APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

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APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

Post by APOD Robot » Sun Sep 23, 2018 4:19 am

Image Equinox: Analemma over the Callanish Stones

Explanation: Does the Sun return to the same spot on the sky every day at the same time? No. 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 an 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. Equinoxes, however, correspond to analemma middle points -- not the intersection point. Today at 1:54 am (UT) 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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vanlewkn

Re: APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

Post by vanlewkn » Sun Sep 23, 2018 4:52 am

Isn't this analemma done at local noon rather than 4 PM?

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Re: APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

Post by alter-ego » Sun Sep 23, 2018 6:24 am

vanlewkn wrote:
Sun Sep 23, 2018 4:52 am
Isn't this analemma done at local noon rather than 4 PM?
Yes. The view is definitely looking south, and the analemma is vertical which requires images be taken at local noon. (But it is 4:00pm somewhere :-))
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Re: APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

Post by neufer » Sun Sep 23, 2018 7:36 am

alter-ego wrote:
Sun Sep 23, 2018 6:24 am
vanlewkn wrote:
Sun Sep 23, 2018 4:52 am

Isn't this analemma done at local noon rather than 4 PM?
Yes. The view is definitely looking south, and the analemma is vertical which requires images be taken at local noon.
(But it is 4:00pm somewhere :-))
A high latitude (75°S vs. 58°N) 4:00pm analemma: https://apod.nasa.gov/apod/ap150923.html
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Re: APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

Post by bjmb » Sun Sep 23, 2018 9:28 am

Today at 1:54 am (UT) is the equinox ("equal night"), when day and night are equal over all of planet Earth.
***
i wish that were true, but it isn't. on the equinoxes the day is on all places on earth longer than the night. for amsterdam it is
Date Sunrise Sunse
23 september 07:28 19:37

and more: on both poles there is midnight sun

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Re: APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

Post by Ayiomamitis » Sun Sep 23, 2018 10:23 am

vanlewkn wrote:
Sun Sep 23, 2018 4:52 am
Isn't this analemma done at local noon rather than 4 PM?
That was my immediate reaction as well since the vertical analemma displayed must be at local noon and certainly not at 4 PM.

I just checked using Edinburgh, Scotland, as an example, and local noon is around 13:06 for today (ie with the Sun at 180 degrees azimuth) ... far different from 16:00 PM. Also, Edinburgh is at 55.9 degrees latitude.

Obviously there is some misunderstanding which led to the comment in the text about 16:00.

Here is a vertical analemma from Greece completed during 2002: http://www.perseus.gr/Astro-Solar-Analemma-102816.htm.
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Lguise

Re: APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

Post by Lguise » Sun Sep 23, 2018 1:20 pm

The shadows are very long for midday on the winter solstice even in Scotland? Perhaps the stones were shot at 4:00pm at some stage, but if so the direction has been shifted to make the composition look good, and it does make a great picture!

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Re: APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

Post by Chris Peterson » Sun Sep 23, 2018 1:53 pm

bjmb wrote:
Sun Sep 23, 2018 9:28 am
Today at 1:54 am (UT) is the equinox ("equal night"), when day and night are equal over all of planet Earth.
***
i wish that were true, but it isn't. on the equinoxes the day is on all places on earth longer than the night. for amsterdam it is
Date Sunrise Sunse
23 september 07:28 19:37

and more: on both poles there is midnight sun
A common complaint, but a rather trivial and pedantic one. Sure, there are all the problems caused by atmospheric refraction, the fact that the Sun isn't a point source, the fact that the equinox occurs at an instant of time, the fact that there are different definitions for sunrise and sunset. But the basic geometry is simple, and the explanation of equal day and night is reasonable and helpful.
Chris

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Re: APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

Post by RJN » Sun Sep 23, 2018 4:53 pm

vanlewkn wrote:
Sun Sep 23, 2018 4:52 am
Isn't this analemma done at local noon rather than 4 PM?
Yes, that is correct. Thanks for pointing this out. This has now been corrected on the main NASA APOD. I apologize for the oversight.

