12 replies to this topic

[mrflibbles]

Just out of pure curiosity: has anyone estimated the length of the visible tail (edit: the dust tail not ion)? Can anyone?

 

I remember seeing it on Oct 14th and thinking to myself how long the tail would have to be to take up several degrees of sky, and it being roughly the distance of Venus. I'm probably not using the right search terms, but I tried looking it up. I wouldn't be able to begin to do the math. I can't be the only one curious though.

 

I'm not looking for pinpoint accuracy, but not stabs in the dark guesses either  but a ballpark figure will do. I know measuring it is also subjective, as in the tail is more visible the longer you expose and if you stack or not stack and blah blah blah yada yada. Lets just go with the "clearly visible" tail, and the average amateurs shot. Please lets not argue over semantics or overthink things. You will be satisfying a silly mans curiosity not writing a scientific paper.

 

For reference I will share share a shot, but it's just to provide context. It was taken @ 84 mm with an m100. I cut a small part of the tail off, but for the most part its all there. It is a bit more zoomed in than what our eyes can see, but its still fairly representative of what was actually naked eye visible that day.

 

Edited by mrflibbles, 14 November 2024 - 02:39 PM.

[Erik Bakker]

Just out of pure curiosity: has anyone estimated the length of the visible tail? Can anyone?

 

I remember seeing it on Oct 14th and thinking to myself how long the tail would have to be to take up several degrees of sky, and it being roughly the distance of Venus. I'm probably not using the right search terms, but I tried looking it up. I wouldn't be able to begin to do the math. I can't be the only one curious though.

 

I'm not looking for pinpoint accuracy, but not stabs in the dark guesses either  but a ballpark figure will do. I know measuring it is also subjective, as in the tail is more visible the longer you expose and if you stack or not stack and blah blah blah yada yada. Lets just go with the "clearly visible" tail, and the average amateurs shot. Please lets not argue over semantics or overthink things. You will be satisfying a silly mans curiosity not writing a scientific paper.

 

For reference I will share share a shot, but it's just to provide context. It was taken @ 84 mm with an m100. I cut a small part of the tail off, but for the most part its all there. It is a bit more zoomed in than what our eyes can see, but its still fairly representative of what was actually naked eye visible that day.

 

 

Beautiful image, representing its visual appearance in those October days well. You likely refer to the dust tail? The yellow-white (even a tint of orange with this comet. The ion tail is a separate, generally much shorter bluish green tail.  With many comets pointing in quite a different direction than the dust tail.

 

On October 14th, under good, but not the best conditions in late twilight, I “measured” the comet including it’s dust tail to be easily visible for roughly 1.5x the field of my 8x42 binocular with a 7.7 degree TFOV. So roughly 12 degrees long, likely a bit longer. Find my uncropped image taken with a 50mm at f/2, 4 seconds and ISO 200, on a FF Nikon DF camera below on that same evening, October 14th 2024. The ion tail was not visible and the anti-tail was only hinted at visually in the 8x42.

 

[Tom Axelsen]

With my binoculars, I estimated the tail length (visually) to 15 degrees on October 15th.

 

Doing the math this translates to a physical tail length of 21 million kilometers or 13 million miles.

Edited by Tom Axelsen, 14 November 2024 - 07:51 AM.

[timelapser]

I guess you're wondering about the length in km of the dust tail.  It's a bit tricky because our view in October was kind of foreshortened - the tail was partly pointing towards us.  So it was longer than you'd get based on the angular length and distance alone.

 

There was some speculation that Earth might pass through the ion tail, though.

[Tom Axelsen]

I guess you're wondering about the length in km of the dust tail.  It's a bit tricky because our view in October was kind of foreshortened - the tail was partly pointing towards us.  So it was longer than you'd get based on the angular length and distance alone.

 

There was some speculation that Earth might pass through the ion tail, though.

My calculated length of the visual tail of 21 million km includes foreshortening.

But that length is just for the 15 degrees of tail that I saw.

[mrflibbles]

Haha yes I got the ion and dust tail confused. Yes I meant the visible "dust" tail. That's not arguing semantics that is getting clarification. so thank you.

 

My calculated length of the visual tail of 21 million km includes foreshortening.
But that length is just for the 15 degrees of tail that I saw.

