 2020 4:50 am

” For example the shadow of a building on the ground is not a sharp boundary…it is fuzzy due to refraction.”
There is no refraction here. The shadow of a building on the ground is not sharp, because the Sun is not a point but has a certain diameter (of half a degree).

menace
Reply to  Jean Meeus
September 23, 2020 10:29 am

Yes, the fuzzy effect is not refraction or diffraction.

There is a small penumbra causing the fuzziness at the edge. If you could be standing on the planet in the fuzzy region you’d see the sun as partially eclipsed.

The penumbra of Io is much smaller than a lunar penumbra on Earth due to the closer proximity of Io and the sun being much more distant (more of a point source but still not quite).

The Moon just happens to be nearly the same apparent diameter as the sun (a coincidence that makes for dazzling total eclipse shows), but Io must be many times the apparent diameter of the sun viewed from Jupiter’s surface.

For many lunar eclipses (the annular ones) the Moon’s apparent diameter is smaller so the shadow is all penumbra but for total eclipses there is a small diameter umbra cast (region of totality), yet still most of the Moon’s shadow is cast on the earth as a very fuzzy penumbra.

If the Moon was like 10x closer to Earth the shadow would look similar to Io’s. Thank goodness it’s not as we’d probably have 200 ft tides then.

Jeff Id
Reply to  Alastair Brickell
September 24, 2020 7:58 am

It is not refraction but the diameter of the source – the sun in this case.

Vuk
Reply to  Carl Friis-Hansen
September 23, 2020 6:04 am

CH-F, Shadow is not falling on the Jupiter’s surface because Jupiter has no surface. The shadow is on the layers of atmosphere of different altitude and transparency which vary with latitude and its temperature (from isolation and internally generated; Jupiter emits more energy than it receives from the sun). Jupiter has the largest planetary atmosphere in the Solar System, rising to over 5,000 km (3,000 mi) in altitude. The Earth’s atmosphere ends at about 100 km but most of its the mass is within 9 km at poles and 17 km at the Equator.
At Jupiter’s orbit (5AU) for all practical purposes of this shadow the sun can be considered to be a ‘near’ point light source.

Carl Friis-Hansen
Reply to  Vuk
September 23, 2020 6:39 am

Thanks Vuk.

Your explanation is even more convincing.

Reply to  Carl Friis-Hansen
September 23, 2020 8:23 am

It’s because of the diameter of the sun!

– JPP

Vuk
Reply to  Jon P Peterson
September 23, 2020 11:16 am

Ratio of solar disk diameter and the distance from Jupiter is 0.00181, as for the very small ratios ArcSin = Sin(angle) = 0.00181 radians = 0.1 degree, i.e for an observer at Jupiter’s distance, the angular diameter of the Sun is = 6 arc min. For all practical purposes this makes sun as viewed from Jupiter a point source.

Ragnaar
Reply to  Jon P Peterson
September 23, 2020 4:27 pm

Looking at the Sun from Jupiter, it’s about 1/5 the apparent diameter of the Earth’s moon. Someone should do a simulation of what it would look like from the top of Jupiters atmosphere.

Alastair Brickell
Reply to  Carl Friis-Hansen
September 23, 2020 1:49 pm

Carl Friis-Hansen
September 23, 2020 at 6:39 am

It’s great you learn things here from a seemingly simple question.

I think they’re all convincing, with mine probably being the least! Next time I see a solar eclipse (2024 hopefully) I will try and see what happens with the shadow of a building just before totality when the sun is reduced to a very small almost point source of light. Certainly at that time you get very sharp shadows from relatively close trees, leaves, etc. which makes all helps to make totality so special.

Also as Vuk says Jupiter’s surface is diffuse, being layers of clouds of varying transparency. It’d be fun to be on Jupiter as you’d see lots of wonderful total eclipses although you might get zapped a bit by the radiation. But what fun to see all those moons zipping around!

