APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

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APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by APOD Robot » Fri Dec 03, 2021 5:05 am

Image Comet Leonard and the Whale Galaxy

Explanation: Sweeping through northern predawn skies, on November 24 Comet Leonard (C/2021 A1) was caught between two galaxies in this composite telescopic image. Sporting a greenish coma the comet's dusty tail seems to harpoon the heart of NGC 4631 (top) also known as the Whale Galaxy. Of course NGC 4631 and NGC 4656 (bottom, aka the Hockey Stick) are background galaxies some 25 million light-years away. On that date the comet was about 6 light-minutes from our fair planet. Its closest approach to Earth (and even closer approach to Venus) still to come, Comet Leonard will grow brighter in December. Already a good object for binoculars and small telescopes, this comet will likely not return to the inner Solar System. Its perihelion, or closest approach to the Sun, will be on January 3, 2022.

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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by De58te » Fri Dec 03, 2021 9:24 am

Something puzzles me. In your second link, Comet Leonard (C/2021 A1) scrolling down to the bottom of their page there is an animation showing the comet approaching the Earth in a counterclockwise direction. The same direction as the planets orbit. However in the wikipedia page their animation shows the comet approaching in the opposite direction of the planets orbit. There the comet is orbiting clockwise. I wonder who is correct?

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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by Chris Peterson » Fri Dec 03, 2021 11:56 am

De58te wrote: Fri Dec 03, 2021 9:24 am Something puzzles me. In your second link, Comet Leonard (C/2021 A1) scrolling down to the bottom of their page there is an animation showing the comet approaching the Earth in a counterclockwise direction. The same direction as the planets orbit. However in the wikipedia page their animation shows the comet approaching in the opposite direction of the planets orbit. There the comet is orbiting clockwise. I wonder who is correct?
Not sure what you're seeing. The comet has an inclination of 133°, so it's in a retrograde orbit. That is, it is orbiting clockwise when viewed from north of the Solar System. Which is what I am seeing in both links.
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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by XgeoX » Fri Dec 03, 2021 12:29 pm

A 70,000 year orbit out to 3,500 AU and is barreling though the solar system now at a 158,000 mph. Some very heady numbers!

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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by orin stepanek » Fri Dec 03, 2021 12:31 pm

ngc4656_4631_A1-LRGB_112421_1024px.jpg
Nice! I'm not two outdoorsy anymore & APOD is a wonderful alternative! 8-)
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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by orin stepanek » Fri Dec 03, 2021 12:35 pm

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Eric; Ya forgot! :mrgreen:
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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by neufer » Fri Dec 03, 2021 12:41 pm

Click to play embedded YouTube video.
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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by smitty » Fri Dec 03, 2021 2:44 pm

". . . this comet will likely not return to the inner Solar System." And the reason for this is . . . ? Would be nice to answer this obvious question! Why/how is this comet different from other comets that do return to the inner Solar System?

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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by Chris Peterson » Fri Dec 03, 2021 2:51 pm

smitty wrote: Fri Dec 03, 2021 2:44 pm ". . . this comet will likely not return to the inner Solar System." And the reason for this is . . . ? Would be nice to answer this obvious question! Why/how is this comet different from other comets that do return to the inner Solar System?
In fact, the comet will not even be a part of our Solar System anymore. Its passage through the inner system has perturbed it into a hyperbolic orbit, meaning its velocity is greater than the Solar System escape velocity. It is being ejected from our system, never to return.

Most long period comets are perturbed significantly as they pass the gas giants and the Sun, which is why most have very different periods after perihelion than before. Most already have eccentricities very near 1, so it isn't all that uncommon for their passage to add just enough extra energy for the eccentricity to be slightly greater than 1, meaning they will be ejected.
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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by smitty » Fri Dec 03, 2021 2:56 pm

Chris Peterson, thank you for this excellent reply/clarification! Makes sense! Might have been good to have added a word about this to the original APOD discussion, but glad to have it now.

