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The NY Times article Saturn’s Rings Are Sculpted by a Crew of Mini-Moons is really interesting and links to the recent paywalled paper in Science Close Cassini flybys of Saturn’s ring moons Pan, Daphnis, Atlas, Pandora, and Epimetheus

But I absolutely can't understand one of the photos in the NY Times article, shown below. Titan seems to be...

  1. behind Saturn's rings, and yet it is
  2. huge relative the spacing of the rings, and yet it
  3. appears to be out of focus while the rings and Epimetheus are in focus.

Can someone help me understand how all of these can be true at the same time?


In the foreground, the moon Epimetheus appearing to hover above Saturn's rings. Epimetheus is dwarfed by Titan in the background.CreditNASA/JPL/Space Science Institute

Epimetheus, Titan and Saturn's Rings

Epimetheus, Titan and Saturn's Rings cropped

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    $\begingroup$ Something that might contribute to this illusion is the extremely narrow angle used. I know Titan is much farther away than the rings, so I intuitively expect it to look much smaller than it is, so it looks enormous in this picture. Because the focal length of the camera is so high, Titan isn't actually that much bigger than it appears. $\endgroup$ – DarthFennec Mar 30 at 0:05
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    $\begingroup$ @DarthFennec yes, the 2nd link in this comment shows that the FOV for this photo is only about 0.35 degrees. $\endgroup$ – uhoh Mar 30 at 0:30
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    $\begingroup$ How are the rings so thin if the moons appear to have vastly different inclinations? That's what looks strange to me. $\endgroup$ – Mazura Mar 30 at 20:55
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    $\begingroup$ Beautiful, stunning image. In case you haven't seen it I would like to recommend Carolyn Porco's classic TED talk about Cassini. It's well worth watching; she puts things in perspective ;-). $\endgroup$ – Peter - Reinstate Monica Mar 31 at 17:43
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The JPL Solar System Simulator doesn't show Epimetheus but does show Titan behind the Encke gap at 2006-04-28 08:12 UTC.

Titan at 08:12, narrow field

The simulated surface texture is probably composed of VIMS images in infrared wavelengths where Titan's atmosphere is relatively transparent. On the real Titan, haze scatters visible light so strongly that the surface is indistinct and the edge looks fuzzy.

If we zoom out, we see that we are looking near the outer edge of the rings at a very shallow angle. This is why they cover less than half of Titan's 10-arcminute apparent diameter.

Since Epimetheus appears above the rings while we are looking from below, it must be in front of them.

Titan at 08:12, wide field

Simulated images courtesy NASA/JPL-Caltech

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    $\begingroup$ Wow I had no idea that JPL had such a Solar System Simulator website. These are quit helpful, thanks! $\endgroup$ – uhoh Mar 29 at 22:10
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This NASA page says this photo was taken on April 28 2006.

Using Celestia, I managed to find the picture from Cassini that best lines up with the photo. It doesn't match up precisely, but that's to be expected as the calculated orbital elements of all these moons (and cassini) in the software won't necessarily match up to reality precisely. cassini celestia shot of titan

Below is the zoomed out version of this shot. You can see Titan in the centre and Epimetheus as a dot on top. enter image description here And here is the top down shot from Cassini to the moons. Circled are Epimetheus and Titan. enter image description here

So to answer your question: Titan is really big compared to epimetheus (about 50x), Titan has an atmosphere and so appears fuzzy (It is actually in focus, everything in space is very far away and so is effectively at infinity for focusing purposes), and the rings are highly oblique, so you only see a small slice of them.

