APOD: Conjunction after Sunset (2020 Dec 19)

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APOD Robot
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APOD: Conjunction after Sunset (2020 Dec 19)

Post by APOD Robot » Sat Dec 19, 2020 5:06 am

Image Conjunction after Sunset

Explanation: How close will Jupiter and Saturn be at their Great Conjunction? Consider this beautiful triple conjunction of Moon, Jupiter and Saturn captured through clouds in the wintry twilight. The telephoto view looks toward the western horizon and the Alborz Mountains in Iran after sunset on December 17. The celestial gathering makes it easy to see Jupiter and fainter Saturn are separated on that date by roughly the diameter of the waxing crescent Moon. On the day of their Great Conjunction, solstice day December 21, Jupiter and Saturn may seem to nearly merge though. In their closest conjunction in 400 years they will be separated on the sky by only about 1/5 the apparent diameter of the Moon. By then the two largest worlds in the Solar System and their moons will be sharing the same field of view in telescopes around planet Earth.

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Re: APOD: Conjunction after Sunset (2020 Dec 19)

Post by orin stepanek » Sat Dec 19, 2020 1:58 pm

ConjunctionPanorama1-S2_1024.jpg
APOD Robot: The celestial gathering makes it easy to see Jupiter and fainter Saturn are separated on that date by roughly the diameter of the waxing crescent Moon.
Quite a bit fainter Saturn; smaller planet & a lot farther away! :eyebrows: :wink: Beautiful photo today! 🤩
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Re: APOD: Conjunction after Sunset (2020 Dec 19)

Post by Constantino » Sat Dec 19, 2020 3:01 pm

that was unfair! the pic that I sent is pretty :cry: You also can see Ganymede and Callisto. https://www.facebook.com/photo?fbid=221 ... 9961954410
:(

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johnnydeep
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Re: APOD: Conjunction after Sunset (2020 Dec 19)

Post by johnnydeep » Sat Dec 19, 2020 4:12 pm

Constantino wrote:
Sat Dec 19, 2020 3:01 pm
that was unfair! the pic that I sent is pretty :cry: You also can see Ganymede and Callisto. https://www.facebook.com/photo?fbid=221 ... 9961954410
:(
Nice shot! I can indeed see the two small dots on either side of Jupiter in roughly the same line as that joining Jupiter and Saturn.

Dumb question time: how do you know those are Ganymede and Callisto and not some other moons?
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orin stepanek
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Re: APOD: Conjunction after Sunset (2020 Dec 19)

Post by orin stepanek » Sat Dec 19, 2020 4:40 pm

Constantino wrote:
Sat Dec 19, 2020 3:01 pm
that was unfair! the pic that I sent is pretty :cry: You also can see Ganymede and Callisto. https://www.facebook.com/photo?fbid=221 ... 9961954410
:(
Were y6u referring to my post? I said the photo was beautiful! :shock: No criticism in my post at all; just explanation why Saturn appears so small!
Orin

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Re: APOD: Conjunction after Sunset (2020 Dec 19)

Post by Chris Peterson » Sat Dec 19, 2020 5:01 pm

johnnydeep wrote:
Sat Dec 19, 2020 4:12 pm
Constantino wrote:
Sat Dec 19, 2020 3:01 pm
that was unfair! the pic that I sent is pretty :cry: You also can see Ganymede and Callisto. https://www.facebook.com/photo?fbid=221 ... 9961954410
:(
Nice shot! I can indeed see the two small dots on either side of Jupiter in roughly the same line as that joining Jupiter and Saturn.

