Comments and questions about the
APOD on the main view screen.
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APOD Robot
- Otto Posterman
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Post
by APOD Robot » Fri Jul 17, 2020 4:06 am
NEOWISE of the North
Explanation: After local midnight on July 14 comet NEOWISE was
still above the horizon for Goldenrod, Alberta, Canada, just north of Calgary, planet Earth. In this snapshot it makes for an awesome night with dancing displays of the northern lights.
The long-tailed comet and auroral displays are beautiful apparitions
in the north these days. Both show the influence of spaceweather and the wind
from the Sun. Skygazers have
widely welcomed the visitor from the Oort cloud, though
C/2020 F3 (NEOWISE) is in an orbit that is now taking it out of the inner Solar System.
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orin stepanek
- Plutopian
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by orin stepanek » Fri Jul 17, 2020 11:45 am
DSC1028_PetersNEOWISEAuroralSpike_800.jpg
Neowise days; that's for sure! Pretty!
Went outside early this morning and noticed a
light near the moon; I presumed I was looking
at Venus! Twas a pretty scene also!
You do not have the required permissions to view the files attached to this post.
Orin
Smile today; tomorrow's another day!
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gvann
- Ensign
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by gvann » Fri Jul 17, 2020 1:55 pm
the APOD explanation describes the comet as a "visitor from the Oort cloud"; but, with an aphelion of just 544 AU (inbound) it does not seem like it ever gets anywhere close to the Oort cloud. What's the reason why it's believed to come from the Oort cloud? Anyone knows?
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Chris Peterson
- Abominable Snowman
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Post
by Chris Peterson » Fri Jul 17, 2020 2:15 pm
gvann wrote: ↑Fri Jul 17, 2020 1:55 pm
the APOD explanation describes the comet as a "visitor from the Oort cloud"; but, with an aphelion of just 544 AU (inbound) it does not seem like it ever gets anywhere close to the Oort cloud. What's the reason why it's believed to come from the Oort cloud? Anyone knows?
Conventionally, the assumption is that long period comets originate in the Oort cloud. But there is no way to tell for sure, since repeated interactions with the planets scramble the orbital parameters thoroughly. The high inclination of this comet is suggestive of an origin in a spherical cloud, not a disc, but certainly not definitive, as gravitational perturbations can alter the inclination of an orbiting body as well as the semi-major axis.
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Sa Ji Tario
Post
by Sa Ji Tario » Fri Jul 17, 2020 4:24 pm
Inside the polar aurora appears a pillar of light
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XgeoX
- Science Officer
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- AKA: Uncle Rico
Post
by XgeoX » Fri Jul 17, 2020 4:36 pm
orin stepanek wrote: ↑Fri Jul 17, 2020 11:45 am!
Went outside early this morning and noticed a
light near the moon; I presumed I was looking
at Venus! Twas a pretty scene also!
Indeed it was! Reminded me of this...
Eric
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orin stepanek
- Plutopian
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Post
by orin stepanek » Fri Jul 17, 2020 6:34 pm
XgeoX wrote: ↑Fri Jul 17, 2020 4:36 pm
orin stepanek wrote: ↑Fri Jul 17, 2020 11:45 am!
Went outside early this morning and noticed a
light near the moon; I presumed I was looking
at Venus! Twas a pretty scene also!
Indeed it was! Reminded me of this...
Eric
Actually; more like this:
venus-moon-mercury-Seattle-WA-Dustin-Guy-3-18-2018-e1521457825834.jpg
picture taken from computer images!
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Orin
Smile today; tomorrow's another day!
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BDanielMayfield
- Don't bring me down
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Post
by BDanielMayfield » Fri Jul 17, 2020 6:43 pm
Chris Peterson wrote: ↑Fri Jul 17, 2020 2:15 pm
gvann wrote: ↑Fri Jul 17, 2020 1:55 pm
the APOD explanation describes the comet as a "visitor from the Oort cloud"; but, with an aphelion of just 544 AU (inbound) it does not seem like it ever gets anywhere close to the Oort cloud. What's the reason why it's believed to come from the Oort cloud? Anyone knows?
Conventionally, the assumption is that long period comets originate in the Oort cloud. But there is no way to tell for sure, since repeated interactions with the planets scramble the orbital parameters thoroughly. The high inclination of this comet is suggestive of an origin in a spherical cloud, not a disc, but certainly not definitive, as gravitational perturbations can alter the inclination of an orbiting body as well as the semi-major axis.
A little more info on this, from wikipedia's comet article:
Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc[92] —a disk of objects in the trans-Neptunian region—whereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesized its existence).[93] Vast swarms of comet-like bodies are thought to orbit the Sun in these distant regions in roughly circular orbits. Occasionally the gravitational influence of the outer planets (in the case of Kuiper belt objects) or nearby stars (in the case of Oort cloud objects) may throw one of these bodies into an elliptical orbit that takes it inwards toward the Sun to form a visible comet.
Just as zero is not equal to infinity, everything coming from nothing is illogical.
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heehaw
Post
by heehaw » Fri Jul 17, 2020 10:15 pm
It has always baffled me how it can possibly be that comets, coming from FAR out in the solar system, can possibly manage (so many of them!) to get so DAMED close to the Sun! (Is it just that we miss most of those that don't?).
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Chris Peterson
- Abominable Snowman
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Post
by Chris Peterson » Fri Jul 17, 2020 10:54 pm
heehaw wrote: ↑Fri Jul 17, 2020 10:15 pm
It has always baffled me how it can possibly be that comets, coming from FAR out in the solar system, can possibly manage (so many of them!) to get so DAMED close to the Sun! (Is it just that we miss most of those that don't?).
What is "close"?
A distant object is barely bound to the Sun; it takes very little energy to knock it into a high eccentricity orbit, which means it gets within a few AU of the Sun. Of course, it's also true that most comets that are outside the inner system go unnoticed because they aren't bright enough to be recorded.
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neufer
- Vacationer at Tralfamadore
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Post
by neufer » Sat Jul 18, 2020 9:48 pm
Chris Peterson wrote: ↑Fri Jul 17, 2020 10:54 pm
heehaw wrote: ↑Fri Jul 17, 2020 10:15 pm
It has always baffled me how it can possibly be that comets, coming from FAR out in the solar system, can possibly manage (so many of them!) to get so DAMED close to the Sun! (Is it just that we miss most of those that don't?).
What is "close"?
A distant object is barely bound to the Sun; it takes very little energy to knock it into a high eccentricity orbit, which means it gets within a few AU of the Sun. Of course, it's also true that most comets that are outside the inner system go unnoticed because they aren't bright enough to be recorded.
https://en.wikipedia.org/wiki/Oort_cloud wrote:
<<The Oort cloud is a theoretical cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 2,000 to 200,000 AU.>>
- Angular momentum grows as the square root of orbital radius.
Angular momentum of a parabolic orbit is sqrt(2) times that of a corresponding circular orbit.
For an Oort cloud planetesimal in a circular orbit at 20,000 AU to drop down to a perihelion of 100 AU it must lose 90% of its angular momentum.
For an Oort cloud planetesimal in a circular orbit at 20,000 AU to drop down to a perihelion of 1 AU it must lose 99% of its angular momentum.
While there should, indeed, be ~10 times (= 10%/1%) as many Oort cloud planetesimal that drop down to a perihelion of 100 AU (or less) than drop down to 1 AU (or less) they must fill a volume a million times greater.
Hence, the density of visible comets with small perihelions is abnormally enhanced over expectations.
Art Neuendorffer