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  • $\begingroup$ Very nice, but we're only half-way there. I've added a cropped version and drawn a rectangle that highlights the rings being almost 40% wider near the left limb of Titan than the right limb. Are you able to adjust the POV of your simulation and make this happen as well? Right now the section of the rings you're showing is so small that there's no slope. $\endgroup$ – uhoh Mar 29 at 9:08
  • $\begingroup$ photojournal.jpl.nasa.gov/catalog/PIA08391 and ciclops.org/iss/iss.php?js=1 $\endgroup$ – uhoh Mar 29 at 9:16
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    $\begingroup$ It's not about the POV, it's about the accuracy of the orbital data in Celestia. There were two conjunctions of epimetheus and Titan on april 28, 2006 from the view of titan, one too early (the one pictured) and one too late (the rings were no longer in view). AFAIK, Celestia does use a database of precise orbital information rather than just using keplerian ellipses, but I guess even that isn't accurate enough to recreate this photo (saturn's moons have very complex orbits). If anybody has other software that tracks these objects with more precision, they're welcome to post an updated answer. $\endgroup$ – Ingolifs Mar 29 at 9:28
  • $\begingroup$ I did a quick plot . It looks like the distances only match up around 08:30 UTC. I have a hunch that through the tiny 0.35 degree FOV of the narrow-angle camera we're looking at the very outer edge of an inner ring, which is why it's getting narrow so rapidly. $\endgroup$ – uhoh Mar 29 at 9:46
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    $\begingroup$ I should have remembered. Epimetheus has a [en.wikipedia.org/wiki/Horseshoe_orbit](horseshoe orbit). That's presumably why it's so inaccurate. $\endgroup$ – Ingolifs Mar 29 at 20:34
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note: This is a supplemental answer adding some details to @ Ingolifs' excellent answer.


At roughly 2006-Apr-28 08:30 UTC Cassini was both 1,800,000 km from Titan and 667,000 km from Epimetheus at the same time.

I used JPL's Horizons and saved the positions in the Saturn body centered coordinates every 5 minutes then ran the python script below to plot. I am not sure how to get the plane of the rings this way easily.

enter image description here

enter image description here

class Body(object):
    def __init__(self, name):
        self.name = name

import numpy as np
import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D

fnames = ['Titan photo Cassini horizons_results.txt',
          'Titan photo Titan horizons_results.txt',
          'Titan photo Epimetheus horizons_results.txt' ]

names  = ['Cassini', 'Titan', 'Epimetheus']

bodies = []

for name, fname in zip(names, fnames):

    with open(fname, 'r') as infile:

        lines = infile.read().splitlines()

    iSOE = [i for i, line in enumerate(lines) if "$$SOE" in line][0]
    iEOE = [i for i, line in enumerate(lines) if "$$EOE" in line][0]

    print iSOE, iEOE, lines[iSOE], lines[iEOE]

    lines = zip(*[line.split(',') for line in lines[iSOE+1:iEOE]])

    JD  = np.array([float(x) for x in lines[0]])
    pos = np.array([[float(x) for x in lines[i]] for i in 2, 3, 4])
    vel = np.array([[float(x) for x in lines[i]] for i in 5, 6, 7])

    body = Body(name)
    bodies.append(body)
    body.JD  = JD
    body.pos = pos
    body.vel = vel

Cassini, Titan, Epimetheus = bodies

r_Titan      = np.sqrt(((Cassini.pos - Titan.pos     )**2).sum(axis=0))
r_Epimetheus = np.sqrt(((Cassini.pos - Epimetheus.pos)**2).sum(axis=0))

hours = 24 * (JD - JD[0])

r_Titan_target      = 1.8E+06 
r_Epimetheus_target = 6.67E+05

hours_Titan      = hours[np.argmax(r_Titan < r_Titan_target)]
hours_Epimetheus = hours[np.argmax(r_Epimetheus[30:] > r_Epimetheus_target)+30]

print hours_Titan, hours_Epimetheus
if True:    
    fig = plt.figure()

    plt.subplot(2, 1, 1)
    plt.plot(hours, r_Titan)
    plt.plot(hours, 1.8E+06 * np.ones_like(r_Titan), '-k')
    plt.ylabel('Cassini-Titan distance (km)', fontsize=16)

    plt.subplot(2, 1, 2)
    plt.plot(hours, r_Epimetheus)
    plt.plot(hours, 6.67E+05 * np.ones_like(r_Epimetheus), '-k')
    plt.ylabel('Cassini-Epimetheus distance (km)', fontsize=16)
    plt.xlabel('2006-Apr-28 hours', fontsize=16)

    plt.show()
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    $\begingroup$ If I set observer location to Epimetheus, then Titan and Cassini have opposite RA at 08:13 UTC. Solar System Simulator matches the image at 08:12. $\endgroup$ – Mike G Mar 29 at 10:30
  • $\begingroup$ @MikeG that's good news! Can you add an answer with a screen shot? $\endgroup$ – uhoh Mar 29 at 10:36

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