Dumb question time: how do you know those are Ganymede and Callisto and not some other moons?
It would be difficult to say if all you had to work with was the image. But if you know the time that the image was made (as I would hope the imager does!) it's easy enough to look up the positions of the moons and have a good chance of determining which ones are seen.
Chris

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Re: APOD: Conjunction after Sunset (2020 Dec 19)

Post by alter-ego » Sat Dec 19, 2020 6:39 pm

Chris Peterson wrote:
Sat Dec 19, 2020 5:01 pm
johnnydeep wrote:
Sat Dec 19, 2020 4:12 pm
Constantino wrote:
Sat Dec 19, 2020 3:01 pm
that was unfair! the pic that I sent is pretty :cry: You also can see Ganymede and Callisto. https://www.facebook.com/photo?fbid=221 ... 9961954410
:(
Nice shot! I can indeed see the two small dots on either side of Jupiter in roughly the same line as that joining Jupiter and Saturn.

Dumb question time: how do you know those are Ganymede and Callisto and not some other moons?
It would be difficult to say if all you had to work with was the image. But if you know the time that the image was made (as I would hope the imager does!) it's easy enough to look up the positions of the moons and have a good chance of determining which ones are seen.
Ganymede and Callisto are the only possibilities for those two moons. Recreating the image in Stellarium puts the time just after sunset on the 16th. The exact time is not needed to identify those moons.
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Re: APOD: Conjunction after Sunset (2020 Dec 19)

Post by Chris Peterson » Sat Dec 19, 2020 7:32 pm

alter-ego wrote:
Sat Dec 19, 2020 6:39 pm
Chris Peterson wrote:
Sat Dec 19, 2020 5:01 pm
johnnydeep wrote:
Sat Dec 19, 2020 4:12 pm


Nice shot! I can indeed see the two small dots on either side of Jupiter in roughly the same line as that joining Jupiter and Saturn.

Dumb question time: how do you know those are Ganymede and Callisto and not some other moons?
It would be difficult to say if all you had to work with was the image. But if you know the time that the image was made (as I would hope the imager does!) it's easy enough to look up the positions of the moons and have a good chance of determining which ones are seen.
Ganymede and Callisto are the only possibilities for those two moons. Recreating the image in Stellarium puts the time just after sunset on the 16th. The exact time is not needed to identify those moons.
But you did need the date to narrow it down. And a degree of luck. Plenty of opportunities for ambiguities in an image of Jupiter with two moons showing.
Chris

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Re: APOD: Conjunction after Sunset (2020 Dec 19)

Post by alter-ego » Sun Dec 20, 2020 3:39 am

Chris Peterson wrote:
Sat Dec 19, 2020 7:32 pm
alter-ego wrote:
Sat Dec 19, 2020 6:39 pm
Chris Peterson wrote:
Sat Dec 19, 2020 5:01 pm


It would be difficult to say if all you had to work with was the image. But if you know the time that the image was made (as I would hope the imager does!) it's easy enough to look up the positions of the moons and have a good chance of determining which ones are seen.
Ganymede and Callisto are the only possibilities for those two moons. Recreating the image in Stellarium puts the time just after sunset on the 16th. The exact time is not needed to identify those moons.
But you did need the date to narrow it down. And a degree of luck. Plenty of opportunities for ambiguities in an image of Jupiter with two moons showing.
Nope. I knew nothing a priori - neither date, time nor location. Simply put, it was educated trial and error. The only the 16th works to locate the moon, and, though not necessary, identifying HIP 98575 helped fine tune the time for Jovian moon certainty. For fun, I roughed in a location to match the image. This is a challenge because I have to assume camera orientation.
A pessimist is nothing more than an experienced optimist

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Re: APOD: Conjunction after Sunset (2020 Dec 19)

Post by Chris Peterson » Sun Dec 20, 2020 3:55 am

alter-ego wrote:
Sun Dec 20, 2020 3:39 am
Chris Peterson wrote:
Sat Dec 19, 2020 7:32 pm
alter-ego wrote:
Sat Dec 19, 2020 6:39 pm


