APOD: The Crown of the Sun (2017 Aug 23)

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APOD: The Crown of the Sun (2017 Aug 23)

Postby APOD Robot » Wed Aug 23, 2017 4:38 am

[img]https://apod.nasa.gov/apod/calendar/S_170823.jpg[/img] The Crown of the Sun

Explanation: During a total solar eclipse, the Sun's extensive outer atmosphere, or corona, is an inspirational sight. Streamers and shimmering features visible to the eye span a brightness range of over 10,000 to 1, making them notoriously difficult to capture in a single photograph. But this composite of telescopic images covers a wide range of exposure times to reveal the crown of the Sun in all its glory. The aligned and stacked digital frames were taken in clear skies above Stanley, Idaho in the Sawtooth Mountains during the Sun's total eclipse on August 21. A pinkish solar prominence extends just beyond the right edge of the solar disk. Even small details on the dark night side of the New Moon can be made out, illuminated by sunlight reflected from a Full Earth.

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heehaw

Re: APOD: The Crown of the Sun (2017 Aug 23)

Postby heehaw » Wed Aug 23, 2017 9:52 am

Today's ESPOD is excellent re eclipses: http://epod.usra.edu/blog/2007/08/2000- ... ipses.html

Steven I Dutch

Re: APOD: The Crown of the Sun (2017 Aug 23)

Postby Steven I Dutch » Wed Aug 23, 2017 2:31 pm

I saw it from Chester, Illinois and this is a perfect reflection of the actual appearance. By far the most intricate corona of the four I've seen (1970, 1972 and 1979). Small cumulus clouds made for the most dramatic eclipse sky I've seen. The sky seemed lighter than previous eclipses.

tomatoherd

Re: APOD: The Crown of the Sun (2017 Aug 23)

Postby tomatoherd » Wed Aug 23, 2017 2:44 pm

Was doing the ratios myself (sun:moon diameters, sun:moon distance from earth), playing with the coincidence of how we, of all systems, have total, but barely total eclipses, and noticed online that the polar and equatorial diameters of the sun differ by only 6 miles!
So..why is that? Doesn't the sun have a pretty good rotational speed/clip? Why no more flattening than that???

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Re: APOD: The Crown of the Sun (2017 Aug 23)

Postby neufer » Wed Aug 23, 2017 3:37 pm

tomatoherd wrote:
Was doing the ratios myself (sun:moon diameters, sun:moon distance from earth), playing with the coincidence of how we, of all systems, have total, but barely total eclipses, and noticed online that the polar and equatorial diameters of the sun differ by only 6 miles!

So..why is that? Doesn't the sun have a pretty good rotational speed/clip? Why no more flattening than that???

Because centrifugal forces scale inversely with the SQUARE of the rotational period:
-----------------------------------------------------------------------------------------------------
Jupiter rotates every 9 h 55 m and has a flattening of 64,870 x 10-6.

If Jupiter were to rotate only every 35 days it should have a flattening of ~9 x 10-6.

The Sun has a flattening of ~9 x 10-6.

Ergo, one should expect the sun to rotate only once every 35 days.

However, the Sun rotates once every 34.4 days at the poles and once every 25.05 days at the equator.
-----------------------------------------------------------------------------------------------------
The Earth rotates every 0.99727 days and has a flattening of 3,353 x 10-6.

If Earth were to rotate only every 243 days it should have a flattening of ~0.0565 x 10-6.

Venus rotates once every 243.025 days and has a flattening of ~0 within the accuracy of measurement.
-----------------------------------------------------------------------------------------------------
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Re: APOD: The Crown of the Sun (2017 Aug 23)

Postby MarkBour » Wed Aug 23, 2017 4:18 pm

capture.jpg
A beautiful image of the corona. Because of the scale of the effect here, I assume the material in the corona does not change shape very fast. This would lead one to expect that the corona (if properly observable) would have looked pretty much the same to all observers across the path of the eclipse. Although they look like wisps of flame to us, these are millions of km long, and the material can't change all that rapidly. On the other hand, what we actually see of the corona is not just the material, but the sunlight reflecting from and the excitations in the material of the corona. For all I know, conditions could cause that to change quite rapidly, so perhaps what the corona looks like can change significantly over the course of an hour or two. Anyway, every high-quality rendering of the corona I have seen from yesterday's eclipse has roughly this same shape (it looks a lot like the starfleet communicators on the Next-gen Star Trek uniforms, turned sideways).
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Re: APOD: The Crown of the Sun (2017 Aug 23)

Postby neufer » Wed Aug 23, 2017 4:56 pm

MarkBour wrote:
Because of the scale of the effect here, I assume the material in the corona does not change shape very fast. This would lead one to expect that the corona (if properly observable) would have looked pretty much the same to all observers across the path of the eclipse.