Vuk
Reply to  Alastair Brickell
September 23, 2020 2:24 pm

“…… although you might get zapped a bit by the radiation. ”
Or instantly electrocuted since there are million Amp currents flowing between Io and Jupiter’s ionosphere. Io is a volcanic moon, it ejects many thousands of tons of sulfur dioxide gas every 24 hours, molecules are broken up, ionised and accelerated into electric currents by the Jupiter’s strong magnetic field.

Curious George
Reply to  Vuk
September 23, 2020 2:26 pm

“Jupiter has no surface.” Maybe. We don’t really know.

Vuk
Reply to  Curious George
September 23, 2020 2:56 pm

Current understanding is Jupiter is made mostly of hydrogen and helium. It is assumed that there is no firm surface so if an object (anything is heavier than H or He) is slowly drooped from a spacecraft into Jovian atmosphere by a parachute, it would slowly sink further and further down to be eventually crushed by the intense pressure inside the planet.

Alastair Brickell
Reply to  Vuk
September 23, 2020 3:57 pm

Vuk
September 23, 2020 at 2:56 pm

Which is exactly what happened to the 1995 Galileo probe which was parachuted into the Jovial atmosphere. It transmitted for about an hour withstanding decelleration of 228g (going from 47km/sec to subsonic in two minutes) before being completely vapourised by the intense heat in the atmosphere. What a brave little spacecraft!

W Zmek
Reply to  Vuk
September 23, 2020 7:16 pm

Vuk, JM and menace are quite correct.
Io is 400 thousand kilometers from Jupiter, in rough numbers. The width of the penumbra surrounding Io’s shadow, for the solar angular diameter of 0.0018 radians (your number is correct), is therefore very roughly 700 kilometers. The diameter of Io’s umbral shadow is (again, roughly) the same as Io’s physical diameter, or about 3600 kilometers. So the width of the penumbra surrounding Io by this argument should be roughly 20% of the width of the umbra. The diameter of the sun is certainly the dominant reason for the lack of a sharp edge to Io’s shadow in the image. (Note that the apparent width of the penumbra will be somewhat narrower to the eye than what the calculation predicts because the sun is round, so the rate of change of shadowing is concentrated at the center of the penumbra, which is what is seen in the image.) Thus, the penumbra should certainly be visible, and it is.

The other claim that I’d like to comment on is that of the apparent geometric strangeness clearly visible in the image. What is being forgotten by some commenters is that the Juno satellite was generally close enough to the planet that such apparent distortions are to be expected. In fact, a fun exercise to try is to estimate the distance of Juno from Jupiter, based on the apparent oversize of Io’s shadow relative to its apparent size in photos taken from Earth, then look up the true distance range of the satellite from official sources online.

Regards,
Bill Zmek

Jim Whelan
Reply to  Carl Friis-Hansen
September 24, 2020 11:12 am

The picture with the article is NOT an actual photo but some kind of artists rendering. If you go to the actual photos you will see that the shadow of IO is extremely small with respect to the size of Jupiter (IO itself is much, much smaller than the planet).

This isn’t really news of any kind since ground photos of Jupiter showing shadows of its moons crossing the planet have been available long before any interplanetary space craft were launched.

Not Chicken Little
Reply to  Jim Whelan
September 24, 2020 6:44 pm

No, it’s an actual photo – as mentioned in the article, the spacecraft that took the picture was only 4885 miles from the top of Jupiter’s clouds in its close flyby when it took the picture. That close to Jupiter the planet appears as a disk but you can’t see all of Jupiter because it’s so close. The size of Io’s shadow is very close to its actual diameter of some 2263 miles. Thus the fish-eye effect appearance.

You can see the same effect in pictures from low Earth orbit, where Earth still appears round but you are seeing only a portion of the surface, not the whole diameter.

ggm
September 23, 2020 3:26 am

haha the movie 2010 (sequel to 2001) predicted this :
comment image

Ron Long
September 23, 2020 3:30 am

Great digital photo from Jupiter/Io, thanks NASA and Kevin M. Gill. Io is often in Science Fiction stories because it has a thin atmosphere, mostly sulfur dioxide, and citizens of Io who have adapted to breathing sulfur gas are really hardy creatures. Sort of like New York City, Portland, and Seattle, which cities were named as sponsors of destructive rioting/gas canisters flying, etc.

