Starship Asterisk*

Chris Peterson wrote: ↑Sat Jul 18, 2020 3:21 pm

johnnydeep wrote: ↑Sat Jul 18, 2020 3:04 pm

APOD Robot wrote: ↑Sat Jul 18, 2020 4:08 am Finding NEOWISE

Explanation: If you can see the stars of the Big Dipper, you can find comet NEOWISE in your evening sky tonight. After sunset look for the naked-eye comet below the bowl of the famous celestial kitchen utensil of the north and above your northwestern horizon. You're looking for a fuzzy 'star' with a tail, though probably not so long a tail as in this clear sky snapshot taken from Los Padres National Forest in California on the evening of July 16. Recent photographs of C/2020 F3 (NEOWISE) often show this comet's broad dust tail and fainter but separate ion tail extending farther than the eye can follow. Skygazers around the world have been delighted to find NEOWISE, surprise visitor from the outer Solar System.

Two questions:

1. Is the statement in blue necessarily true? I would think it would depend on how high the big dipper is in the sky. If it's not high enough, the comet will still be below the horizon, right? That is, whether you can see the comet or not still depends on your latitude.

2. The stars of the big dipper look enhanced to me. Are they? The image details didn't say. They are never that glaring to the eye that I've ever seen.

Statement 1 is broadly accurate. The comet is currently at a declination of +48°, which is less than that of the Big Dipper. As you travel south, the Dipper will disappear over the horizon before the comet. But certainly, the fact that they have different right ascensions also means they have different rising and setting times, so depending on the time the Dipper may be visible and the comet not. What the statement really means is that if you can see the Dipper in your sky, you can also see the comet... but not necessarily at the same time.

And yes, the stars of the Dipper have clearly been enhanced.

Chris Peterson wrote: ↑Sat Jul 18, 2020 9:21 pm

neufer wrote: ↑Sat Jul 18, 2020 9:17 pm

Chris Peterson wrote: ↑Sat Jul 18, 2020 3:03 pm
The material ejected from a comet does not follow the same orbit as the nucleus. It follows a similar orbit, which becomes less similar over time.

Gravity drops off inversely with solar radial distance squared.

Radiation pressure also drops off inversely with solar radial distance squared.

Ergo:
similar particles will travel on elliptical orbits that (roughly) match up with the initial nucleus velocity at their time of ejection. (Solar wind is negligible for the dust tail.)

Oh no, they don't. They move outward and they slow down.

I've run enough million particle models of comet ejecta to have a very good idea of how those particles behave.

Chris Peterson wrote: ↑Sat Jul 18, 2020 9:21 pm

neufer wrote: ↑Sat Jul 18, 2020 9:17 pm
(Solar wind is negligible for the dust tail.)

Nor is solar wind neglible- it certainly can't be ignored in models if you want to know where material ends up- especially after a few orbits of short period comets, the ones that are responsible for meteor showers.

https://en.wikipedia.org/wiki/Electric_sail wrote:

<<The electric solar wind sail gets its momentum from the solar wind ions,
whilst a photonic sail is propelled by photons.

Thus, the available pressure is only about 1% of photon pressure.>>

De58te wrote: ↑Sat Jul 18, 2020 11:27 pm I can never be a real astronomer since I can't get my head around some concepts. Many sites have said that Neowise has passed the sun and is now heading out of the solar System. They also say Neowise is approaching Earth and will be the closest on July 23. Then it will pass Earth by. Now I was taught in school that the comet tail always points away from the Sun regardless of its true direction. So if it is approaching Earth, shouldn't the tail actually be pointing more or less towards Earth until July 24? Why is the tail in the photo at 90 degrees to Earth?

neufer wrote: ↑Sat Jul 18, 2020 9:17 pm

Chris Peterson wrote: ↑Sat Jul 18, 2020 3:03 pm

Sa Ji Tario wrote: ↑Sat Jul 18, 2020 2:58 pm
Each particle that belongs to the body of the comet retains the movement, if it divides, all the pieces would go the same way if there were no other force to modify it. In the case of comet dust, this is the arrangement of the nucleus by solar heat and UV radiation, once outside the body, the solar wind slows it down and is slightly in orbit, but it follows the same path as the nucleus. As it rotates, the front jets are faster than the side jets and the rear jets are the slowest, so extensive flow forms and all of the lost material remains in the same orbit through which the core passed , if the orbit is curved the flow also. -
When the Earth crosses it, it generates the "star showers"

The material ejected from a comet does not follow the same orbit as the nucleus. It follows a similar orbit, which becomes less similar over time.

Gravity drops off inversely with solar radial distance squared.

Radiation pressure also drops off inversely with solar radial distance squared.

Ergo:
similar particles will travel on elliptical orbits that (roughly) match up with the initial nucleus velocity at their time of ejection.
(Solar wind is negligible for the dust tail.)