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Re: APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

Post by Cousin Ricky » Sun Sep 23, 2018 8:15 pm

Lguise wrote:
Sun Sep 23, 2018 1:20 pm
The shadows are very long for midday on the winter solstice even in Scotland? Perhaps the stones were shot at 4:00pm at some stage, but if so the direction has been shifted to make the composition look good, and it does make a great picture!
Scotland is further north than you think. This scene was shot just 8.4° shy of the Arctic Circle.

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Re: APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

Post by bjmb » Sun Sep 23, 2018 10:16 pm

Chris Peterson wrote:
Sun Sep 23, 2018 1:53 pm
bjmb wrote:
Sun Sep 23, 2018 9:28 am
Today at 1:54 am (UT) is the equinox ("equal night"), when day and night are equal over all of planet Earth.
***
i wish that were true, but it isn't. on the equinoxes the day is on all places on earth longer than the night. for amsterdam it is
Date Sunrise Sunse
23 september 07:28 19:37

and more: on both poles there is midnight sun
A common complaint, but a rather trivial and pedantic one. Sure, there are all the problems caused by atmospheric refraction, the fact that the Sun isn't a point source, the fact that the equinox occurs at an instant of time, the fact that there are different definitions for sunrise and sunset. But the basic geometry is simple, and the explanation of equal day and night is reasonable and helpful.
***
pedantic it may be, but that doesn't make it wrong. here are a few equinox sunrises and -sets that tell you that 'equally long' is a fairy tale.
07:13 – 19:25 reykjavic
06:34 – 19:01 longyearbyen
00:00 – 23:59 south pole
on the contrary, the equinoxes are the only days that the day is longer than the night everywhere on earth.
***
>>the fact that there are different definitions for sunrise and sunset
***
never heard of them. why peddle a fairy tale, when you can say 'almost equally long'? does that hurt anyone?
***
>>A common complaint
***
yes, and a common error. isn't that a reason to think it over once again?
bjmb

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Re: APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

Post by Chris Peterson » Sun Sep 23, 2018 10:25 pm

bjmb wrote:
Sun Sep 23, 2018 10:16 pm
Chris Peterson wrote:
Sun Sep 23, 2018 1:53 pm
bjmb wrote:
Sun Sep 23, 2018 9:28 am
Today at 1:54 am (UT) is the equinox ("equal night"), when day and night are equal over all of planet Earth.
***
i wish that were true, but it isn't. on the equinoxes the day is on all places on earth longer than the night. for amsterdam it is
Date Sunrise Sunse
23 september 07:28 19:37

and more: on both poles there is midnight sun
A common complaint, but a rather trivial and pedantic one. Sure, there are all the problems caused by atmospheric refraction, the fact that the Sun isn't a point source, the fact that the equinox occurs at an instant of time, the fact that there are different definitions for sunrise and sunset. But the basic geometry is simple, and the explanation of equal day and night is reasonable and helpful.
***
pedantic it may be, but that doesn't make it wrong.
Nor did I suggest that. Just rather missing the point.
>>the fact that there are different definitions for sunrise and sunset
***
never heard of them.
Really? Top of the Sun on the horizon? Bottom? Center? Refracted or unrefracted?
why peddle a fairy tale, when you can say 'almost equally long'?
Because it isn't a "fairy tale". It's a geometric condition that people can actually understand. You deviate from that, and you exclude a large audience from a learning opportunity.
Chris

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Re: APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

Post by geckzilla » Sun Sep 23, 2018 10:37 pm

Did you realize that bjmb has been dying on this hill for over ten years now? Check the user post history.
Just call me "geck" because "zilla" is like a last name.

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Re: APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

Post by gwrede » Sun Sep 23, 2018 11:47 pm

Sirs!

While the explanation to this Apod picture is technically right, I take serious exception to publishing a picture of the analemma that shows the Winter Solstice when the actual date is the Autumn Equinox.

