APOD: The Tail of a Christmas Comet (2021 Dec 25)

Comments and questions about the APOD on the main view screen.
User avatar
APOD Robot
Otto Posterman
Posts: 5547
Joined: Fri Dec 04, 2009 3:27 am

APOD: The Tail of a Christmas Comet (2021 Dec 25)

Post by APOD Robot » Sat Dec 25, 2021 5:06 am

Image The Tail of a Christmas Comet

Explanation: The tail of a comet streams across this three degree wide telescopic field of view captured under dark Namibian skies on December 21. In outburst only a few days ago and just reaching naked eye visibility Comet Leonard (C/2021 A1) is this year's brightest comet. Binoculars will make the diffuse comet easier to spot though, close to the western horizon after sunset. Details revealed in the sharp image show the comet's coma with a greenish tinge, and follow the interaction of the comet's ion tail with magnetic fields in the solar wind. After passing closest to Earth on December 12 and Venus on December 18, Comet Leonard is heading toward perihelion, its closest approach to the Sun on January 3rd. Appearing in late December's beautiful evening skies after sunset, Comet Leonard has also become known as 2021's Christmas Comet.

<< Previous APOD This Day in APOD Next APOD >>

David G

Re: APOD: The Tail of a Christmas Comet (2021 Dec 25)

Post by David G » Sat Dec 25, 2021 7:08 am

An ellipse has two focal points. Many comets' orbits are elliptical. For those comets in our solar system, the sun is one focal point ... but where's the other? For those comets outside our solar system, has anyone been able to establish the two foci?

User avatar
orin stepanek
Plutopian
Posts: 8200
Joined: Wed Jul 27, 2005 3:41 pm
Location: Nebraska

Re: APOD: The Tail of a Christmas Comet (2021 Dec 25)

Post by orin stepanek » Sat Dec 25, 2021 1:06 pm

C2021A1_211221_1024.jpg
Merry Christmas Everyone! 🤶🏻 🧑🏻🎄 🎅🏻
You do not have the required permissions to view the files attached to this post.
Orin

Smile today; tomorrow's another day!

User avatar
Chris Peterson
Abominable Snowman
Posts: 18529
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: The Tail of a Christmas Comet (2021 Dec 25)

Post by Chris Peterson » Sat Dec 25, 2021 2:09 pm

David G wrote: Sat Dec 25, 2021 7:08 am An ellipse has two focal points. Many comets' orbits are elliptical. For those comets in our solar system, the sun is one focal point ... but where's the other? For those comets outside our solar system, has anyone been able to establish the two foci?
This comet is in the process of being perturbed from an elliptical orbit to a hyperbolic one. While both ellipses and hyperbolas have two foci, only one of them is physically meaningful. If you know the orbital parameters, you can trivially identify the location of the empty focus, but it has no physical significance, so nobody bothers. All that is important is the focus that lies at the Sun.

Keplerian orbits are fundamentally the product of two-body interactions- which don't physically exist anywhere in the Universe except as approximations. As this comet gets farther and farther from the Sun, it will be increasingly perturbed into a complex path that the word "orbit" only marginally describes (of course, the comet might still be reasonably seen as in a near Keplerian elliptical orbit around the center of the galaxy).
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
https://www.cloudbait.com

User avatar
neufer
Vacationer at Tralfamadore
Posts: 18805
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: The Tail of a Christmas Comet (2021 Dec 25)

Post by neufer » Sat Dec 25, 2021 5:53 pm

Chris Peterson wrote: Sat Dec 25, 2021 2:09 pm
(of course, the comet might still be reasonably seen as in a near Keplerian elliptical orbit around the center of the galaxy).
Keplerian elliptical orbits require a 1/R2 force field.