Gordon A. Dressler
Reply to  Ron Long
September 23, 2020 8:03 am

Nice! I agree with you, with the caveat that it is a great FAKED digital photo.

Adrian Mann
Reply to  Gordon A. Dressler
September 23, 2020 4:16 pm

In what way is it faked? Sorry – FAKED! Looking forward to seeing that incontrovertible evidence and data that you have to prove your point.

J Cuttance
Reply to  Adrian Mann
September 23, 2020 6:20 pm

There is no way Io casts a shadow that big on Jupiter. You only have to compare the radius of the shadow to that of the planet to know that the photo has a scale modifcation of some kind.

Adrian Mann
Reply to  J Cuttance
September 24, 2020 12:22 am

It is absolutely possible for the shadow to appear that large, given the type of camera, the field of view, the distance from the shadow/Jupiter, and the angles involved. You can only see a very small portion of Jupiter in that image, and the lens is a fisheye lens.
Read more here:
https://www.syfy.com/syfywire/what-would-a-solar-eclipse-look-like-on-jupiter-but-with-more-volcanoes
Here’s a paper on the camera involved: https://www.missionjuno.swri.edu/pub/e/downloads/JunoCam_Junos_Outreach_Camera.pdf
Also, compare the size of the shadow to the width of the belt it’s passing over, and you’ll find that it’s actually correct. It’s your understanding of it that’s incorrect.

Gordon A. Dressler
Reply to  Adrian Mann
September 24, 2020 9:19 am

See my separate posting below.

SAMURAI
September 23, 2020 3:51 am

Cool photo of a moon shadow, which reminded me of:

beng135
Reply to  SAMURAI
September 24, 2020 7:30 am

Thanks, that’s what came to my mind right away. Except in this post the moon shadow is outta this world….

Loydo
September 23, 2020 4:08 am
Tom in Florida
September 23, 2020 4:37 am

“The shadow is about 2,200 miles (3,600 kilometers) wide, approximately the same width as Io, but appears much larger relative to Jupiter.”

The diameter of Jupiter is approx 88,900 miles so this image seems a bit out of wack, even though they say the shadow appears larger. Just wondering if this is an actual image or not.

Gordon A. Dressler
Reply to  Tom in Florida
September 23, 2020 7:44 am

Tom,
I was going to make the same point. The diameter of Jupiter is 86,880 miles (139,820 km). At the distance that Jupiter is from the Sun, the Sun will subtend an angle of only 0.10 degrees, or about one-fifth the angle it subtends at Earth’s orbital distance.

Therefore, all rays of sunlight arriving at Io will be essentially parallel and thus Io’s shadow that falls on Jupiter should have a planar diameter very close to Io’s diameter. There will be some VERY SLIGHT enlargement due to light refraction from Io’s atmosphere (essentially plumes of the sulfur dioxide gas bursts from multiple active volcanoes that can reach up to 300 miles, or 480 kilometers, above the moon’s surface, although the atmospheric depth is observed to vary greatly and sporadically over time). As other’s have noted, this atmospheric refraction will soften the edges of the shadow (in addition to the softening that results from the shadow falling on various heights of Jupiter’s cloud tops along it’s circumference).

So, given the above one would expect the shadow on Jupiter to cover about 2,200/86,880 = .025 = 2.5% of Jupiter’s diameter. In the photo given in the lead-in to the above article, this is clearly not the case (I estimate the shadow to be somewhere around 15% of Jupiter’s diameter). Note that the shadow is elliptical in shape, as would be expected due to falling on Jupiter’s nearly spherical atmosphere on a vector that is not passing through Jupiter’s center.

Overall, IMHO, based on the above calculations, I agree with you there is something seriously wrong with the photo above . . . I suspect it has been altered to increase the “shock value” for public attention.