This will cause every off-guard uneducated reader to leave with the impression that the picture shows an Equinox. -- Isn't the very point of Apod to educate the uneducated!

As a former middle-school, high-school and university teacher, I cannot stress this enough. In extensive discussions with students and even adults, it often turns up that a sloppy detail in an article, or even a school book has caused a lifelong misunderstanding of something fundamental.

Please try to avoid carelessness or ambiguity in the future.

Thank you.
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Re: APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

Post by neufer » Sun Sep 23, 2018 11:55 pm

geckzilla wrote:
Sun Sep 23, 2018 10:37 pm

Did you realize that bjmb has been dying on this hill for over ten years now? Check the user post history.
At least bjmb has managed ~0.01 posts per day ...which is more than can be said for gwrede.
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Re: APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

Post by Chris Peterson » Mon Sep 24, 2018 12:08 am

gwrede wrote:
Sun Sep 23, 2018 11:47 pm
While the explanation to this Apod picture is technically right, I take serious exception to publishing a picture of the analemma that shows the Winter Solstice when the actual date is the Autumn Equinox.
In what way do you perceive this image showing the winter solstice? It is a multiple exposure of many dates over an entire year. In the analemma you can see the position of the Sun at both solstices and both equinoxes. The only connection to the winter solstice is that the foreground shot was probably exposed around then.

An analemma shot is pretty relevant to APOD on any of the quarter-days.
Chris

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Re: APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

Post by neufer » Mon Sep 24, 2018 2:54 am

Chris Peterson wrote:
Mon Sep 24, 2018 12:08 am
gwrede wrote:
Sun Sep 23, 2018 11:47 pm

While the explanation to this Apod picture is technically right, I take serious exception to publishing a picture of the analemma that shows the Winter Solstice when the actual date is the Autumn Equinox.
In what way do you perceive this image showing the winter solstice? It is a multiple exposure of many dates over an entire year. In the analemma you can see the position of the Sun at both solstices and both equinoxes. The only connection to the winter solstice is that the foreground shot was probably exposed around then. An analemma shot is pretty relevant to APOD on any of the quarter-days.
The equinoxes represent the middle of the analemma vertically
which is somewhat difficult to determine without either checking the dates or using a ruler.

The most noticeable feature of an analemma is the crossing point at around April 12 & August 29.

So... I'm sorta with gwrede on this one: poor choice for an equinox.

(Note that a noontime analemma is not entirely vertical.)
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Re: APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

Post by alter-ego » Mon Sep 24, 2018 5:41 am

neufer wrote:
Mon Sep 24, 2018 2:54 am
...
(Note that a noontime analemma is not entirely vertical.)
The tilt and asymmetry of the figure-8 is due entirely to the Earth's orbital eccentricity. Assuming a due south view, the analemma tilt is very close to 1°. Solar images taken ~14 minutes after local mean noon would yield a vertical analemma, but the view would be 1°off from south. I don't know the APOD view heading that accurately, and the exact orientation of the analemma is not clear.
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Re: APOD: Equinox: Analemma over the Callanish... (2018 Sep 23)

Post by neufer » Mon Sep 24, 2018 12:39 pm

alter-ego wrote:
Mon Sep 24, 2018 5:41 am
neufer wrote:
Mon Sep 24, 2018 2:54 am
...
(Note that a noontime analemma is not entirely vertical.)
The tilt and asymmetry of the figure-8 is due entirely to the Earth's orbital eccentricity. Assuming a due south view, the analemma tilt is very close to 1°. Solar images taken ~14 minutes after local mean noon would yield a vertical analemma, but the view would be 1°off from south. I don't know the APOD view heading that accurately, and the exact orientation of the analemma is not clear.
Lacking accurate clocks, the Neolithic builders of the Callanish Stones would have been totally
unaware of the shape & orientation of an analemma (even if they took careful note of the equinoxes).

:arrow: But what are we to make of the the shape & orientation of the Callanish Stones :?:
Art Neuendorffer