APOD: In the Shadow of Saturn (2013 Nov 13)

[MarkBour]

Thanks to all of you for the answers to several of my questions.
Based on Chris Peterson's advice, I plotted a 166-pixel diameter white disk about where I'd guess the sun was.



(I am partly just wondering if I can plot an image in this way.)

I don't have enough experience with photography to know any reason to place the sun off of the location of being centered behind Saturn. If the ring disk is tilted down about 27 degrees, then the rings are seen with sunlight shining through them. I don't see any tilt of the rings in this image either to the left or right, so I'm thinking that it is currently near the middle of Saturn's northern hemisphere summer at this time.

Why does the nearly-outermost ring appear like a bright white circle, while the rings within it are varying shades of khaki, or olive?

[geckzilla]

Mark, I saw a simulated view of the shot once and it indicated that the sun was just about peeking out from the lower left of the planet. I found the image again here:
http://saturn.jpl.nasa.gov/news/waveatsaturn/

I'm not sure how accurate the shadows for the simulated picture are but I would guess the positions of the planets and moons are accurate. Another thing to keep in mind is that both Saturn and Cassini are moving about as Cassini is taking all of the exposures necessary to complete the mosaic. It is be more accurate to say that the sun and Saturn's many satellites were in multiple places for the picture. One thing that makes these mosaics difficult to compose is that movement.

[Chris Peterson]

Thanks to all of you for the answers to several of my questions.
Based on Chris Peterson's advice, I plotted a 166-pixel diameter white disk about where I'd guess the sun was.

That was based on the pixel scale of the narrow field camera. From the image description, it sounds as if this composite may be at the scale of the wide field camera, in which case the Sun would appear ten times smaller. Since this is a composite, some other scale may have been used, as well. To be sure, we'd need to calculate the actual scale from some physical reference, like the known angle between a pair of planets.

[MarkBour]

Thank you sirs for the additional info. And I'm wondering about posting images. It appears that if I want to post an image in this discussion, I need to place it on a web server. Of course, as a typical contributor, I will probably drop or move the image after a time. So, it will eventually vanish from this discussion. Alternatively, if APOD were to allow me to upload it, it would be stable, but I can certainly imagine why APOD would not want people uploading images. So, am I going about this the right way?

[geckzilla]

You've made 10 posts now so you're out of the new users group. There should be an option for uploading file attachments on the full editor when making posts.

[Nitpicker]

Thanks to all of you for the answers to several of my questions.
Based on Chris Peterson's advice, I plotted a 166-pixel diameter white disk about where I'd guess the sun was.

That was based on the pixel scale of the narrow field camera. From the image description, it sounds as if this composite may be at the scale of the wide field camera, in which case the Sun would appear ten times smaller. Since this is a composite, some other scale may have been used, as well. To be sure, we'd need to calculate the actual scale from some physical reference, like the known angle between a pair of planets.

I estimate the diameter of the Sun should appear as about 0.5% (half of one percent) of the apparent distance between Mars and Earth. (Based on the fact that from Saturn, Mars and Earth were separated at the time by ~10.8° and the Sun has an apparent diameter of 3.25 minutes of arc. This neglects the relatively small distance between Cassini and Saturn, which was about 10 Saturn diameters, not including rings.)

[alter-ego]

Thanks to all of you for the answers to several of my questions.
Based on Chris Peterson's advice, I plotted a 166-pixel diameter white disk about where I'd guess the sun was.

That was based on the pixel scale of the narrow field camera. From the image description, it sounds as if this composite may be at the scale of the wide field camera, in which case the Sun would appear ten times smaller. Since this is a composite, some other scale may have been used, as well. To be sure, we'd need to calculate the actual scale from some physical reference, like the known angle between a pair of planets.

I estimate the diameter of the Sun should appear as about 0.5% (half of one percent) of the apparent distance between Mars and Earth. (Based on the fact that from Saturn, Mars and Earth were separated at the time by ~10.8° and the Sun has an apparent diameter of 3.25 minutes of arc. This neglects the relatively small distance between Cassini and Saturn, which was about 10 Saturn diameters, not including rings.)

The separations of multiple object references yield a pixel angular resolution of ≈ 12.4", exactly the WAC. Therefore the sun would illuminate 15.8 pixels.

Edit: Of course the Sun is not in only one spot. Wrt Saturn, it drifts about 1° per hour. That's about 18.5 solar diameters per hour.

[Nitpicker]

The separations of multiple object references yield a pixel angular resolution of ≈ 12.4", exactly the WAC. Therefore the sun would illuminate 15.8 pixels.

Edit: Of course the Sun is not in only one spot. Wrt Saturn, it drifts about 1° per hour. That's about 18.5 solar diameters per hour.

And according to the JPL's Solar System Simulator, a few hours after this four-hour photo session, Cassini drifted out of alignment with the Sun-Saturn line, revealing the Sun as shown in this link:

http://space.jpl.nasa.gov/cgi-bin/wspac ... -1&bfov=30