Vuk
Reply to  Gordon A. Dressler
September 23, 2020 12:45 pm

This is Io and its true shadow on the Jupiter
comment image

Vuk
Reply to  Gordon A. Dressler
September 23, 2020 1:11 pm
Gordon A. Dressler
Reply to  Vuk
September 23, 2020 1:56 pm

Vuk,
The image you linked at 12:45 pm today is believable. The one that you linked most recently (at 1:11 pm today) is not. There is a world of difference between the two shadow diameters, and this cannot be attributed to orbital changes nor to relative sun/shadow angle.

The basic problem with any photo now being traced back to Juno Mission images (particularly “JunoCam” images) is that NASA specifically says “JunoCam’s raw images are available for the public to peruse and process into image products”, as stated in the very last sentence of the above article.

There is no end of people that will “photoshop” an image to “enhance” it without regards for maintaining scientific integrity, if it means their processed image(s) will stand out and gain publicity. “Citizen scientist” Kevin M. Gill, cited in the above article, being the current case in point.

NASA, of course, willing plays along with this game.

It is somewhat interesting to find, based on just the comments noted under this WUWT article, that this image serves as the equivalent of a Rorschach test of gullibility of the general populace to be taken in based solely on what they see and read.

Vuk
Reply to  Gordon A. Dressler
September 23, 2020 1:48 pm

May be or may be not
Juno’s orbit is a very elongated ellipse with eccentricity of 39 (1 and 39 Jupiter radii); hence, when Juno is between Io and Jupiter (larger shadow) and viceversa, as in the two examples above (?)

Gordon A. Dressler
Reply to  Vuk
September 23, 2020 2:06 pm

Vuk,
That variation in Io’s orbital distance from the center-of-mass of Jupiter pales into insignificance compared to the Sun-Jupiter average distance. In other words, Io’s highly elliptic orbit will have negligible effect on Io’s shadow as projected on Jupiter’s spherical atmosphere since the Sun at all times can be considered (from Jupiter’s perspective) as a point source of illumination.

Vuk
Reply to  Gordon A. Dressler
September 23, 2020 3:27 pm

I know that, see: Vuk September 23, 2020 at 11:16 am.
What I had in mind is somewhat complicated to explain.
In shaded area temperature would drop significantly (e.g. solar eclipse), the Jovian clouds average temperature is about 120K, and it is much lower in the outer layers. With lack of sunlight atmosphere would get more dense – perhaps even condense (?) at 33 K, loosing its normally full transparency at an altitude of thousand or more miles higher than the surrounding area and so capturing Io’s shadow. When Juno is in the distant part of the orbit it would make no difference and the image would be as expected, but when Juno is in the low section of the orbit just above the shadow, the shadow would appear to be much larger compared to the Jupiter’s dimensions., i.e is an optical illusion of relative distances between camera, shadow and the sunlit clouds.
… just musing of an idle mind.

Gordon A. Dressler
Reply to  Gordon A. Dressler
September 23, 2020 5:39 pm

For essentially parallel light rays falling on an object, the shadow cast by that object is fixed at the object’s dimensions . . . the distance to a second body upon which that shadow falls won’t make any difference, the shadow dimensions (in a cross-sectional plane normal to the incoming light vector) will remain fixed.

As for possible effects of shadow cooling associated with low orbital altitude above Jupiter’s cloud tops, keep in mind that for an elliptical orbit the closer to the focus of the ellipse defined by the governing mass the faster the orbiting object moves . . . there will be much more persistent cooling at the umbral position at apoapsis than at periapsis.

Tom in Florida
Reply to  Vuk
September 23, 2020 3:21 pm

I guess only the Shadow knows.

Not Chicken Little
Reply to  Gordon A. Dressler
September 24, 2020 6:50 pm

Nothing wrong with the photo – you are missing the fact that the spacecraft that took the photo is in a close flyby orbit and as stated above it was only 4885 miles from the top of Jupiter’s clouds when the photo was taken. That close to Jupiter it is impossible to see its whole diameter – just like pictures taken from low Earth orbit, only a portion of the disk can be seen, it’s just geometry.

