APOD: A Path North (2020 Apr 07)

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APOD: A Path North (2020 Apr 07)

Post by APOD Robot » Tue Apr 07, 2020 4:06 am

Image A Path North

Explanation: What happens if you keep going north? The direction north on the Earth, the place on your horizon below the northern spin pole of the Earth -- around which other stars appear to slowly swirl, will remain the same. This spin-pole-of-the-north will never move from its fixed location on the sky -- night or day -- and its height will always match your latitude. The further north you go, the higher the north spin pole will appear. Eventually, if you can reach the Earth's North Pole, the stars will circle a point directly over your head. Pictured, a four-hour long stack of images shows stars trailing in circles around this north celestial pole. The bright star near the north celestial pole is Polaris, known as the North Star. 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. Actually, the Earth has two spin poles -- and much the same would happen if you started below the Earth's equator and went south.

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Re: APOD: A Path North (2020 Apr 07)

Post by Boomer12k » Tue Apr 07, 2020 6:09 am

Wonderful shot... but now I am dizzy... and have a strange feeling to..."obey"....

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Re: APOD: A Path North (2020 Apr 07)

Post by Ann » Tue Apr 07, 2020 6:29 am

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

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Re: APOD: A Path North (2020 Apr 07)

Post by heehaw » Tue Apr 07, 2020 9:58 am

Boomer beat me to it...

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Re: APOD: A Path North (2020 Apr 07)

Post by Indigo_Sunrise » Tue Apr 07, 2020 10:44 am

The colors of the star trails are pretty amazing! 8-)
Forget the box, just get outside.

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Re: APOD: A Path North (2020 Apr 07)

Post by orin stepanek » Tue Apr 07, 2020 12:08 pm

I feel like I'm at the carnival! :mrgreen:
PathNorth_Konang_960.jpg
Round and round we go; where we stop,
nobody know! :lol2:
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Re: APOD: A Path North (2020 Apr 07)

Post by Tszabeau » Tue Apr 07, 2020 12:40 pm

Zoom-in on the horizon, to the right of the clump of trees. There’s a bright orange “snake” running along the fence line. Perhaps a partial glimpse of the Worm Orouborous?

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Re: APOD: A Path North (2020 Apr 07)

Post by RC Davison » 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.

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Re: APOD: A Path North (2020 Apr 07)

Post by Chris Peterson » Tue Apr 07, 2020 1:15 pm

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.
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.
Chris

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Re: APOD: A Path North (2020 Apr 07)

Post by neufer » 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:
<<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.>>
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Re: APOD: A Path North (2020 Apr 07)

Post by Chris Peterson » Tue Apr 07, 2020 2:44 pm

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:
<<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.>>
And, of course, if we look closely enough, it goes beyond not being a perfect sphere. Earth is also not a perfectly rigid body, and it has density variations throughout it which are constantly shifting. All of which go towards making the spin axis somewhat variable. And all of which are insignificant in the context of a image like today's.
Chris

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Re: APOD: A Path North (2020 Apr 07)

Post by neufer » Tue Apr 07, 2020 3:01 pm

Image
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].
A desperate search for a place to relieve
oneself after one too many Jägermeisters :?:
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Re: APOD: A Path North (2020 Apr 07)

Post by MarkBour » 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.)

capture.gif
To have a steady pace that would get 10 degrees north in about 6 hours or so, you'd have to be going north at about 1111 km/ 6 hr, or 185 km/hr. I guess this could be done in a fast car ... but as we must often caution our readers of APOD, it is not recommended that you do any astrophotography while driving. :-)
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Re: APOD: A Path North (2020 Apr 07)

Post by De58te » 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?

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A Prolate Trochoid

Post by neufer » Tue Apr 07, 2020 6:52 pm

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.)
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Re: A Prolate Trochoid

Post by TheZuke! » Tue Apr 07, 2020 8:10 pm

neufer wrote:
Tue Apr 07, 2020 6:52 pm
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,
I've seen a similar graphic, only it "explained" that part of a railroad locomotive must "go backward" in order for the locomotive to move forward.
i.e. the wheel flange that is below the surface of the rail is travelling "backward". Without the flange, the train would not stay on the track.
Though, I suppose, instead of the train moving on a rail, it could have wheels that travel in a trough...

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Re: APOD: A Path North (2020 Apr 07)

Post by TheZuke! » Tue Apr 07, 2020 8:13 pm

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?
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.

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Re: APOD: A Path North (2020 Apr 07)

Post by neufer » Tue Apr 07, 2020 10:58 pm

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.
There might be an eclipse for a while.

https://apod.nasa.gov/apod/ap120620.html
https://apod.nasa.gov/apod/ap170730.html
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Re: A Prolate Trochoid

Post by MarkBour » Mon Apr 13, 2020 5:54 am

neufer wrote:
Tue Apr 07, 2020 6:52 pm
A prolate trochoid ...
Of course you're correct. After I thought about it a little longer, I had that each star would be tracing a trochoid over a 24-hour period if this could be sustained. If we progress 10 degrees North in 6 hours, Polaris is rising 1 2/3 degrees per hour in the sky. Here's a point I'm pretty unsure of so far: 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. (Closer and farther here meaning apparent angular separation on the dome of the sky.) The threshold is that angular distance such that the rate that Polaris is moving in 24 hours equals one full revolution. So, if we had Polaris moving 1.6667 degrees per hour, that's 40 degrees in 24 hours, so 40/2*pi = about 7 degrees.

But we wouldn't see good examples of trochoids in the image I'd capture. Each star would only trace out 6 hours of progress and (unlike in the standard fixed-location star tracks), where the end of one trail meets the beginning of another trail, those would be two rather different trails. I don't know when I'll ever have the time to find a way to simulate it and plot it out. I wonder if something like Mathematica or GNU Octave would make it really easy to do.

I don't think this would ever be imaged from a highway. And I don't know of any ship that can go as fast as prescribed. A train would be nice ... most long-distance train tracks run East-West, which would be totally boring and just give faster or slower circular star trails. To go North-South a long distance by train, one might try the City of New Orleans, which gets up to 160 km/hr some times. An airplane would be ideal, too.
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Re: A Prolate Trochoid

Post by neufer » Mon Apr 13, 2020 8:30 pm

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.
Of course Polaris [declination = +89° 15′ 50.8″ ] is not at the true pole.

For Polaris, itself, to move cycloidally one must sail north (or south) at ~11.57 knots.

For Sigma Octantis (Polaris Australis) to move cycloidally one must sail north (or south) at ~16.42 knots.
Art Neuendorffer