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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by DL MARTIN » Fri Dec 03, 2021 4:59 pm

Astronomers may not be perturbed by this misrepresentation of scientific rigour, but I still find a problem with viewing an object 6 light-minutes distant as if it was in context with an object 25 million light years away. Since we are referring to the past, how is it reconciled that the galaxy, as shown 25 million years ago, is positioned currently contexed with the comet's position 6 minutes ago? How do we know where the galaxy currently resides?

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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by Chris Peterson » Fri Dec 03, 2021 5:25 pm

DL MARTIN wrote: Fri Dec 03, 2021 4:59 pm Astronomers may not be perturbed by this misrepresentation of scientific rigour, but I still find a problem with viewing an object 6 light-minutes distant as if it was in context with an object 25 million light years away. Since we are referring to the past, how is it reconciled that the galaxy, as shown 25 million years ago, is positioned currently contexed with the comet's position 6 minutes ago? How do we know where the galaxy currently resides?
We are observing both as they are now, at this point in their evolution.
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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by Fred the Cat » Fri Dec 03, 2021 5:47 pm

Perhaps what happened to Comet Holmes may happen to Comet Leonard. If so, we should celebrate with a frozen and fried desert. :content:

They will be keeping an eye on it. :roll: It would be a Merry Christmas!
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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by DL MARTIN » Fri Dec 03, 2021 6:37 pm

As much as I'm in awe of Chris Peterson's understanding of astronomy matters, I simply can't agree with his 'looking out the window and declaring the world is flat' point of view. A lot of evolution takes place between 25 million years ago and 6 minutes ago, The failure of astronomy to emphasize the past nature of observation is a fundamental flaw in science. Astrophysics, for example, does not fall into this categorization since it makes no such reference that I'm aware of.

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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by Chris Peterson » Fri Dec 03, 2021 6:40 pm

DL MARTIN wrote: Fri Dec 03, 2021 6:37 pm As much as I'm in awe of Chris Peterson's understanding of astronomy matters, I simply can't agree with his 'looking out the window and declaring the world is flat' point of view. A lot of evolution takes place between 25 million years ago and 6 minutes ago, The failure of astronomy to emphasize the past nature of observation is a fundamental flaw in science. Astrophysics, for example, does not fall into this categorization since it makes no such reference that I'm aware of.
You are wrong. I can't put it more simply than that. It's like rejecting an image of your grandfather taken when he was a boy because it doesn't show what he looks like today.

An astronomical image typically shows an object at a certain age. Not at a certain time in the past.
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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by Ann » Fri Dec 03, 2021 8:57 pm


I love today's APOD! I find the contrast between the elongated green comet and the elongated blue and blue-and-yellow background galaxies so very beautiful.

However, I must admit that Virindi's image here at Starship Asterisk* may give a "truer" picture of the contrast between the comet and the galaxies. Note that the surface brightness of the comet's coma is much higher than the surface brightness of the galaxies. Also note that the green color of the comet's coma is much more saturated than the colors of the galaxies.

Both Gregg Ruppel's and Virindi's images are very interesting!

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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by johnnydeep » Fri Dec 03, 2021 9:52 pm

Chris Peterson wrote: Fri Dec 03, 2021 6:40 pm
DL MARTIN wrote: Fri Dec 03, 2021 6:37 pm As much as I'm in awe of Chris Peterson's understanding of astronomy matters, I simply can't agree with his 'looking out the window and declaring the world is flat' point of view. A lot of evolution takes place between 25 million years ago and 6 minutes ago, The failure of astronomy to emphasize the past nature of observation is a fundamental flaw in science. Astrophysics, for example, does not fall into this categorization since it makes no such reference that I'm aware of.
You are wrong. I can't put it more simply than that. It's like rejecting an image of your grandfather taken when he was a boy because it doesn't show what he looks like today.

