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

[APOD Robot]

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.

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[David G]

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?

[orin stepanek]

Merry Christmas Everyone! ‍

[Chris Peterson]

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).

[neufer]

(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.

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.

[Chris Peterson]

(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).

[De58te]

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!

[neufer]

Click to play embedded YouTube video.

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!

• http://asterisk.apod.com/viewtopic.php? ... 50#p319272

[neufer]

160,000 seconds

[johnnydeep]

160,000 seconds

Some context would have helped:

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.