 

Since you put some thought into it instead of just guessing, I will take your word for it. Thank you!

 

To measure the suns width scientists just have to agree on parameters. For instance if you measure in the visible light spectrum, and then measure in UV, IR etc. you would get different results. I think your parameters are acceptable Tom. I think 21 million give or take a million is a good ballpark.

[BrooksObs]

In point of fact, it is quite impossible to estimate the true length of Comet 2023 A3 visible tail simply from images, or visual observations. This is because when the comet was at its best, our view was essentially in alignment up through/along the comet's orbital plane, making the tail look perfectly straight instead of its actual shape. In fact, the tail was undoubtedly quite curved based on determination of its shape from images obtained months earlier when we were well above the comet's orbital plane.

 

In reality, the visible dust tail likely curved strongly in the direction of Earth during the evening phase of the comet's apparition. However, we as amateurs, do not know the actual degree of curvature the dust tail presented. Therefore, one can only calculate  the tail's 'apparrent length' which likely would be significantly different (shorter) than the real tail length.

 

If instead of our Earthly view, we could have viewed Comet 2023 A3 from 90 degrees above the comet's orbital plane we likely would have seen an object far more resembling 1976's Comet West, or 1970's Comet Bennett, with a strongly curved tail trailing the comet's head, instead of the apparently arrow-straight look we did have from Earth.

 

BrooksObs 

Edited by BrooksObs, 16 November 2024 - 06:06 PM.

[timelapser]

That's a good point, Brooks.  We know how far away the nucleus was.  But we don't know very well how far away (some determination of) the tip of the tail was, since that depends on how curved the tail was.  It's possible the far end of the tail was aligned quite closely towards Earth in which case you couldn't say how far away the tip was.

 

This would've been easier to do for the ion tail which is normally much straighter than the dust tail, but of course we didn't see much of that with C/2023 A3.

 

It's not clear what Tom Axelsen assumed about the orientation and curvature of the dust tail in his length estimate above - maybe he assumed it was straight and directed away from the sun, like the ion tail?

Edited by timelapser, 16 November 2024 - 07:27 PM.

[gwfbmd]

It’s good to know some rough estimates just so we can think about it. The reality was that the tail length observed by any individual or camera depended on a lot of factors. I was impressed with the length in binoculars, but then I realized my DSLR was showing much more. And, I had one 135mm lens in particular which showed the antitail and which put the dust tail out there the most. Trying to be very exact with something this tenuous doesn’t seem possible. I am surprised the estimates here are as low as they are. I may try my own if I find the time. Glen

[Tom Axelsen]

That's a good point, Brooks.  We know how far away the nucleus was.  But we don't know very well how far away (some determination of) the tip of the tail was, since that depends on how curved the tail was.  It's possible the far end of the tail was aligned quite closely towards Earth in which case you couldn't say how far away the tip was.

 

This would've been easier to do for the ion tail which is normally much straighter than the dust tail, but of course we didn't see much of that with C/2023 A3.

 

It's not clear what Tom Axelsen assumed about the orientation and curvature of the dust tail in his length estimate above - maybe he assumed it was straight and directed away from the sun, like the ion tail?

My assumption was a foreshortened, but straight tail, so I have not included any curvature of the dust tail in the estimate, because as Brooks writes, we have no way of knowing how much the tail has curved. My length estimate is therefore the minimum length. 

I should have made that clear in my post.

[kbart0791]

My calculated length of the visual tail of 21 million km includes foreshortening.

But that length is just for the 15 degrees of tail that I saw.

I would have to agree with you.  Oct 14 was a good night to observe and the tail was well placed.  The foreshortening of the comets tail had a big effect of the tail surface brightness.  I could directly see a bright 10 to 12 degrees of its tail.  Using averted imagination I could detect another 2-3 degrees.  After perihelion the angle of the comet was not as favorable to see the gas tail.  Both tails were blended together in the forward back scatter and the foreshortening. Idid manage to see a couple of photos from individuals that were able to catch images of the gas tail in rare moments.

[bobjonesisthebest]

wouldnt the stereo spacecraft observations of it be ideal for this due to the different viewing angle allowing the curve to be seen?

Edited by bobjonesisthebest, 02 January 2025 - 05:55 PM.

[timelapser]

Which stereo observations do you mean?