APOD: Earth and Moon through Saturn's Rings (2020 May 27)

Comments and questions about the APOD on the main view screen.
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APOD: Earth and Moon through Saturn's Rings (2020 May 27)

Post by APOD Robot » Wed May 27, 2020 4:05 am

Image Earth and Moon through Saturn's Rings

Explanation: What are those dots between Saturn's rings? Our Earth and Moon. Just over three years ago, because the Sun was temporarily blocked by the body of Saturn, the robotic Cassini spacecraft was able to look toward the inner Solar System. There, it spotted our Earth and Moon -- just pin-pricks of light lying about 1.4 billion kilometers distant. Toward the right of the featured image is Saturn's A ring, with the broad Encke Gap on the far right and the narrower Keeler Gap toward the center. On the far left is Saturn's continually changing F Ring. From this perspective, the light seen from Saturn's rings was scattered mostly forward , and so appeared backlit. After more than a decade of exploration and discovery, the Cassini spacecraft ran low on fuel in 2017 and was directed to enter Saturn's atmosphere, where it surely melted.

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Re: APOD: Earth and Moon through Saturn's Rings (2020 May 27)

Post by Ann » Wed May 27, 2020 5:37 am

It's fascinating to see the Earth and Moon like this.

The respective color, brightness and sizes of the Earth and Moon looks just "right" to me. (Well, me the expert. :wink: ) The Earth looks basically all white, as a consequence of our planet's rather high albedo (reflectiveness). But the faint diffraction spikes centered on the Earth are slightly bluish, which is a testament of the bluish color of our planet's oceans. The Moon, by contrast, is clearly not only smaller, but its surface brightness is lower, because of its very low albedo. Its color is beige.

Earth and Moon from Saturn.png
Earth and Moon through Saturn's Rings. Image credit: NASA, ESA, JPL-Caltech,
SSI, Cassini Imaging Team; Processing & License: Kevin M. Gill




















Are the Earth and Moon resolved? That is, does the Earth look bigger than the Earth only because of its brightness, so that the larger size of the Earth is caused by "pixel bleeding", or is the Earth sufficiently resolved that the round shape of the Earth is due to the fact that at least, say, four pixels are involved in reacting to the light from the Earth?

And what about the distance between the Earth and the Moon? The average distance between the Earth and the Moon is (if I remember correctly) about 30 Earth diameters. (Or was it 60?) The distance between the Earth and the Moon in the picture at right strikes me as slightly smaller than 30 Earth diameters, let alone 60, and in the APOD the apparent distance is very small. Of course, the Earth-Moon system could be seen at an angle, which would make the distance between the two bodies look much smaller than it really is.

Note that, in the picture at right, the image of the Moon has been artificially brightened to make it look brighter. The Moon really is quite dark.

And to think that the bright white little disk in the APOD encompasses 100% of the history of humanity, with the exception of the 12 men who have walked on the Moon.

Ann
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heehaw

Re: APOD: Earth and Moon through Saturn's Rings (2020 May 27)

Post by heehaw » Wed May 27, 2020 9:22 am

Am I in that picture, or ... was I on the other side of the Earth when the picture was taken? We need to know the exact time the picture was taken!

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Re: APOD: Earth and Moon through Saturn's Rings (2020 May 27)

Post by orin stepanek » Wed May 27, 2020 12:32 pm

SaturnEarthMoon_Cassini_960.jpg
Simply an awesome photo! It's amazing what these satellite
cameras see when photographing out there! 8-)
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Re: APOD: Earth and Moon through Saturn's Rings (2020 May 27)

Post by neufer » Wed May 27, 2020 1:28 pm

heehaw wrote:
Wed May 27, 2020 9:22 am

Am I in that picture, or ... was I on the other side of the Earth when the picture was taken?

We need to know the exact time the picture was taken!
  • Πᾷ βῶ, καὶ χαριστίωνι τὰν γᾶν κινήσω πᾶσαν
    [Pā bō, kai kharistiōni tan gān kinēsō pāsan].
Give me a good place to stand and a long enough selfie stick and I will photobomb the earth.
- Archimedes of Syracuse (c. 287 BC – c. 212 BC
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Re: APOD: Earth and Moon through Saturn's Rings (2020 May 27)

Post by Sa Ji Tario » Wed May 27, 2020 1:42 pm

The Moon's orbital plane and the ring plane are almost coincident with the solar equatorial plane where all the planetary orbits are. So why do the rings appear to be visually normal to Earth?