Ganymede and Callisto are the only possibilities for those two moons. Recreating the image in Stellarium puts the time just after sunset on the 16th. The exact time is not needed to identify those moons.
But you did need the date to narrow it down. And a degree of luck. Plenty of opportunities for ambiguities in an image of Jupiter with two moons showing.
Nope. I knew nothing a priori - neither date, time nor location. Simply put, it was educated trial and error. The only the 16th works to locate the moon, and, though not necessary, identifying HIP 98575 helped fine tune the time for Jovian moon certainty. For fun, I roughed in a location to match the image. This is a challenge because I have to assume camera orientation.
Location is irrelevant. But you depend upon more information than just Jupiter and two moons in an image, for which there are an infinite number of possible times, all the more given that you can't reliably identify which of the four Galilean moons make up the two dots.
Chris

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Re: APOD: Conjunction after Sunset (2020 Dec 19)

Post by kao_yuan » Sun Dec 20, 2020 12:44 pm

also a view of great conjunction in Taiwan on Dec. 17, 2020
at a high-speed railway station :D

https://www.facebook.com/photo?fbid=101 ... 9240346113
https://www.facebook.com/photo?fbid=101 ... 9240346113

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Re: APOD: Conjunction after Sunset (2020 Dec 19)

Post by johnnydeep » Sun Dec 20, 2020 4:03 pm

alter-ego wrote:
Sun Dec 20, 2020 3:39 am
Chris Peterson wrote:
Sat Dec 19, 2020 7:32 pm
alter-ego wrote:
Sat Dec 19, 2020 6:39 pm


Ganymede and Callisto are the only possibilities for those two moons. Recreating the image in Stellarium puts the time just after sunset on the 16th. The exact time is not needed to identify those moons.
But you did need the date to narrow it down. And a degree of luck. Plenty of opportunities for ambiguities in an image of Jupiter with two moons showing.
Nope. I knew nothing a priori - neither date, time nor location. Simply put, it was educated trial and error. The only the 16th works to locate the moon, and, though not necessary, identifying HIP 98575 helped fine tune the time for Jovian moon certainty. For fun, I roughed in a location to match the image. This is a challenge because I have to assume camera orientation.
Uh, "identifying HIP 98575"? I don't see ANY stars in Constantino's image. Of course, I had no idea what HIP 98575 was, but Universeguide.com has a surprising amount of info about it: https://www.universeguide.com/star/55390/hip55390"

Code: Select all

Alternative Names			HD 98575, TYC 4926-207-1, 02 3325
Spectral Type				A1mF0-F2
Binary or Multiple Star System		Yes
Star Type based on Spectral Type	Star
Colour					Blue
Galaxy					Milky Way
Constellation				Leo
Constellation's Main Star		No
Absolute Magnitude			-2.18 / -0.18
Visual / Apparent Magnitude		9.12
Naked Eye Visible			Requires a 7x50 Binoculars
Right Ascension (R.A.)			11h 20m 32.05
Declination (Dec.)			-02° 53` 53.3
Galactic Latitude			52.82441629 degrees
Galactic Longitude			263.25061553 degrees
1997 Distance from Earth		0.55000 Parallax (milliarcseconds)
					5930.24 Light Years
					1818.18 Parsecs
2007 Distance from Earth		1.38000 Parallax (milliarcseconds)
					2363.50 Light Years
					724.64 Parsecs
					149,466,687.70 Astronomical Units
Proper Motion Dec.			-10.70000 ± 0.87000 milliarcseconds/year
Proper Motion RA.			-16.66000 ± 1.37000 milliarcseconds/year
B-V Index				0.32
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Re: APOD: Conjunction after Sunset (2020 Dec 19)

Post by alter-ego » Tue Dec 29, 2020 2:50 am

Chris Peterson wrote:
Sun Dec 20, 2020 3:55 am
alter-ego wrote:
Sun Dec 20, 2020 3:39 am
Chris Peterson wrote:
Sat Dec 19, 2020 7:32 pm

 