Gordon A. Dressler
Reply to  Not Chicken Little
September 25, 2020 5:56 pm

NCL, at 4,885 above Jupiter’s cloud tops, the full visible limb of Jupiter (note: its full diameter is not visible at such low altitude, just as you stated) would subtend a total angle of about 129°

At that same altitude a shadow of Io on Jupiter, if located in Io’s projected orbital plane would subtend an angle of at most—i.e., looking straight down on it—atan[(2,200/4,885)] = 24°. However, this is clearly not the case in the above photo. I calculate the look angle, given the reported 4,885 mile camera altitude above Jupiter’s cloud tops, to the center of the eclipse shadow, as shown, to be 63° degrees above a horizontal plane (relative to the photo) that bisects the planet. In turn, this means that the slant range to the center of the eclipse center is approximately 14,370 miles from the orbiting camera. For a true 2,200 mile-wide shadow, the total angle subtended by the shadow would be about atan[2,200/14,370] = 8.7°.

The resulting ratio of true shadow subtended angle to Jupiter’s limb subtended angle would thus be 8.7°/129°, or about 6%. This is clearly not the case in the above photo, where the ratio is more like 16%.

Working the calculations to account for the 4,885 claimed spacecraft altitude and the fact that Jupiter’s full diameter is not visible at the altitude has improved the ratio over previous simplified calculations, but it still falls far short of what is needed to be geometrically correct.

To reiterate my previous assertion, the above image has been FAKED.

Of course, if you can provide the original JunoCam image that “citizen scientist” Kevin M. Gill claims to have used for this photo— in the above article, the associated specific date and time are given, as is the website for downloading JunoCam images— I might be forced to change my conclusion. But I have been totally unsuccessful locating any such original image.

Hans Erren
Reply to  Tom in Florida
September 23, 2020 11:13 am

The image is zoomed in, here is a projection on Jupiter disk

https://www.missionjuno.swri.edu/junocam/processing?id=9295

Gordon A. Dressler
Reply to  Hans Erren
September 23, 2020 12:41 pm

The overall image is not zoomed-in . . . note the disk of Jupiter in the photo.

Hans, I believe what you meant to say was that there is a close-up photo of Io’s shadow on Jupiter’s cloud tops that was superimposed over a separate photo showing the entire disk of Jupiter, and that composite image was cropped to the boundaries of the presented photograph.

The scale of Io’s shadow average diameter to the diameter of Jupiter is totally WRONG.

Hans Erren
Reply to  Gordon A. Dressler
September 24, 2020 8:43 am

That is not the disk, that is the low orbit horizon of the juno-satellite, that is why ios shadow is out of proportion.

Gordon A. Dressler
Reply to  Hans Erren
September 24, 2020 9:49 am

“. . . that is the low orbit horizon of the juno-satellite . . .”

Maybe, maybe not. The portion of the Jupiter’s limb that is in the photo is about 57 degrees of arc, or about one-sixth of the limb’s full circumference. To infer the altitude at which the Jupiter portion of the image was taken would require knowledge of the associated camera’s focal length.

The actual altitude and camera focal length (as well as perspective angle of the camera) would NOT cause the distortion of Io shadow diameter to Jupiter’s diameter that is so obvious in the photo in question.

Hans Erren
Reply to  Hans Erren
September 26, 2020 5:37 am

Juno orbit parameters:
https://en.wikipedia.org/wiki/Juno_(spacecraft)

Perijove altitude:
4,200 km (2,600 mi) above the clouds
75,600 km (47,000 mi) above Jupiter center

So at perijove you’ll see only a fraction of Jupiters disk, ergo it is a horizon.

Sara
September 23, 2020 4:50 am

Nice shot. Very nice. Makes up for a lot of unsettling things going on. Thanks for posting that!

JoeG
September 23, 2020 5:53 am

Our Moon causes a total solar eclipse because it just covers the Sun completely. It is 400x smaller than the Sun but also 400x closer to us than the Sun. The apparent size in the sky is the same. That means we can see the Sun’s corona. That isn’t how it is on Jupiter, though. The solar eclipse isn’t the same for both planets.

Hans Erren
September 23, 2020 8:19 am

Great, you can watch Ios shadow on Jupiter from Earth with a small telescope.