An astronomical image typically shows an object at a certain age. Not at a certain time in the past.
That's an interesting way to think about it, and one I don't think I've seen you state before so succinctly. But isn't seeing an image of an object at a certain age (i.e., point in its evolution over time) - either a snapshot of your grandfather, or a CCD sensor image of a galaxy - just another way of saying that you are seeing it as it was at a certain time in the past?
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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by Chris Peterson » Fri Dec 03, 2021 10:48 pm

johnnydeep wrote: Fri Dec 03, 2021 9:52 pm
Chris Peterson wrote: Fri Dec 03, 2021 6:40 pm
DL MARTIN wrote: Fri Dec 03, 2021 6:37 pm As much as I'm in awe of Chris Peterson's understanding of astronomy matters, I simply can't agree with his 'looking out the window and declaring the world is flat' point of view. A lot of evolution takes place between 25 million years ago and 6 minutes ago, The failure of astronomy to emphasize the past nature of observation is a fundamental flaw in science. Astrophysics, for example, does not fall into this categorization since it makes no such reference that I'm aware of.
You are wrong. I can't put it more simply than that. It's like rejecting an image of your grandfather taken when he was a boy because it doesn't show what he looks like today.

An astronomical image typically shows an object at a certain age. Not at a certain time in the past.
That's an interesting way to think about it, and one I don't think I've seen you state before so succinctly. But isn't seeing an image of an object at a certain age (i.e., point in its evolution over time) - either a snapshot of your grandfather, or a CCD sensor image of a galaxy - just another way of saying that you are seeing it as it was at a certain time in the past?
But that is not usually a useful concept. You can say it that way, but to what purpose? There are times where it matters... for instance, when we're looking back at cosmological time scales, and knowing the age of something tells us a bit about what the Universe was like then. But even in that case, we're not worrying about what that object might look like today. We look at (for example) galaxy evolution by observing many of them at different stages. That is, different ages from their formation, not different times from now back to when the photons left them.
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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by spaced4ever » Sat Dec 04, 2021 2:18 am

Looks like a Starship firing a photon torpedo.

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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by alter-ego » Sat Dec 04, 2021 5:26 am

Chris Peterson wrote: Fri Dec 03, 2021 10:48 pm
johnnydeep wrote: Fri Dec 03, 2021 9:52 pm
Chris Peterson wrote: Fri Dec 03, 2021 6:40 pm

You are wrong. I can't put it more simply than that. It's like rejecting an image of your grandfather taken when he was a boy because it doesn't show what he looks like today.

An astronomical image typically shows an object at a certain age. Not at a certain time in the past.
That's an interesting way to think about it, and one I don't think I've seen you state before so succinctly. But isn't seeing an image of an object at a certain age (i.e., point in its evolution over time) - either a snapshot of your grandfather, or a CCD sensor image of a galaxy - just another way of saying that you are seeing it as it was at a certain time in the past?
But that is not usually a useful concept. You can say it that way, but to what purpose? There are times where it matters... for instance, when we're looking back at cosmological time scales, and knowing the age of something tells us a bit about what the Universe was like then. But even in that case, we're not worrying about what that object might look like today. We look at (for example) galaxy evolution by observing many of them at different stages. That is, different ages from their formation, not different times from now back to when the photons left them.
Yes, that is the not only the most valuable way to look at past events now, but it's also the only way really. Scientifically speaking, "now" is all we have to work from. Bottom line, we cannot predict the future both for events that haven't occurred yet, and for past (cosmic) events are observed now. Thinking about how the present, past view of a complex system (e.g., galaxy) will look in the future can be interesting and entertaining :b: :b: but, given our present inability to model the complex myriad of real-Universe details, the reality of these predictions is as far from science as alchemy is. We should be able make better and more detailed predictions in the future; just can't say when or how much.
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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by Ann » Sat Dec 04, 2021 7:26 am

Does it matter a lot that NGC 4631 and NGC 4656 are located circa 25 million light-years away from us, so that the light that reaches us from them was emitted some 25 million years ago? Is it likely that the overall appearance of these two galaxies will have changed significantly in 25 million years?

It isn't, if you ask me.


Take a look at these two galaxies. M101 at right is believed to be some 21 million light-years distant, whereas the distance to NGC 2336 is about 100 million light-years. So one galaxy is about five times as distant as the other, but they look much the same.