Milky Way dark matter, however, produces an approximate 1/R force field [= a logarithmic potential]
which balances out the V2/R centrifugal force with an observed radially constant orbital Velocity.
https://arxiv.org/abs/1209.1342 wrote:
A Study of the Orbits of the Logarithmic Potential for Galaxies
S. R. Valluri, P. A. Wiegert, J. Drozd, M. DaSilva

The logarithmic potential is of great interest and relevance in the study of the dynamics of galaxies. Some small corrections to the work of Contopoulos & Seimenis (1990) who used the method of Prendergast (1982) to find periodic orbits and bifurcations within such a potential are presented. The solution of the orbital radial equation for the purely radial logarithmic potential is then considered using the p-ellipse (precessing ellipse) method pioneered by Struck (2006). This differential orbital equation is a special case of the generalized Burgers equation. The apsidal angle is also determined, both numerically as well as analytically by means of the Lambert W and the Polylogarithm functions. The use of these functions in computing the gravitational lensing produced by logarithmic potentials is discussed.
Art Neuendorffer

User avatar
Chris Peterson
Abominable Snowman
Posts: 18529
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: The Tail of a Christmas Comet (2021 Dec 25)

Post by Chris Peterson » Sat Dec 25, 2021 5:59 pm

neufer wrote: Sat Dec 25, 2021 5:53 pm
Chris Peterson wrote: Sat Dec 25, 2021 2:09 pm
(of course, the comet might still be reasonably seen as in a near Keplerian elliptical orbit around the center of the galaxy).
Keplerian elliptical orbits require a 1/R2 force field.

Milky Way dark matter, however, produces an approximate 1/R force field [= a logarithmic potential]
which balances out the V2/R centrifugal force with an observed radially constant orbital Velocity.
Good point... although the main idea that the comet is in multiple orbits remains the same (a substantially Keplerian orbit, currently elliptical but soon to be hyperbolic, around the Sun as well as an orbit around the galaxy's center of mass that is nearly identical to the Sun's).
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
https://www.cloudbait.com

De58te
Commander
Posts: 584
Joined: Mon Sep 30, 2013 6:35 pm

Re: APOD: The Tail of a Christmas Comet (2021 Dec 25)

Post by De58te » Sat Dec 25, 2021 6:49 pm

Nice comet. Sort of off topic but nobody has mentioned it yet. The James Webb telescope was successfully launched this morning from French Guiana. It will take 12 to 14 days for it to unfurl its solar panels and sun shield so everybody at NASA and some 16 other countries involved are holding their breaths. What a merry Christmas present for astronomers and would be astronomers!

User avatar
neufer
Vacationer at Tralfamadore
Posts: 18805
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: The Tail of a Christmas Comet (2021 Dec 25)

Post by neufer » Sat Dec 25, 2021 7:59 pm

Click to play embedded YouTube video.
De58te wrote: Sat Dec 25, 2021 6:49 pm
Nice comet. Sort of off topic but nobody has mentioned it yet. The James Webb telescope was successfully launched this morning from French Guiana. It will take 12 to 14 days for it to unfurl its solar panels and sun shield so everybody at NASA and some 16 other countries involved are holding their breaths. What a merry Christmas present for astronomers and would be astronomers!
Art Neuendorffer

User avatar
neufer
Vacationer at Tralfamadore
Posts: 18805
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: The Tail of a Christmas Comet (2021 Dec 25)

Post by neufer » Thu Dec 30, 2021 10:28 pm

Art Neuendorffer

User avatar
johnnydeep
Commodore
Posts: 3149
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: The Tail of a Christmas Comet (2021 Dec 25)

Post by johnnydeep » Fri Dec 31, 2021 2:57 pm

neufer wrote: Thu Dec 30, 2021 10:28 pm 160,000 seconds
Some context would have helped:
https://www.syfy.com/syfy-wire/bad-astronomy-comet-heads-are-green-but-not-their-tails-due-to-c2 wrote:This part is cool: With that in hand, they could calculate how long, on average, a molecule of C2 takes to break apart when hit by sunlight if a comet were at the same distance from the Sun as Earth (150 million kilometers). It's actually a very complicated multi-part process to do this, involving at least two photons of light from the Sun plus a couple of more steps where the molecule changes configuration, so it takes time to proceed. What they found is that on average it takes very roughly 160,000 seconds, or two days. That means a lot of the C2 is gone by the time it gets from 50–100,000 km from the comet.
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}