Climate believer
Reply to  Hans Erren
September 23, 2020 9:02 am

I’ve been watching Jupiter and Saturn for a few weeks right out the front of my house in the early night sky. No light pollution where I live so when it’s clear (and very little moon) I get the whole show….. blows me away every time.

Gordon A. Dressler
Reply to  Hans Erren
September 23, 2020 11:00 am

Well, actually not so small a telescope.

The closest Earth-Jupiter distance is about 588 million kilometers. Therefore, the angle subtended by Io (or its shadow on Jupiter) is approximately atan(3600/588e+6) = 1.26 arcseconds.

At a telescope’s best resolution possible (i.e., diffraction-limited optics), the telescope aperture needed to resolve Io—or its shadow—in yellow light (as a dot) theoretically would be at least 8 inches. See https://en.wikipedia.org/wiki/Diffraction-limited_system for a very convenient graph giving required aperture dependence versus desired diffraction limited resolution and wavelength received by a given telescope.

And the above would be with perfect “seeing” conditions through Earth’s typically turbulent atmosphere.

Better shoot for at least a 10-inch diameter telescope of you want to “observe” Io or its eclipse shadow from Earth’s surface.

Gordon A. Dressler
September 23, 2020 8:39 am

The same photo of Io casting a shadow on Jupiter appears on a NASA website, with the same photo attribution to “Citizen scientist Kevin M. Gill” (see https://www.nasa.gov/image-feature/moon-shadow-over-jupiter ).

Here’s the thing though: if you go to above-cited website where “JunoCam’s raw images are available for the public to peruse and process into image products” and enter the claimed date for the raw imagery (Sept. 11, 2019, or even+/- a couple of days on either side of such), there is no imagery available from JunoCam.

Caveat emptor.

ralfellis
September 23, 2020 8:52 am

That shadow is FAR too large to be of a Jovian moon.

Reply to  ralfellis
September 23, 2020 10:04 am

That’s what I was thinking:
Diameter of IO = 2,263.8 miles
Diameter of Jupiter = 86,881 miles
or 1 to 38.378 diameter difference

– JPP

Reply to  ralfellis
September 23, 2020 10:08 am
Vuk
Reply to  ralfellis
September 23, 2020 12:24 pm

Hi Ralf
This is Io and its shadow on the Jupiter
comment image
Periapsis 420000 km
Apoapsis 423400 km
not much difference between two to justify change in the size of the shadow
In this image distance between Io and Jupiter is small compared to distance to the sun, but Io is closer to Juno’s camera so it appears to be larger than its shadow. On my monitor Io’s shadow is about 1cm and I estimated Jupiter’s diameter about 36cm which appears to be similar ratio to the ratio of the diameters of Io and Jupiter.
May be it is not Io after all

Hans Erren
Reply to  Vuk
September 27, 2020 3:06 am

It is the proximity of Juno spacecraft to the clouds of Jupiter (8000 km) that gives the effect seen in the picture.

Karl
September 23, 2020 9:13 am

Stunning image. Thanks very much for sharing it.

Olen
September 23, 2020 9:33 am

How shadows are cast. Take them for granted yet they are complex. Thanks

Me and my shadow, strolling down he avenue…

menace
September 23, 2020 10:48 am

Re: 2nd paragraph

The tilt of a planet’s axis has no direct impact on the chance of a moon eclipsing.

It is the tilt of the moon’s orbital plane with respect to the planets orbital plane around the sun that is important. If they are nearly in the same plane an eclipse would occur every orbit, regardless of the planet’s axial tilt.

The planet’s diameter is also a big factor. If Earth was the diameter of Jupiter I suspect the Moon would eclipse >50% of its orbits instead of the present <10% rate.

Vuk
September 23, 2020 11:25 am

… about 6 x the angular diameter of Venus as seen from the Earth at its closest.

Gordon A. Dressler
September 23, 2020 2:07 pm

Vuk,
That variation in Io’s orbital distance from the center-of-mass of Jupiter pales into insignificance compared to the Sun-Jupiter average distance. In other words, Io’s highly elliptic orbit will have negligible effect on Io’s shadow as projected on Jupiter’s spherical atmosphere since the Sun at all times can be considered (from Jupiter’s perspective) as a point source of illumination.