But you did need the date to narrow it down. And a degree of luck. Plenty of opportunities for ambiguities in an image of Jupiter with two moons showing.
Nope. I knew nothing a priori - neither date, time nor location. Simply put, it was educated trial and error. The only the 16th works to locate the moon, and, though not necessary, identifying HIP 98575 helped fine tune the time for Jovian moon certainty. For fun, I roughed in a location to match the image. This is a challenge because I have to assume camera orientation.
Location is irrelevant. But you depend upon more information than just Jupiter and two moons in an image, for which there are an infinite number of possible times, all the more given that you can't reliably identify which of the four Galilean moons make up the two dots.
Certainly there are images for which what you say is true. However, this particular image has enough analyzable information to not only reliably identify the Callisto as the second moon, but also identify a 2000km path within which the image could have been taken over roughly a 10-minute time window.
  1. Only looking at Jupiter and the two moons, it can be determined that Callisto is the only solution for the second moon:
    → A measured 8.4' separation between the two moons corresponds to an Image time ~12/16 23:30 UT
    → Moon's separation =97% of the maximum possible. Of the 4 Jovian moons, Callisto and Ganymede are the only pair that reach separation.
    • Identifying Dec 16th UT as the day is determined from the moon position wrt the planet
    • Comparing the plot of Jovian moons orbital positions over time against the measured 8.4-arcmin separation between Ganymede and the second moon in the image, it's readily apparent that Callisto must the moon

    → Given the measured 50% diameter of Jupiter's over-exposed disk ≈ 2.7' (5x larger than actual size), the plot shows the inner moons, Io and Europa, are essentially buried by Jupiter's oversized disk when the image was captured.
    → Given 1) A reasonable image measurement accuracy of 1', and 2)That either Io and Europa would be visible if they were located far enough outside of Jupiter's glare (Callisto is the faintest of the 4 main Jovian moons), I estimate that the capture-time window ≥10 hours that the inner moons would not be visible.
    There is no doubt in my mind that Callisto is the only reasonable candidate for the second moon.

     
     
     
     
     

    Lacking observer location and image details, this method is the easiest and most direct approach to establishing Callisto is the moon in question. Even the large, 10-hr uncertainty in the capture time does not change the answer.
     
  2. To predict observer locations, I did a another, totally independent (and more tedious), analysis to also find the image-capture time:
    For many input image-times, the closest observer locations (Lat & Long) were empirically solved such that, as viewed from each location, the Moon at the same apparent RA & Dec as in the image (I used identifiable stars in the image to more accurately determine the imaged lunar coordinates, and thus observer locations) The results:
    • A 2000-km path cutting across South America (Argentina) where twilight conditions existed, as well as an image-capture window of ~10 minutes, and
    • Best estimate solution time ~ 12/16/23:53UT

    → Regarding location on Earth, it is irrelevant wrt Jovian moon positions and timings. However, when extracting an observation time from the apparent lunar RA & Dec, observer locations are relevant. The primary reason why there's a locus, instead of a single-point location, is because the image did not have a horizon reference. The specific time shown assumes a horizon parallel to the horizontal image frame edge.
     
     
     
     
     
     
     
     
     
     
     
    I was quite surprised by only a ½-hour difference in image-time. Even though much more tedious, the more involved location search method resulted in a much smaller image-capture time uncertainty.
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Re: APOD: Conjunction after Sunset (2020 Dec 19)

Post by alter-ego » Tue Dec 29, 2020 2:52 am

johnnydeep wrote:
Sun Dec 20, 2020 4:03 pm
...
Uh, "identifying HIP 98575"? I don't see ANY stars in Constantino's image. Of course, I had no idea what HIP 98575 was, but Universeguide.com has a surprising amount of info about it: https://www.universeguide.com/star/55390/hip55390"
There are actually 3 stars I used: HIP 97944, HIP 98575 and HIP 98399. These are the stars I used to establish the moon's celestial coordinates.

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