APOD: An Analemma of the Sun (2019 Sep 28)

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APOD: An Analemma of the Sun (2019 Sep 28)

Post by APOD Robot » Sat Sep 28, 2019 4:07 am

Image An Analemma of the Sun

Explanation: This week the equinox found the Sun near the middle, but not at the crossing point, of an analemma in its annual trek through planet Earth's skies. In this scenic view, that graceful, figure-8-shaped curve was intentionally posed above the iconic Danube River and the capital city of Hungary. Looking south from Budapest's Margaret Bridge it combines digital frames taken at exactly the same time of day (11:44 CET) on dates between 2018 September 24 and 2019 September 15. That puts the metropolitan Pest on the left, regal Buda on the right, and the positions of the Sun on the solstice dates at the top and bottom of the analemma curve. December's near solstice Sun is just hidden behind a dramatic cloud bank.

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heehaw

Re: APOD: An Analemma of the Sun (2019 Sep 28)

Post by heehaw » Sat Sep 28, 2019 9:55 am

The ESPOD, https://epod.usra.edu/blog/ , possibly shows an analemma for a planet in a multiple-star system?

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Re: APOD: An Analemma of the Sun (2019 Sep 28)

Post by orin stepanek » Sat Sep 28, 2019 11:19 am

Aww; you can't tell me that that cloud bank was there every day for a year! So the picture must have been added at a different date!
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Re: APOD: An Analemma of the Sun (2019 Sep 28)

Post by Chris Peterson » Sat Sep 28, 2019 2:28 pm

orin stepanek wrote:
Sat Sep 28, 2019 11:19 am
Aww; you can't tell me that that cloud bank was there every day for a year! So the picture must have been added at a different date!
All analemma images collect the solar images at periodic intervals and then add a single exposure of the landscape for the background. What's different here is that the landscape image appears to have been taken when the Sun was behind a cloud. In most images, it is taken when the Sun is outside the frame completely (that is, at a different time of day).
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Re: APOD: An Analemma of the Sun (2019 Sep 28)

Post by mickwilson20 » Sat Sep 28, 2019 4:13 pm

Since when has Budapest been on the equator? I thought the long axis of the analemma meets the local horizon at the complement of the site's latitude. Budapest is at nearly 50N so the axis ought be at about 40 degrees to the horizon.

Or have I missed something here?

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Re: APOD: An Analemma of the Sun (2019 Sep 28)

Post by Chris Peterson » Sat Sep 28, 2019 5:18 pm

mickwilson20 wrote:
Sat Sep 28, 2019 4:13 pm
Since when has Budapest been on the equator? I thought the long axis of the analemma meets the local horizon at the complement of the site's latitude. Budapest is at nearly 50N so the axis ought be at about 40 degrees to the horizon.

Or have I missed something here?
The angle of the long axis of the analemma with respect to the horizon depends upon the time of day that it is imaged.
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Cheers

Re: APOD: An Analemma of the Sun (2019 Sep 28)

Post by Cheers » Sat Sep 28, 2019 11:03 pm

Fascinating image. Can you explain why the distances between the positions of the sun vary from day to day when they were taken at the same time?

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Re: APOD: An Analemma of the Sun (2019 Sep 28)

Post by BDanielMayfield » Sun Sep 29, 2019 1:06 am

Chris Peterson wrote:
Sat Sep 28, 2019 2:28 pm
All analemma images collect the solar images at periodic intervals and then add a single exposure of the landscape for the background. What's different here is that the landscape image appears to have been taken when the Sun was behind a cloud.
Dang, cursed cloud! It was that analemma's anathema. :ssmile:
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Re: APOD: An Analemma of the Sun (2019 Sep 28)

Post by alter-ego » Sun Sep 29, 2019 4:25 am

Cheers wrote:
Sat Sep 28, 2019 11:03 pm
Fascinating image. Can you explain why the distances between the positions of the sun vary from day to day when they were taken at the same time?
There are multiple factors contributing to the nonuniformity of solar positions. First, even though the image times were the same on each day an image was taken, there are not an equal (integer) number of days between all the images. If the sun isn't visible at the image time, it won't be in the analemma. Second, due to the Earth's tilt (trigonometry) and elliptical orbit (varying orbital speed) around the Sun, apparent sun positions won't be equally spaced anyway. Lastly, if the camera were repositioned for every image, there'll be some (hopefully small) random position error from ideal.
This simulated APOD analemma contains 36 images taken every 10 days at 10:44:00 UT. You see the images are closest together around the solstices, and furthest apart around the equinoxes. Note: The figure 8 pattern is fundamentally due to Earth's obliquity (axis tilt). The slight tilt and asymmetry of the "8" is due to Earth's eccentricity. For a perfectly circular orbit, the crossing point would be on the celestial equator. The equinoxes would therefore occur at the crossing point.
 
Budapest Analemma Every 10 Days_2.jpg
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