Starship Asterisk*

Boomer12k wrote: ↑Tue Apr 07, 2020 6:09 am Wonderful shot... but now I am dizzy... and have a strange feeling to..."obey"....

:---[===] *

RC Davison wrote: ↑Tue Apr 07, 2020 12:48 pm Wonderful picture!

Unless I’m missing something I don’t think this statement is quite true:
“This spin-pole-of-the-north will never move from its fixed location on the sky“
With the precession of Earth, the direction of the celestial pole will drift over a period of 26,000 years.

Chris Peterson wrote: ↑Tue Apr 07, 2020 1:15 pm

RC Davison wrote: ↑Tue Apr 07, 2020 12:48 pm
Unless I’m missing something I don’t think this statement is quite true:

“This spin-pole-of-the-north will never move from its fixed location on the sky“

With the precession of Earth, the direction of the celestial pole will drift over a period of 26,000 years.

Well, the "spin pole" will never move from the frame of an observer on Earth. What will move is the particular background of stars relative to that pole. But I actually read the statement to mean it wouldn't move as you traveled, other than shifting to reflect your latitude.

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

Weebles wobble, but they don't fall down

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<<The Chandler wobble or variation of latitude is a small deviation in the Earth's axis of rotation relative to the solid earth, which was discovered by American astronomer Seth Carlo Chandler in 1891. It amounts to change of about 9 metres in the point at which the axis intersects the Earth's surface and has a period of 433 days. This wobble, which is a nutation, combines with another wobble with a period of one year, so that the total polar motion varies with a period of about 7 years.

The Chandler wobble is an example of the kind of motion that can occur for a spinning object that is not a sphere; this is called a free nutation. Somewhat confusingly, the direction of the Earth's spin axis relative to the stars also varies with different periods, and these motions—caused by the tidal forces of the Moon and Sun—are also called nutations, except for the slowest, which are precessions of the equinoxes.

The existence of Earth's free nutation was predicted by Isaac Newton in Corollaries 20 to 22 of Proposition 66, Book 1 of the Philosophiæ Naturalis Principia Mathematica, and by Leonhard Euler in 1765 as part of his studies of the dynamics of rotating bodies. Based on the known ellipticity of the Earth, Euler predicted that it would have a period of 305 days. Several astronomers searched for motions with this period, but none was found. Chandler's contribution was to look for motions at any possible period; once the Chandler wobble was observed, the difference between its period and the one predicted by Euler was explained by Simon Newcomb as being caused by the non-rigidity of the Earth. The full explanation for the period also involves the fluid nature of the Earth's core and oceans—the wobble, in fact, produces a very small ocean tide with an amplitude of approximately 6 mm, called a "pole tide", which is the only tide not caused by an extraterrestrial body. Despite the small amplitude, the gravitational effect of the pole tide is easily detected by the superconducting gravimeter.

The International Latitude Observatories were established in 1899 to measure the wobble. These provided data on the Chandler and annual wobble for most of the 20th century, though they were eventually superseded by other methods of measurement. Monitoring of the polar motion is now done by the International Earth Rotation Service.

The wobble's amplitude has varied since its discovery, reaching its largest size in 1910 and fluctuating noticeably from one decade to another. In 2009, Malkin & Miller's analysis of International Earth Rotation and Reference Systems Service (IERS) Pole coordinates time series data from January 1946 to January 2009 showed three phase reversals of the wobble, in 1850, 1920, and 2005.

Since the Chandler wobble should die down in a matter of decades or centuries, there must be influences that continually re-excite it. While it must be due to changes in the mass distribution or angular momentum of the Earth's outer core, atmosphere, oceans, or crust (from earthquakes), for a long time the actual source was unclear, since no available motions seemed to be coherent with what was driving the wobble.

An investigation was done in 2001 by Richard Gross at the Jet Propulsion Laboratory managed by the California Institute of Technology. He used angular momentum models of the atmosphere and the oceans in computer simulations to show that from 1985 to 1996, the Chandler wobble was excited by a combination of atmospheric and oceanic processes, with the dominant excitation mechanism being ocean‐bottom pressure fluctuations. Gross found that two-thirds of the "wobble" was caused by fluctuating pressure on the seabed, which, in turn, is caused by changes in the circulation of the oceans caused by variations in temperature, salinity, and wind. The remaining third is due to atmospheric fluctuations.>>

neufer wrote: ↑Tue Apr 07, 2020 2:34 pm

Chris Peterson wrote: ↑Tue Apr 07, 2020 1:15 pm

RC Davison wrote: ↑Tue Apr 07, 2020 12:48 pm
Unless I’m missing something I don’t think this statement is quite true:

“This spin-pole-of-the-north will never move from its fixed location on the sky“

With the precession of Earth, the direction of the celestial pole will drift over a period of 26,000 years.

Well, the "spin pole" will never move from the frame of an observer on Earth. What will move is the particular background of stars relative to that pole. But I actually read the statement to mean it wouldn't move as you traveled, other than shifting to reflect your latitude.

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

Weebles wobble, but they don't fall down

---

<<The Chandler wobble or variation of latitude is a small deviation in the Earth's axis of rotation relative to the solid earth, which was discovered by American astronomer Seth Carlo Chandler in 1891. It amounts to change of about 9 metres in the point at which the axis intersects the Earth's surface and has a period of 433 days. This wobble, which is a nutation, combines with another wobble with a period of one year, so that the total polar motion varies with a period of about 7 years.