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Re: APOD: Earth and Moon through Saturn's Rings (2020 May 27)

Post by Chris Peterson » Wed May 27, 2020 1:49 pm

Ann wrote:
Wed May 27, 2020 5:37 am
Are the Earth and Moon resolved? That is, does the Earth look bigger than the Earth only because of its brightness, so that the larger size of the Earth is caused by "pixel bleeding", or is the Earth sufficiently resolved that the round shape of the Earth is due to the fact that at least, say, four pixels are involved in reacting to the light from the Earth?
The camera resolution is 1.2 arcseconds per pixel. On the date the image was taken, the Earth, from Saturn, subtended an angle of 2 arcseconds. So absent diffraction and other optical effects, it should cover just under two pixels. Which is technically on the edge of "resolved", but clearly not what we're seeing here. Because Earth is overexposed, we're seeing well out onto the diffraction tails, making it appear larger than it actually should. Another clue that we're not seeing a resolved image is that the Earth was near its maximum elongation (as seen from Saturn) on that date, meaning that its phase was 50%. If you looked at it through a telescope, you'd see a half-Earth. If the image was resolved, that's what we'd see, not a round spot.
Chris

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Ann
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Re: APOD: Earth and Moon through Saturn's Rings (2020 May 27)

Post by Ann » Wed May 27, 2020 2:59 pm

Chris Peterson wrote:
Wed May 27, 2020 1:49 pm
Ann wrote:
Wed May 27, 2020 5:37 am
Are the Earth and Moon resolved? That is, does the Earth look bigger than the Earth only because of its brightness, so that the larger size of the Earth is caused by "pixel bleeding", or is the Earth sufficiently resolved that the round shape of the Earth is due to the fact that at least, say, four pixels are involved in reacting to the light from the Earth?
The camera resolution is 1.2 arcseconds per pixel. On the date the image was taken, the Earth, from Saturn, subtended an angle of 2 arcseconds. So absent diffraction and other optical effects, it should cover just under two pixels. Which is technically on the edge of "resolved", but clearly not what we're seeing here. Because Earth is overexposed, we're seeing well out onto the diffraction tails, making it appear larger than it actually should. Another clue that we're not seeing a resolved image is that the Earth was near its maximum elongation (as seen from Saturn) on that date, meaning that its phase was 50%. If you looked at it through a telescope, you'd see a half-Earth. If the image was resolved, that's what we'd see, not a round spot.
Thank, Chris.

Of course I meant to say, "Does the Earth look bigger than the Moon only because of its brightness..."

Ann
Last edited by Ann on Wed May 27, 2020 4:08 pm, edited 1 time in total.
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Saturn - Rings, Distant Earth & moon mere pinpricks

Post by BaldEagle » Wed May 27, 2020 3:04 pm

..
Today's (5/27/20) APOD photo...

It is a shot thru one of Saturn's rings, and seeing "pin-pricks of light lying about 1.4 billion kilometers distant" that are the earth and moon.

Question: Those two objects (our earth and moon) seem so close and easy to see from Saturn. I don't find Saturn that "close and easy to see" from Earth.

What gives?

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Re: Saturn - Rings, Distant Earth & moon mere pinpricks

Post by Chris Peterson » Wed May 27, 2020 3:11 pm

BaldEagle wrote:
Wed May 27, 2020 3:04 pm
..
Today's (5/27/20) APOD photo...

It is a shot thru one of Saturn's rings, and seeing "pin-pricks of light lying about 1.4 billion kilometers distant" that are the earth and moon.

Question: Those two objects (our earth and moon) seem so close and easy to see from Saturn. I don't find Saturn that "close and easy to see" from Earth.

What gives?
Saturn, viewed from Earth, looks quite similar to the Earth viewed from Saturn, at the resolution of our eyes (which is similar to the resolution of this camera). Saturn is incredibly easy to see from the Earth!