And take a look at this galactic pair:


The large galaxy is NGC 7753 and the small galaxy is NGC 7752. How far away would you say that this galactic pair is located? 25 million light-years? 50? 100?

The answer according to Wikipedia is "approximately 272 million light-years". Yes, we may question the very exact distance estimate. Nevertheless, this galactic pair may be ten times as distant as NGC 4631 and NGC 4656. Yet judging by their appearance only, NGC 7752/NGC 7753 may easily have been as close to us as the galaxies in this APOD.

In my opinion, we don't have to worry about what NGC 4631 and NGC 4656 look like "now" (or, more exactly, what they look like 25 million years after the light that we can see was emitted). The answer is, in my opinion, that they look the same, because the differences in their appearance in such a short time (by cosmic standards) will be so small that we can disregard them.

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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by johnnydeep » Sat Dec 04, 2021 1:47 pm

I don't know. It seems to me that what Chris, alter-ego, and Ann seem to be arguing above comes uncomfortable close to saying that the passage of time is (mostly) irrelevant when it comes to understanding astronomical images. I liken that to being satisfied with the idea that we can get a full understand of what happens to life on Earth solely from a collection of snapshots. But perhaps my cognitive dissonance stems from the ability to actually SEE change happening in real-time on Earth, whereas with astronomical events more than a light-minute away there is essentially no "real time" to observe. But of course, even with some things that happen on Earth, we are indeed constrained to theorize from snapshots only: long term evolution and geological processes being the prime examples. However, even there, we CAN see viral and bacterial evolution in real-time; beach erosion happening before our eyes; and earthquakes toppling buildings.

So, I'm still mulling things over: is this all much ado about nothing, a "distinction without a difference", or something more profound that I still don't appreciate. Not sure yet :?
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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by Chris Peterson » Sat Dec 04, 2021 2:06 pm

johnnydeep wrote: Sat Dec 04, 2021 1:47 pm I don't know. It seems to me that what Chris, alter-ego, and Ann seem to be arguing above comes uncomfortable close to saying that the passage of time is (mostly) irrelevant when it comes to understanding astronomical images.
Speaking for myself (and I guess the others, too) I'm not remotely saying that. What I'm saying is that in most contexts, "now" is the moment we observe something. It is pointless to ask "what is that object like 'now' because it took time for the light to reach us?" One of the most fundamental ideas of relativity is that "now" is different in different places, because of the nature of spacetime.

That does not mean that time isn't important! We look at pretty much everything in terms of how it has evolved and changed with time. And we may well use that information to try and understand what it will look like in the future. But we don't treat our observations as somehow false or incomplete because our frame of reference is different from that of the object in some different part of spacetime.
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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by Fred the Cat » Sat Dec 04, 2021 4:19 pm

Let’s say a nearby star is going supernova today. Predicting that has some scientific evidence. If IK Pegasi gave us a forewarning, calculating its distance in our current time might have value to Earthlings.

Are there any other situations knowing what a system would be doing at this moment that would add useful information :?: (rather than extrapolating from the multitude of examples readily seeable in their previous state)
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Re: APOD: Comet Leonard and the Whale Galaxy (2021 Dec 03)

Post by Chris Peterson » Sat Dec 04, 2021 4:40 pm

Fred the Cat wrote: Sat Dec 04, 2021 4:19 pm Let’s say a nearby star is going supernova today. Predicting that has some scientific evidence. If IK Pegasi gave us a forewarning, calculating its distance in our current time might have value to Earthlings.

Are there any other situations knowing what a system would be doing at this moment that would add useful information :?: (rather than extrapolating from the multitude of examples readily seeable in their previous state)
But we don't try to predict a supernova that happens "today". That doesn't matter, especially as we don't necessarily know the distance accurately. And we don't need to know the distance. What we need to do is to know how long after the current observation the star will supernova. Because all that matters is when the supernova light will reach us. If we understand enough about supernovas, we can look at the star and say, "Our observations allow us to predict that this star will go supernova in 15 years." That is useful, and does not depend at all on distance, or knowing what day, on our calendar, the star blew up or will blow up.
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