The Chandler wobble is an example of the kind of motion that can occur for a spinning object that is not a sphere; this is called a free nutation. Somewhat confusingly, the direction of the Earth's spin axis relative to the stars also varies with different periods, and these motions—caused by the tidal forces of the Moon and Sun—are also called nutations, except for the slowest, which are precessions of the equinoxes.

The existence of Earth's free nutation was predicted by Isaac Newton in Corollaries 20 to 22 of Proposition 66, Book 1 of the Philosophiæ Naturalis Principia Mathematica, and by Leonhard Euler in 1765 as part of his studies of the dynamics of rotating bodies. Based on the known ellipticity of the Earth, Euler predicted that it would have a period of 305 days. Several astronomers searched for motions with this period, but none was found. Chandler's contribution was to look for motions at any possible period; once the Chandler wobble was observed, the difference between its period and the one predicted by Euler was explained by Simon Newcomb as being caused by the non-rigidity of the Earth. The full explanation for the period also involves the fluid nature of the Earth's core and oceans—the wobble, in fact, produces a very small ocean tide with an amplitude of approximately 6 mm, called a "pole tide", which is the only tide not caused by an extraterrestrial body. Despite the small amplitude, the gravitational effect of the pole tide is easily detected by the superconducting gravimeter.

The International Latitude Observatories were established in 1899 to measure the wobble. These provided data on the Chandler and annual wobble for most of the 20th century, though they were eventually superseded by other methods of measurement. Monitoring of the polar motion is now done by the International Earth Rotation Service.

The wobble's amplitude has varied since its discovery, reaching its largest size in 1910 and fluctuating noticeably from one decade to another. In 2009, Malkin & Miller's analysis of International Earth Rotation and Reference Systems Service (IERS) Pole coordinates time series data from January 1946 to January 2009 showed three phase reversals of the wobble, in 1850, 1920, and 2005.

Since the Chandler wobble should die down in a matter of decades or centuries, there must be influences that continually re-excite it. While it must be due to changes in the mass distribution or angular momentum of the Earth's outer core, atmosphere, oceans, or crust (from earthquakes), for a long time the actual source was unclear, since no available motions seemed to be coherent with what was driving the wobble.

An investigation was done in 2001 by Richard Gross at the Jet Propulsion Laboratory managed by the California Institute of Technology. He used angular momentum models of the atmosphere and the oceans in computer simulations to show that from 1985 to 1996, the Chandler wobble was excited by a combination of atmospheric and oceanic processes, with the dominant excitation mechanism being ocean‐bottom pressure fluctuations. Gross found that two-thirds of the "wobble" was caused by fluctuating pressure on the seabed, which, in turn, is caused by changes in the circulation of the oceans caused by variations in temperature, salinity, and wind. The remaining third is due to atmospheric fluctuations.>>

APOD Robot wrote: ↑Tue Apr 07, 2020 4:06 am
Explanation: The bright path was created by the astrophotographer's headlamp as he zigzagged up a hill just over a week ago in Lower Saxony, Germany. The astrophotographer can be seen, at times, in shadow.[/url].

MarkBour wrote: ↑Tue Apr 07, 2020 5:37 pm
The caption suggests what might make an interesting variation on these images. If one could, over the course of one night, get the camera to move a significant amount northward, say 10 degrees north, all the while otherwise keeping a steady frame, then the time lapse should show Polaris rise by 10 degrees and all of the other stars taking an apparent path that might be elliptical. (I'll think about it, I don't know what shape, but it would certainly elongate some of the arcs.)

neufer wrote: ↑Tue Apr 07, 2020 6:52 pm

A prolate trochoid

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MarkBour wrote: ↑Tue Apr 07, 2020 5:37 pm
The caption suggests what might make an interesting variation on these images. If one could, over the course of one night, get the camera to move a significant amount northward, say 10 degrees north, all the while otherwise keeping a steady frame, then the time lapse should show Polaris rise by 10 degrees and all of the other stars taking an apparent path that might be elliptical. (I'll think about it, I don't know what shape, but it would certainly elongate some of the arcs.)

De58te wrote: ↑Tue Apr 07, 2020 5:48 pm That's a beautiful picture. My compliments to the photographer. I especially like the zigzag lines you made. This explanation though got me to thinking. If the camera was truly at the north pole, then the circular pattern would be overhead, and now that the season is spring the midnight sun would be on the horizon? So what would that look like? The colourful stars making half a circle from the top frame and the horizon would have dozens of red or yellow sunrises running across the horizon?

TheZuke! wrote: ↑Tue Apr 07, 2020 8:13 pm

De58te wrote: ↑Tue Apr 07, 2020 5:48 pm
This explanation though got me to thinking. If the camera was truly at the north pole, then the circular pattern would be overhead, and now that the season is spring the midnight sun would be on the horizon? So what would that look like? The colourful stars making half a circle from the top frame and the horizon would have dozens of red or yellow sunrises running across the horizon?

I'm not sure if I understand your comment properly, but I have seen a video of a sun tracking camera following the course of The Sun slightly above the horizon for a 24 hour period. As The Sun was "up", I don't know if any (other) stars were visible.

neufer wrote: ↑Tue Apr 07, 2020 6:52 pm A prolate trochoid ...

MarkBour wrote: ↑Mon Apr 13, 2020 5:54 am
Which stars would just stop exactly as Polaris passed them to their East? Whatever that threshold is, stars that are "closer" to Polaris than that should be tracing a curtate trochoid, and those that are "farther" from Polaris than that will trace a prolate trochoid.