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.
[quote=Cheers post_id=295617 time=1569711815]
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?
[/quote]
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. [b]Note:[/b] 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.
[attachment=0]Budapest Analemma Every 10 Days_2.jpg[/attachment]