Chris Peterson wrote: ↑Sat Jul 18, 2020 3:03 pm

Sa Ji Tario wrote: ↑Sat Jul 18, 2020 2:58 pm
Each particle that belongs to the body of the comet retains the movement, if it divides, all the pieces would go the same way if there were no other force to modify it. In the case of comet dust, this is the arrangement of the nucleus by solar heat and UV radiation, once outside the body, the solar wind slows it down and is slightly in orbit, but it follows the same path as the nucleus. As it rotates, the front jets are faster than the side jets and the rear jets are the slowest, so extensive flow forms and all of the lost material remains in the same orbit through which the core passed , if the orbit is curved the flow also. -
When the Earth crosses it, it generates the "star showers"

The material ejected from a comet does not follow the same orbit as the nucleus. It follows a similar orbit, which becomes less similar over time.

Chris Peterson wrote: ↑Sat Jul 18, 2020 3:21 pm

johnnydeep wrote: ↑Sat Jul 18, 2020 3:04 pm

APOD Robot wrote: ↑Sat Jul 18, 2020 4:08 am Finding NEOWISE

Explanation: If you can see the stars of the Big Dipper, you can find comet NEOWISE in your evening sky tonight. After sunset look for the naked-eye comet below the bowl of the famous celestial kitchen utensil of the north and above your northwestern horizon. You're looking for a fuzzy 'star' with a tail, though probably not so long a tail as in this clear sky snapshot taken from Los Padres National Forest in California on the evening of July 16. Recent photographs of C/2020 F3 (NEOWISE) often show this comet's broad dust tail and fainter but separate ion tail extending farther than the eye can follow. Skygazers around the world have been delighted to find NEOWISE, surprise visitor from the outer Solar System.

Two questions:

1. Is the statement in blue necessarily true? I would think it would depend on how high the big dipper is in the sky. If it's not high enough, the comet will still be below the horizon, right? That is, whether you can see the comet or not still depends on your latitude.

2. The stars of the big dipper look enhanced to me. Are they? The image details didn't say. They are never that glaring to the eye that I've ever seen.

Statement 1 is broadly accurate. The comet is currently at a declination of +48°, which is less than that of the Big Dipper. As you travel south, the Dipper will disappear over the horizon before the comet. But certainly, the fact that they have different right ascensions also means they have different rising and setting times, so depending on the time the Dipper may be visible and the comet not. What the statement really means is that if you can see the Dipper in your sky, you can also see the comet... but not necessarily at the same time.

And yes, the stars of the Dipper have clearly been enhanced.

johnnydeep wrote: ↑Sat Jul 18, 2020 3:04 pm

APOD Robot wrote: ↑Sat Jul 18, 2020 4:08 am Finding NEOWISE

Explanation: If you can see the stars of the Big Dipper, you can find comet NEOWISE in your evening sky tonight. After sunset look for the naked-eye comet below the bowl of the famous celestial kitchen utensil of the north and above your northwestern horizon. You're looking for a fuzzy 'star' with a tail, though probably not so long a tail as in this clear sky snapshot taken from Los Padres National Forest in California on the evening of July 16. Recent photographs of C/2020 F3 (NEOWISE) often show this comet's broad dust tail and fainter but separate ion tail extending farther than the eye can follow. Skygazers around the world have been delighted to find NEOWISE, surprise visitor from the outer Solar System.

Two questions:

1. Is the statement in blue necessarily true? I would think it would depend on how high the big dipper is in the sky. If it's not high enough, the comet will still be below the horizon, right? That is, whether you can see the comet or not still depends on your latitude.

2. The stars of the big dipper look enhanced to me. Are they? The image details didn't say. They are never that glaring to the eye that I've ever seen.

APOD Robot wrote: ↑Sat Jul 18, 2020 4:08 am Finding NEOWISE

Explanation: If you can see the stars of the Big Dipper, you can find comet NEOWISE in your evening sky tonight. After sunset look for the naked-eye comet below the bowl of the famous celestial kitchen utensil of the north and above your northwestern horizon. You're looking for a fuzzy 'star' with a tail, though probably not so long a tail as in this clear sky snapshot taken from Los Padres National Forest in California on the evening of July 16. Recent photographs of C/2020 F3 (NEOWISE) often show this comet's broad dust tail and fainter but separate ion tail extending farther than the eye can follow. Skygazers around the world have been delighted to find NEOWISE, surprise visitor from the outer Solar System.

Sa Ji Tario wrote: ↑Sat Jul 18, 2020 2:58 pm Each particle that belongs to the body of the comet retains the movement, if it divides, all the pieces would go the same way if there were no other force to modify it. In the case of comet dust, this is the arrangement of the nucleus by solar heat and UV radiation, once outside the body, the solar wind slows it down and is slightly in orbit, but it follows the same path as the nucleus. As it rotates, the front jets are faster than the side jets and the rear jets are the slowest, so extensive flow forms and all of the lost material remains in the same orbit through which the core passed , if the orbit is curved the flow also. -
When the Earth crosses it, it generates the "star showers"

Chris Peterson wrote: ↑Sat Jul 18, 2020 2:04 pm

Robot wrote: ↑Sat Jul 18, 2020 1:50 pm
What causes the apparent curvature of the dust trail?

The dust trail is curved. As particles move outward from the comet's orbit, they orbit slower (basic orbital mechanics). So the tail curves.

Robot wrote: ↑Sat Jul 18, 2020 1:50 pm What causes the apparent curvature of the dust trail?