APOD: Between the Rings (2017 Apr 22)

[APOD Robot]

Between the Rings

Explanation: On April 12, as the Sun was blocked by the disk of Saturn the Cassini spacecraft camera looked toward the inner Solar System and the gas giant's backlit rings. At the top of the mosaicked view is the A ring with its broader Encke and narrower Keeler gaps visible. At the bottom is the F ring, bright due to the viewing geometry. The point of light between the rings is Earth, 1.4 billion kilometers in the distance. Look carefully and you can even spot Earth's large moon, a pinprick of light to the planet's left. Today Cassini makes its final close approach to Saturn's own large moon Titan, using Titan's gravity to swing into the spacecraft's Grand Finale, the final set of orbits that will bring Cassini just inside Saturn's rings.

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[daddyo]

How can such a little pinprick of light contain so much life?

[Ann]

Click to view full size image

Pale blue dot, Earth seen through rings of Saturn
by Voyager 1, 1990.

I prefer the Pale Blue Dot picture from 1990.

Click to view full size image

[c]Moon and Earth between rings of Saturn.[/c]

Of course, a good thing about the new image is that Saturn's rings look so much better, and you can actually see the Moon.

Ann

[Nitpicker]

Of course, a good thing about the new image is that Saturn's rings look so much better, and you can actually see the Moon.

Saturn's rings look better than what? Saturn is not in the famous "Pale Blue Dot" image from 1990 (taken from beyond the orbits of Pluto and Neptune) which shows bands of scattered sunlight.

I love both images, but I have to say I prefer this new one. Beautiful. Grand and humbling at once.

[Nitpicker]

(I also quietly enjoy the fact that I can get more resolution on an image of Saturn from my back yard, than Cassini can on an image of Earth.)

[Ann]

Of course, a good thing about the new image is that Saturn's rings look so much better, and you can actually see the Moon.

Saturn's rings look better than what? Saturn is not in the famous "Pale Blue Dot" image from 1990 (taken from beyond the orbits of Pluto and Neptune) which shows bands of scattered sunlight.

Oh! Sorry. And thanks for telling me.

Ann

[Czerno O]

A significant (to us Earthlings) image ! My first thought was how much Galileo would have loved to see a picture like today's confirm the soundness of a life's studies, findings and speculations !

[Karen]

What a great photo on Earth day! Thanks

[Jim Armstrong]

Another great Cassini photograph. I am going to miss them.
I am wondering why the Earth and Moon don't show any phase in the pix, i.e., they seem to be "full."
One chart I looked at this morning shows that Saturn is toward the closer to the Earth part of its travels. This would make full solar illumination unlikely.
I am sure that I will have my mistake gently but firmly pointed out within hours.
Happy Science Day!

[neufer]

I am wondering why the Earth and Moon don't show any phase in the pix, i.e., they seem to be "full."

I can't tell about the Moon but...
the Earth is an overexposed Airy disk in order that the back-lit rings are clearly visible.

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

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

[Case]

I am wondering why the Earth and Moon don't show any phase in the pix, i.e., they seem to be "full."

So lets do the calculation. In the Pale Blue Dot photo, the Earth is only 0.12 pixels in size, per Wikipedia description. You’re seeing it already at least 8 times enlarged.
In the Cassini image, the best tool to use is the distance Earth-Moon. 65 pixels in the pixel-doubled zoom image. On April 12, 2017 at 10:41 p.m. PDT, the angle between Saturn and the Moon was 41.4°, as seen from Earth. The Earth-Moon distance at that time was 401242 km (249320 mi). Because of the angle, it would look shorter (× sin α) from a Saturn vantage point: 265346 km (164878 mi). 65 pixels equals 265346 km, then 1 pixel equals 4082 km (2537 mi). The earth diameter is 12735 km (7913 mi), so 3 pixels. The Moon would be 0.85 pixel. Even besides optical effects, you couldn't see phases.