(Of course, this is a narrow field image. At this scale, if Saturn were located where we see the Earth/Moon, its ring system would nearly span the two.)
Chris

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Re: APOD: Earth and Moon through Saturn's Rings (2020 May 27)

Post by MarkBour » Wed May 27, 2020 4:32 pm

Really love this image. Look Homeward, Angel !

And thanks for the additional info, Ann and Chris.
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Re: APOD: Earth and Moon through Saturn's Rings (2020 May 27)

Post by Cousin Ricky » Wed May 27, 2020 4:41 pm

Ann wrote:
Wed May 27, 2020 5:37 am
And to think that the bright white little disk in the APOD encompasses 100% of the history of humanity, with the exception of the 12 men who have walked on the Moon.
Do Eugene Shoemaker’s remains count?
Click to play embedded YouTube video.

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Re: APOD: Earth and Moon through Saturn's Rings (2020 May 27)

Post by neufer » Wed May 27, 2020 5:02 pm

wspace.jpg
wspace2.jpg
Chris Peterson wrote:
Wed May 27, 2020 1:49 pm


The camera resolution is 1.2 arcseconds per pixel. On the date the image was taken, the Earth, from Saturn, subtended an angle of 2 arcseconds. So absent diffraction and other optical effects, it should cover just under two pixels. Which is technically on the edge of "resolved", but clearly not what we're seeing here. Because Earth is overexposed, we're seeing well out onto the diffraction tails, making it appear larger than it actually should. Another clue that we're not seeing a resolved image is that the Earth was near its maximum elongation (as seen from Saturn) on that date, meaning that its phase was 50%. If you looked at it through a telescope, you'd see a half-Earth. If the image was resolved, that's what we'd see, not a round spot.
Earth subtended an angle of 1.8"
Earth & Moon both at phase: 116º
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Re: APOD: Earth and Moon through Saturn's Rings (2020 May 27)

Post by neufer » Wed May 27, 2020 5:36 pm

Cousin Ricky wrote:
Wed May 27, 2020 4:41 pm
Ann wrote:
Wed May 27, 2020 5:37 am

And to think that the bright white little disk in the APOD encompasses 100% of the history of humanity, with the exception of the 12 men who have walked on the Moon.
Do Eugene Shoemaker’s remains count?
  • Clyde Tombaugh is off in the other direction:
https://en.wikipedia.org/wiki/Clyde_Tombaugh#Death wrote:
wspace1.jpg
<<Clyde William Tombaugh (February 4, 1906 – January 17, 1997) died when he was in Las Cruces, New Mexico, at the age of 90. He was cremated. A small portion of his ashes was placed aboard the New Horizons spacecraft [which flewby of Pluto on July 14, 2015]. The container includes the inscription: "Interred herein are remains of American Clyde W. Tombaugh, discoverer of Pluto and the Solar System's 'third zone'. Adelle and Muron's boy, Patricia's husband, Annette and Alden's father, astronomer, teacher, punster, and friend: Clyde W. Tombaugh (1906–1997)".>>
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Re: APOD: Earth and Moon through Saturn's Rings (2020 May 27)

Post by ATM Bob » Thu May 28, 2020 3:36 am

I encounter the same issue.
Either expose for Saturn and lose the moons, or expose for the moons, and the planet floods the sensor.
But, wait... Shouldn't the Earth/Moon be backlit or crescent?

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Re: APOD: Earth and Moon through Saturn's Rings (2020 May 27)

Post by Chris Peterson » Thu May 28, 2020 4:04 am

ATM Bob wrote:
Thu May 28, 2020 3:36 am
I encounter the same issue.
Either expose for Saturn and lose the moons, or expose for the moons, and the planet floods the sensor.
But, wait... Shouldn't the Earth/Moon be backlit or crescent?
Both are in crescent phases. You just can't see it because Earth is only a couple of pixels and the Moon is smaller than a pixel. Both are just overexposed diffraction spots.
Chris

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Re: APOD: Earth and Moon through Saturn's Rings (2020 May 27)

Post by TheZuke! » Thu May 28, 2020 1:54 pm

heehaw wrote:
Wed May 27, 2020 9:22 am
Am I in that picture, or ... was I on the other side of the Earth when the picture was taken? We need to know the exact time the picture was taken!
You are in the picture, but you were moving at the time, so your image is quite blurred.
Next time, stand still when your picture is being taken!
:ssmile: