APOD: Analemma over the Callanish Stones (2022 Sep 18)

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APOD: Analemma over the Callanish Stones (2022 Sep 18)

Post by APOD Robot » Sun Sep 18, 2022 4:05 am

Image Analemma over the Callanish Stones

Explanation: If you went outside at the same time every day and took a picture that included the Sun, how would the Sun's position change? A more visual answer to that question is an analemma, a composite image taken from the same spot at the same time over the course of a year. The featured analemma was composed from images taken every few days at noon near the village of Callanish in the Outer Hebrides in Scotland, UK. In the foreground are the Callanish Stones, a stone circle built around 2700 BC during humanity's Bronze Age. It is not known if the placement of the Callanish Stones has or had astronomical significance. The ultimate causes for the figure-8 shape of this and all analemmas are the tilt of the Earth axis and the ellipticity of the Earth's orbit around the Sun. At the solstices, the Sun will appear at the top or bottom of an analemma. The featured image was taken near the December solstice and so the Sun appears near the bottom. Equinoxes, however, correspond to analemma middle points -- not the intersection point. This coming Friday at 1:04 am (UT) -- Thursday in the Americas -- is the equinox ("equal night"), when day and night are equal over all of planet Earth. Many cultures celebrate a change of season at an equinox.

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RocketRon

Re: APOD: Analemma over the Callanish Stones (2022 Sep 18)

Post by RocketRon » Sun Sep 18, 2022 4:26 am

Marvelously illustrative photo - thanks !
APOD Robot wrote: Sun Sep 18, 2022 4:05 am It is not known if the placement of the Callanish Stones has or had astronomical significance.
All (or nearly all ?) the stones at Stonehenge have been removed/replaced/concreted into place, so it is no longer viable to study any significant alignments there. (they were falling over/falling down/fallen down though).

Do we know if Callinish has been thus altered, or may it be a bit more original. ?

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Re: APOD: Analemma over the Callanish Stones (2022 Sep 18)

Post by VictorBorun » Sun Sep 18, 2022 5:07 am

somehow counter-intuitive is that noon suns are not exactly in the meridian.
Do Easts and Wests shift during a year?

RocketRon

Re: APOD: Analemma over the Callanish Stones (2022 Sep 18)

Post by RocketRon » Sun Sep 18, 2022 8:01 am

https://en.wikipedia.org/wiki/Equation_of_time

How far the actual sun varies from a 'perfect fictional' sun.

Earth's orbit around the Sun is not a perfect circle,
so the Sun does not keep 'perfect' time.
(although its the Earth doing the gymnastics/stretching/shrinking )
The analemma's shape illustrates that ...

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Re: APOD: Analemma over the Callanish Stones (2022 Sep 18)

Post by orin stepanek » Sun Sep 18, 2022 12:31 pm

CallanishAnalemma_Petricca_960_Dates.jpg
tutulemma_tezel_960.jpg
Where these stones a religious symbol at one time or a scientic
study of the sun?
You do not have the required permissions to view the files attached to this post.
Orin

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Re: APOD: Analemma over the Callanish Stones (2022 Sep 18)

Post by JWP456 » Sun Sep 18, 2022 3:57 pm

Alternatively, you could go outside each day and record the time when the sun was on the meridian. If you then graphed those times over the course of a year, it would illustrate the same thing - that the length of the day measured as the time between meridian passages of the sun varies throughout the year.

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Re: APOD: Analemma over the Callanish Stones (2022 Sep 18)

Post by johnnydeep » Sun Sep 18, 2022 7:50 pm

Well hello there again to my old nemesis the Analemma. I'll probably be dead before I really understand thee. Caveat lector: no need for helpful others to venture to explain it further: there have already been many who've tried in vain before you. You will not fare any better. What might help is an app that allows you to change the angle of the tilt of the Earth's axis and the eccentricity of the Earth's orbit to see how the shape of the analemma changes as a result.
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Re: APOD: Analemma over the Callanish Stones (2022 Sep 18)

Post by bystander » Sun Sep 18, 2022 8:23 pm

johnnydeep wrote: Sun Sep 18, 2022 7:50 pm Well hello there again to my old nemesis the Analemma. I'll probably be dead before I really understand thee. Caveat lector: no need for helpful others to venture to explain it further: there have already been many who've tried in vain before you. You will not fare any better. What might help is an app that allows you to change the angle of the tilt of the Earth's axis and the eccentricity of the Earth's orbit to see how the shape of the analemma changes as a result.
https://www.analemma.com/other-analemmas.html
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alive to the gentle breeze of communication, and please stop being such a jerk.
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RocketRon

Re: APOD: Analemma over the Callanish Stones (2022 Sep 18)

Post by RocketRon » Sun Sep 18, 2022 10:19 pm

johnnydeep wrote: Sun Sep 18, 2022 7:50 pm that allows you to change the angle of the tilt of the Earth's axis and the eccentricity of the Earth's orbit to see how the shape of the analemma changes as a result.
IF the Earths orbit was perfectly circular, the 'analemma' would merely be a perfectly straight line in the sky.
On the meridian, as suggested above.

And the 'equation of time' would evaporate, since the fictional and real suns would perfectly coincide.
The sun would always be where it was expected to be, rather than distorted because of odd orbit(s).

Would such an app be a best seller, we wonder.
Even as a giveaway ...

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Re: APOD: Analemma over the Callanish Stones (2022 Sep 18)

Post by johnnydeep » Mon Sep 19, 2022 3:49 pm

Thanks for trying. Apparently, the only case I am capable of understanding is if the Earth had a perfectly circular orbit, a 0 degree axis tilt, and was tidally locked so that one side faced the Sun at all times (such that the rotation rate matched the yearly orbital period). In that case, the position of the Sun in the sky would never change at all for any observer anywhere on the Earth, and the "analemma" would be a single point.

Now, let's see, if the orbit was allowed to be a proper Newtonian ellipse (all other aforementioned hypotheticals remaining in effect), would the analemma still be a point? Yes, right?
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Re: APOD: Analemma over the Callanish Stones (2022 Sep 18)

Post by Chris Peterson » Mon Sep 19, 2022 4:13 pm

johnnydeep wrote: Mon Sep 19, 2022 3:49 pm Thanks for trying. Apparently, the only case I am capable of understanding is if the Earth had a perfectly circular orbit, a 0 degree axis tilt, and was tidally locked so that one side faced the Sun at all times (such that the rotation rate matched the yearly orbital period). In that case, the position of the Sun in the sky would never change at all for any observer anywhere on the Earth, and the "analemma" would be a single point.

Now, let's see, if the orbit was allowed to be a proper Newtonian ellipse (all other aforementioned hypotheticals remaining in effect), would the analemma still be a point? Yes, right?
Maybe you can slowly devolve your "perfect" system.

If the Earth had no axial tilt with respect to its orbital plane, and an orbital eccentricity of zero (i.e. a circular orbit), the analemma would be a point. The Sun would appear in the same spot each day at a give time. (Noon is easiest to understand, with the Sun right on the meridian and its altitude determined only by the latitude of the observer.)

Now lets give the Earth some axial tilt. This basically just introduces seasons, so the Sun is higher or lower in the sky depending on which part of our annual orbit we're on. So now the analemma has become a line perpendicular to the horizon. The Sun is still always on the meridian at noon, but its altitude depends both upon the observer's latitude and the position in our orbit.

Now lets make our orbit a little eccentric. That means our orbital speed isn't uniform. Per Kepler's Second Law our orbital speed will be greater when we're closer to the Sun. So this makes the Sun a little before or a little after the meridian at noon, depending on where we are in our orbit. If we had no axial tilt, the analemma would again be a line, but in this case parallel to the horizon. With both tilt and eccentricity, we have deviation vertically and horizontally, which is why real world analemmas look like closed roundish or figure-eight paths.
Chris

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Re: APOD: Analemma over the Callanish Stones (2022 Sep 18)

Post by johnnydeep » Mon Sep 19, 2022 8:51 pm

Chris Peterson wrote: Mon Sep 19, 2022 4:13 pm
johnnydeep wrote: Mon Sep 19, 2022 3:49 pm Thanks for trying. Apparently, the only case I am capable of understanding is if the Earth had a perfectly circular orbit, a 0 degree axis tilt, and was tidally locked so that one side faced the Sun at all times (such that the rotation rate matched the yearly orbital period). In that case, the position of the Sun in the sky would never change at all for any observer anywhere on the Earth, and the "analemma" would be a single point.

Now, let's see, if the orbit was allowed to be a proper Newtonian ellipse (all other aforementioned hypotheticals remaining in effect), would the analemma still be a point? Yes, right?
Maybe you can slowly devolve your "perfect" system.

If the Earth had no axial tilt with respect to its orbital plane, and an orbital eccentricity of zero (i.e. a circular orbit), the analemma would be a point. The Sun would appear in the same spot each day at a give time. (Noon is easiest to understand, with the Sun right on the meridian and its altitude determined only by the latitude of the observer.)

Now lets give the Earth some axial tilt. This basically just introduces seasons, so the Sun is higher or lower in the sky depending on which part of our annual orbit we're on. So now the analemma has become a line perpendicular to the horizon. The Sun is still always on the meridian at noon, but its altitude depends both upon the observer's latitude and the position in our orbit.

Now lets make our orbit a little eccentric. That means our orbital speed isn't uniform. Per Kepler's Second Law our orbital speed will be greater when we're closer to the Sun. So this makes the Sun a little before or a little after the meridian at noon, depending on where we are in our orbit. If we had no axial tilt, the analemma would again be a line, but in this case parallel to the horizon. With both tilt and eccentricity, we have deviation vertically and horizontally, which is why real world analemmas look like closed roundish or figure-eight paths.
Thanks. My 3D sense has mostly vanished from when I had it as a math major 40 years ago. Or maybe I never had it at all when it comes to astronomical motions. What does it mean to take a photo of the Sun at the "same time" each day? How is a day defined here? Using solar time as opposed to sidereal time I suppose? I feel like I'm regressing to the point of needing a course in remedial astronomy...
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Re: APOD: Analemma over the Callanish Stones (2022 Sep 18)

Post by Chris Peterson » Mon Sep 19, 2022 8:58 pm

johnnydeep wrote: Mon Sep 19, 2022 8:51 pm
Chris Peterson wrote: Mon Sep 19, 2022 4:13 pm
johnnydeep wrote: Mon Sep 19, 2022 3:49 pm Thanks for trying. Apparently, the only case I am capable of understanding is if the Earth had a perfectly circular orbit, a 0 degree axis tilt, and was tidally locked so that one side faced the Sun at all times (such that the rotation rate matched the yearly orbital period). In that case, the position of the Sun in the sky would never change at all for any observer anywhere on the Earth, and the "analemma" would be a single point.

Now, let's see, if the orbit was allowed to be a proper Newtonian ellipse (all other aforementioned hypotheticals remaining in effect), would the analemma still be a point? Yes, right?
Maybe you can slowly devolve your "perfect" system.

If the Earth had no axial tilt with respect to its orbital plane, and an orbital eccentricity of zero (i.e. a circular orbit), the analemma would be a point. The Sun would appear in the same spot each day at a give time. (Noon is easiest to understand, with the Sun right on the meridian and its altitude determined only by the latitude of the observer.)

Now lets give the Earth some axial tilt. This basically just introduces seasons, so the Sun is higher or lower in the sky depending on which part of our annual orbit we're on. So now the analemma has become a line perpendicular to the horizon. The Sun is still always on the meridian at noon, but its altitude depends both upon the observer's latitude and the position in our orbit.

Now lets make our orbit a little eccentric. That means our orbital speed isn't uniform. Per Kepler's Second Law our orbital speed will be greater when we're closer to the Sun. So this makes the Sun a little before or a little after the meridian at noon, depending on where we are in our orbit. If we had no axial tilt, the analemma would again be a line, but in this case parallel to the horizon. With both tilt and eccentricity, we have deviation vertically and horizontally, which is why real world analemmas look like closed roundish or figure-eight paths.
Thanks. My 3D sense has mostly vanished from when I had it as a math major 40 years ago. Or maybe I never had it at all when it comes to astronomical motions. What does it mean to take a photo of the Sun at the "same time" each day? How is a day defined here? Using solar time as opposed to sidereal time I suppose? I feel like I'm regressing to the point of needing a course in remedial astronomy...
You can work backwards to time. If we had a perfectly circular orbit, and measured the time from one meridian crossing of the Sun to the next (solar noon to solar noon) that's the time we're interested in. Solar time. A synodic day. And that's what we continue to use once we have tilt and eccentricity.
Chris

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Re: APOD: Analemma over the Callanish Stones (2022 Sep 18)

Post by johnnydeep » Mon Sep 19, 2022 9:07 pm

Chris Peterson wrote: Mon Sep 19, 2022 8:58 pm
johnnydeep wrote: Mon Sep 19, 2022 8:51 pm
Chris Peterson wrote: Mon Sep 19, 2022 4:13 pm

Maybe you can slowly devolve your "perfect" system.

If the Earth had no axial tilt with respect to its orbital plane, and an orbital eccentricity of zero (i.e. a circular orbit), the analemma would be a point. The Sun would appear in the same spot each day at a give time. (Noon is easiest to understand, with the Sun right on the meridian and its altitude determined only by the latitude of the observer.)

Now lets give the Earth some axial tilt. This basically just introduces seasons, so the Sun is higher or lower in the sky depending on which part of our annual orbit we're on. So now the analemma has become a line perpendicular to the horizon. The Sun is still always on the meridian at noon, but its altitude depends both upon the observer's latitude and the position in our orbit.

Now lets make our orbit a little eccentric. That means our orbital speed isn't uniform. Per Kepler's Second Law our orbital speed will be greater when we're closer to the Sun. So this makes the Sun a little before or a little after the meridian at noon, depending on where we are in our orbit. If we had no axial tilt, the analemma would again be a line, but in this case parallel to the horizon. With both tilt and eccentricity, we have deviation vertically and horizontally, which is why real world analemmas look like closed roundish or figure-eight paths.
Thanks. My 3D sense has mostly vanished from when I had it as a math major 40 years ago. Or maybe I never had it at all when it comes to astronomical motions. What does it mean to take a photo of the Sun at the "same time" each day? How is a day defined here? Using solar time as opposed to sidereal time I suppose? I feel like I'm regressing to the point of needing a course in remedial astronomy...
You can work backwards to time. If we had a perfectly circular orbit, and measured the time from one meridian crossing of the Sun to the next (solar noon to solar noon) that's the time we're interested in. Solar time. A synodic day. And that's what we continue to use once we have tilt and eccentricity.
Ok. This explains it well - from https://en.wikipedia.org/wiki/Sidereal_ ... solar_time:

So if sidereal time was used, would the shape of the analemma change much? And how about if the Earth rotated the opposite way (at the same rate)?
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Re: APOD: Analemma over the Callanish Stones (2022 Sep 18)

Post by Chris Peterson » Mon Sep 19, 2022 9:13 pm

johnnydeep wrote: Mon Sep 19, 2022 9:07 pm So if sidereal time was used, would the shape of the analemma change much? And how about if the Earth rotated the opposite way (at the same rate)?
If you used sidereal time the Sun wouldn't even be in the sky for about half the year. I'd say that's a change to the analemma!
Chris

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Re: APOD: Analemma over the Callanish Stones (2022 Sep 18)

Post by johnnydeep » Tue Sep 20, 2022 3:31 pm

Chris Peterson wrote: Mon Sep 19, 2022 9:13 pm
johnnydeep wrote: Mon Sep 19, 2022 9:07 pm So if sidereal time was used, would the shape of the analemma change much? And how about if the Earth rotated the opposite way (at the same rate)?
If you used sidereal time the Sun wouldn't even be in the sky for about half the year. I'd say that's a change to the analemma!
Yeah, I hadn't thought of that. And how about if the Earth rotated the opposite way?
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Re: APOD: Analemma over the Callanish Stones (2022 Sep 18)

Post by Mathew32df » Thu Oct 27, 2022 4:08 am

APOD Robot wrote: Sun Sep 18, 2022 4:05 am Image Analemma over the Callanish Stones

Explanation: If you went outside at the same time every day and took a picture that included the Sun, how would the Sun's position change? A more visual answer to that question is an analemma, a composite image taken from the same spot at the same time over the course of a year. The featured analemma was composed from images taken every few days at noon near the village of Callanish in the Outer Hebrides in Scotland, UK. In the foreground are the Callanish Stones, a stone circle built around 2700 BC during humanity's Bronze Age. It is not known if the placement of the Callanish Stones has or had astronomical significance. The ultimate causes for the figure-8 shape of this and all analemmas are the tilt of the Earth axis and the ellipticity of the Earth's orbit around the Sun fnf . At the solstices, the Sun will appear at the top or bottom of an analemma. The featured image was taken near the December solstice and so the Sun appears near the bottom. Equinoxes, however, correspond to analemma middle points -- not the intersection point. This coming Friday at 1:04 am (UT) -- Thursday in the Americas -- is the equinox ("equal night"), when day and night are equal over all of planet Earth. Many cultures celebrate a change of season at an equinox.

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Great picture, can I use it?

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Re: APOD: Analemma over the Callanish Stones (2022 Sep 18)

Post by bystander » Thu Oct 27, 2022 4:45 am

Mathew32df wrote: Thu Oct 27, 2022 4:08 am
APOD Robot wrote: Sun Sep 18, 2022 4:05 am Image Analemma over the Callanish Stones

Great picture, can I use it?

Ask the photographer: Giuseppe Petricca
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
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