William Zmek
September 23, 2020 7:27 pm

Gordon, you and H. Erren are both correct. Your (Vuk’s) statements about resolving the shadow are sound, but the notion that the shadow can be *detected* with a smaller diameter scope is certainly correct as well, as thousands of amateur astronomers can confirm.
Regards,
Bill Zmek

Gordon A. Dressler
Reply to  William Zmek
September 23, 2020 9:10 pm

William, I concur that there is a distinction to be made between being able to DETECT a pinpoint of light (such as sunlight reflected off the four largest moons of Jupiter, one of which is Io) and being able to RESOLVE the disk of the object that is reflecting the sunlight, and equivalently the shadow that said disk casts upon a bright surface (i.e., the cloud tops of Jupiter).

I have no doubt that 4-inch refractors or reflectors—or even good binoculars—are sufficient to magnify the pinpoints of light and their separation from the planet Jupiter to the point of detection by the human eye at the receiving end (numerous Web articles confirm this), but this is fundamentally different from optically resolving the disks of each of these moons.

According to https://www.space.com/23153-jupiter-moons-shadow-dance.html#:~:text=Io%20may%20cause%20an%20eclipse,90mm%20aperture%20to%20see%20it :
“. . . Io’s shadow is very small; you need a telescope with at least 90mm aperture to see it.” But again, that is detecting a black point on a bright surface, not resolving the disk DIAMETER of that dark shadow.

Here’s a crude analogy to see the difference. Assume you have somewhat less that 20/20 visual acuity and your optometrist asks you to read the Snellen chart line corresponding to 20/20 acuity. You can DETECT that there are eight fuzzy black areas on that line, but you cannot RESOLVE the actual letters corresponding to each of those fuzzy black areas.

Adrian Mann
Reply to  Gordon A. Dressler
September 24, 2020 12:29 am

I don’t think you understand the term ‘resolve’ – as if it were important. I have personally seen shadows on Jupiter with an 80mm refractor. Not sure what point you’re trying to make.

Gordon A. Dressler
Reply to  Adrian Mann
September 24, 2020 10:10 am

I have personally seen (i.e., detected) the starlight from Polaris, 323 light-years away, with my dark-adapted, naked eyes . . . pupil aperture somewhere in the range of 4-8 mm.

I have not been able to see (i.e., resolve) the stellar disk of Polaris, given that small of an aperture.

That is the point that I am making, and it is physically important.

This whole kerfuffle regarding Io’s shadow cast on Jupiter originated with being able to RESOLVE the diameter of Io’s shadow on Jupiter relative to the absolute size of Jupiter itself. As I have stated previously, I have no doubt that observers on Earth can DETECT Io’s shadow with small-aperture telescopes, or even binoculars.

W Zmek
Reply to  Gordon A. Dressler
September 25, 2020 8:24 am

Gordon,
I agree with your statements concerning resolving vs. detecting 100%, though low power binoculars such as those for birding probably would give us fits trying to see an eclipse on Jupiter. So you’ve suggested a fun little observing project! Namely, what magnification and objective lens diameter is necessary for my old eyes to detect an eclipse on Jupiter. Too bad that Jupiter is so far South on the ecliptic now (for those of us living in the northern hemisphere!) Got to wait a few years for Jupiter to climb out of the murk.
Bill

griff
September 24, 2020 12:25 am

Did nobody else think ‘that’s the gate which the alien spaceships come out of!’?

Adrian Mann
Reply to  griff
September 24, 2020 12:29 am

No. Nobody did that.

PhilJourdan
September 24, 2020 5:20 pm

It looks like 2010 Odyssey! Impressive!

Gordon A. Dressler
September 26, 2020 7:43 pm

Well, to wrap this up, I finally located positive proof that the image of Io’s shadow on Jupiter that is presented at the top of the above article has been grossly manipulated (i.e., FAKED).