Two of large scale projects to observe the ~90 minutes of totality across the U.S. (via multiple telescopes on the ground and from balloons) were designed to observe coronal changes over that time period (specifically to look for changing features that might be responsible for the heating of the corona).
https://en.wikipedia.org/wiki/Corona#Va ... the_corona wrote:
A portrait as diversified as the one already pointed out for the coronal features is emphasized by the analysis of the dynamics of the main structures of the corona, which evolve in times very different among them. Studying the coronal variability in its complexity is not easy because the times of evolution of the different structures can vary considerably: from seconds to several months. The typical sizes of the regions where coronal events take place vary in the same way, as it is shown in the following table.

Code: Select all

Coronal event                         Typical time-scale    Typical length-scale (Mm)
-------------------------------------------------------------------------------------------
Active region flare                   10 to 10,000 seconds       10–100
X-ray bright point                         minutes                 1–10
Transient in large-scale structures   from minutes to hours        ~100
Transient in interconnecting arcs     from minutes to hours        ~100
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Re: APOD: The Crown of the Sun (2017 Aug 23)

Postby De58te » Wed Aug 23, 2017 7:51 pm

Looking forward to tomorrow's APOD. Pig cells in Space as my Windows Narrator pronounced. I hope most of you can remember the Muppet Show's Pigggs in Spaaaccccccce. This must be a spinoff cell from the Cosmoporkers, Firstmate Miss Piggy and the handsome Captain Link Hogthrob, and the Doctor Julius Strangepork. Join the adventures of the Swine Trek on a five year mission where no little piggy has gone before.

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Re: APOD: The Crown of the Sun (2017 Aug 23)

Postby JohnD » Wed Aug 23, 2017 9:57 pm

tomatoherd wrote:Was doing the ratios myself (sun:moon diameters, sun:moon distance from earth), playing with the coincidence of how we, of all systems, have total, but barely total eclipses, and noticed online that the polar and equatorial diameters of the sun differ by only 6 miles!
So..why is that? Doesn't the sun have a pretty good rotational speed/clip? Why no more flattening than that???


No coincidence, tomatoherd, just that you are alive today, which along with 7.5 billions others on Earth and 400 million in the US makes it fairly good odds that somone will be here to see it. And the Moon isn't where it always was. As it became tidally bound to the Earth it moved out, and the tidal stresses are still moving it out as the Earth becomes tidally locked to the Moon. Eventually it will be twice as far away and there will be no total eclipses.

Also, the Moons orbit is not circular, but elliptical as are all orbits, and this makes the Moon appear to vary in diameter by 14%. Some totals show "Bailey's Beads" as the Sun shines through valleys on the Moon's circumference, this one didn't.

John

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Re: APOD: The Crown of the Sun (2017 Aug 23)

Postby neufer » Wed Aug 23, 2017 11:02 pm

JohnD wrote:
tomatoherd wrote:
... the coincidence of how we, of all systems, have total, but barely total eclipses

No coincidence, tomatoherd, just that you are alive today, which along with 7.5 billions others on Earth and 400 million in the US makes it fairly good odds that somone will be here to see it. And the Moon isn't where it always was. As it became tidally bound to the Earth it moved out, and the tidal stresses are still moving it out as the Earth becomes tidally locked to the Moon. Eventually it will be twice as far away and there will be no total eclipses.

Also, the Moons orbit is not circular, but elliptical as are all orbits, and this makes the Moon appear to vary in diameter by 14%. Some totals show "Bailey's Beads" as the Sun shines through valleys on the Moon's circumference, this one didn't.

    It's a pretty big coincidence :!:
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Re: APOD: The Crown of the Sun (2017 Aug 23)

Postby Chris Peterson » Thu Aug 24, 2017 12:04 am

JohnD wrote:Also, the Moons orbit is not circular, but elliptical as are all orbits, and this makes the Moon appear to vary in diameter by 14%.

nit

All orbits are elliptical, including circular ones. A circle is an ellipse with eccentricity zero. The Moon's orbit has an eccentricity greater than zero.

/nit
Chris

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Re: APOD: The Crown of the Sun (2017 Aug 23)

Postby neufer » Thu Aug 24, 2017 3:26 am

Chris Peterson wrote:
JohnD wrote:
Also, the Moons orbit is not circular, but elliptical as are all orbits, and this makes the Moon appear to vary in diameter by 14%.

All orbits are elliptical, including circular ones. A circle is an ellipse with eccentricity zero.
The Moon's orbit has an eccentricity greater than zero.

In what way are you disagreeing with JohnD :?:
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Re: APOD: The Crown of the Sun (2017 Aug 23)

Postby Chris Peterson » Thu Aug 24, 2017 3:37 am

neufer wrote:
Chris Peterson wrote:
JohnD wrote:Also, the Moons orbit is not circular, but elliptical as are all orbits, and this makes the Moon appear to vary in diameter by 14%.

All orbits are elliptical, including circular ones. A circle is an ellipse with eccentricity zero.
The Moon's orbit has an eccentricity greater than zero.

In what way are you disagreeing with JohnD :?:

The implication is that circular orbits are not elliptical (note the use of "but"). The wording could easily lead a person to believe that there are no circular orbits. People tend to use "elliptical" when they mean "eccentric".
Chris

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