Just visit this webpage: https://www.sciencealert.com/yes-this-is-actually-the-shadow-of-io-passing-across-the-surface-of-jupiter

At the top of the webpage you will find a photo of an nearly circular shadow (assumed that from Io) that appears to be at correct scale vis-a-vis the Jupiter cloudtop features (swirls, waves and circulation “spots”). The near-circular shadow implies the JunoCam that took the photo must have been looking almost directly downward on the shadow. Pay particular attention to the different patterns (“features”) and colors in this photo in relation to the shadow.

Then look at the next non-advertising image down the page . . . the image at the top of the above article is just a cropped portion of this second image!

And here is the “smoking gun”: if you compare this image to the first image on the referenced webpage you can fairly easily determine that the second image is just the first image that has been FLIPPED HORIZONTALLY then FLIPPED VERTICALLY, then “mapped” (aka MORPHED) to a spherical surface and finally cropped into a full disc that simulates the visible limb of Jupiter from the supposed perspective of the orbitingJuno spacecraft. These multiple processing steps distort the shape of the shadow and the relative separation distances and scales of all other features.

In particular, note that with considerations for the flipping, inversion, tilting and differential stretching of various areas in the full disc image, the basic features are identical in the common areas of these two separate images. Identical features are found in the common areas of light blue cloud formations, but with “perspective” distortion. Identical features are found in the common areas have elliptical-type storm cloud types, and clouds close to these, but with “perspective” distortion. Identical features are found in the common areas immediately surrounding the shadow, but with perspective distortion.

The correct image has a round shadow . . . the intentionally distorted image (that of the disk of Jupiter) necessarily caused the shadow to become elliptical. It is the FAKED image with the overly large, elliptical shadow (of Io) that has been cropped and is presented at the top of the above article.

I rest my case.

Hans Erren
Reply to  Gordon A. Dressler
September 27, 2020 3:14 am

Nope not faked, just close to the cloud tops.
If you don’t believe me Just make a drawing on scale.

Shadow diameter Io 3642 km
Jupiter diameter 142984 km
Juno altitude 8000 km above clouds

Gordon A. Dressler
September 27, 2020 11:35 am

Hans Erren,
I did make a “sketch”—actually a to-scale drawing on a computer—to derive the numerical values (especially slant range) and resulting conclusions for the Kevin Gill-faked Io shadow, which I posted far above on September 25, 2020 at 5:56 pm. It substantiated that the photo as presented in the above article is FAKE.

But thanks for giving me the opportunity to present the following independent, incontrovertible evidence that that photo has been “photoshopped”. The JunoCam has a fixed focal length lens projecting an image onto a square CCD array to give a resulting square field-of-view that is 58° wide both vertically and horizontally. However, that camera is rigidly mounted with its optical axis perpendicular to the spin axis of the Juno spacecraft. In turn, this give the camera the ability to make very wide (“horizontal” dimension) images, even 360°panoramas, that retain the 58°-wide “vertical” field-of-view.

The key point is that a 58° field-of-view is not large enough to even qualify as being a wide-angle lens (“A lens is considered wide-angle when it covers the angle of view between 64° and 84°” — https://en.wikipedia.org/wiki/Wide-angle_lens ). As such, it cannot possibly produce the optical “barrel distortion” that is so characteristic of fisheye lenses.

Now, it is a simple fact that Jupiter’s cloud tops are oriented above and below Jupiter’s equator in generally latitudinal bands due to the fast rotation of the planet (about 10 hours for one revolution).

However, if you examine the faked full horizon (“disk”) image in question at the website I referenced in my post immediately above, you can obviously detect what appears to be massive barrel distortion of cloud layers . . . the cloud top bands appear to converge on the left and right and to balloon out in the center. This type of distortion cannot possibly be created by the JunoCam camera, and hence it most assuredly can be attributed to the mapping of the original (correct) close-up photo of Io’s shadow onto a spherical surface, just as I speculated had been done.

Cropping the faked photo to the extent seen in the photo presented at the top of the above article helps to cover up this “photoshopping” artifact, but nevertheless the truth is now out there for all to see.

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