APOD: Star Trails and the Equinox Sunrise (2019 Mar 21)

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APOD: Star Trails and the Equinox Sunrise (2019 Mar 21)

Post by APOD Robot » Thu Mar 21, 2019 4:07 am

Image Star Trails and the Equinox Sunrise

Explanation: Stars trail and the Sun rises in this night and day composite panorama made on March 19. The view looks toward the eastern horizon from La Nava de Santiago, Spain. To create it, a continuous series of digital frames was recorded for about two hours and combined to trace the concentric motion of the stars through the night sky. A reflection of the Earth's rotation, star trails curve around the north celestial pole toward upper left and the south celestial pole toward the lower right. Of course on that day the Sun was near the celestial equator, a diagonal straight line in the wide-angle projection. A dense dimming filter was used to capture the Sun's image every two minutes. Superimposed on the star trails it rose due east in the morning sky. In the scene, foreground landscape and a local prehistoric monument were illuminated by full moonlight, though. The monument's corridor faces nearly to the east and the equinox sunrise.

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Re: APOD: Star Trails and the Equinox Sunrise (2019 Mar 21)

Post by Ann » Thu Mar 21, 2019 5:43 am

Nice. I like it. The heavily dimmed Sun rising in a straight line is quite striking.

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Re: APOD: Star Trails and the Equinox Sunrise (2019 Mar 21)

Post by De58te » Thu Mar 21, 2019 9:57 am

Question. If that yellow dotted trail is the Sun, why does it appear no more than twice the size as the star trails? Even if the largest star in the sky (not including the Sun) to my view is actually the planet Venus and not a star, still, when I look at the actual sky (with Sun protection glasses of course) the Sun appears at about 10 times the diameter of Venus. (just an estimate, I haven't scientifically measured it). Noticeably bigger than it is represented here.

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Re: APOD: Star Trails and the Equinox Sunrise (2019 Mar 21)

Post by Ann » Thu Mar 21, 2019 10:39 am

De58te wrote:
Thu Mar 21, 2019 9:57 am
Question. If that yellow dotted trail is the Sun, why does it appear no more than twice the size as the star trails? Even if the largest star in the sky (not including the Sun) to my view is actually the planet Venus and not a star, still, when I look at the actual sky (with Sun protection glasses of course) the Sun appears at about 10 times the diameter of Venus. (just an estimate, I haven't scientifically measured it). Noticeably bigger than it is represented here.
Chris or Art or some of the other math whizzes should answer this one, but I think that the main problem is not the Sun looks too small, but rather that the star trails look too broad due to pixel bleeding.

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Re: APOD: Star Trails and the Equinox Sunrise (2019 Mar 21)

Post by Chris Peterson » Thu Mar 21, 2019 12:52 pm

Ann wrote:
Thu Mar 21, 2019 10:39 am
De58te wrote:
Thu Mar 21, 2019 9:57 am
Question. If that yellow dotted trail is the Sun, why does it appear no more than twice the size as the star trails? Even if the largest star in the sky (not including the Sun) to my view is actually the planet Venus and not a star, still, when I look at the actual sky (with Sun protection glasses of course) the Sun appears at about 10 times the diameter of Venus. (just an estimate, I haven't scientifically measured it). Noticeably bigger than it is represented here.
Chris or Art or some of the other math whizzes should answer this one, but I think that the main problem is not the Sun looks too small, but rather that the star trails look too broad due to pixel bleeding.

Ann
"Bleeding" isn't quite the right word, but yeah, that's right. The Sun is the only object here that's resolved... that actually shows a disk. The stars are- optically- point sources, meaning they are all much smaller than a single pixel. But diffraction and scattering makes them occupy more than one pixel, and just what that apparent size is depends only on their brightness. So bright stars look big, dim ones look small. (The Sun would do the same thing if it were overexposed.)
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Re: APOD: Star Trails and the Equinox Sunrise (2019 Mar 21)

Post by orin stepanek » Thu Mar 21, 2019 12:56 pm

Anyway; it's spring now! :D :?
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Re: APOD: Star Trails and the Equinox Sunrise (2019 Mar 21)

Post by Cousin Ricky » Thu Mar 21, 2019 1:06 pm

I understand that yesterday in some parts of the world, people welcomed a change in seasons.

That is so adorable!

Greetings from the Virgin Islands! :mrgreen:

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Re: APOD: Star Trails and the Equinox Sunrise (2019 Mar 21)

Post by Chris Peterson » Thu Mar 21, 2019 1:49 pm

Cousin Ricky wrote:
Thu Mar 21, 2019 1:06 pm
I understand that yesterday in some parts of the world, people welcomed a change in seasons.

That is so adorable!

Greetings from the Virgin Islands! :mrgreen:
Gosh, I'm sorry.
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Re: APOD: Star Trails and the Equinox Sunrise (2019 Mar 21)

Post by neufer » Thu Mar 21, 2019 3:03 pm

https://earthsky.org/human-world/origin-europes-ancient-megaliths-stonehenge-brittany wrote:
Stonehenge mystery solved?
By Deborah Byrd in Human World, EarthSky, Feb. 17, 2019

<<A new study suggests that Europe’s ancient megaliths, including Stonehenge, might have stemmed from a single hunter-gatherer culture in what’s now northwestern France. A new study published February 11, 2019, in the peer-reviewed journal Proceedings of the National Academy of Sciences sheds light on these mysteries. The new study was conducted by Bettina Schulz Paulsson, a prehistoric archaeologist at the University of Gothenburg in Sweden. Her work suggests these European megaliths can be traced back to a single hunter-gatherer culture that originated nearly 7,000 years ago in what’s today the Brittany region of northwestern France. The study also argues for a cultural exchange over sea routes emanating from northwest France, suggesting a more advanced seafaring technology for that ancient time than was previously believed. Schulz Paulson spent 10 years creating what she called a megalith evolution using radiocarbon dating from thousands of historic sites across Europe. She wrote in her study: We have thus been able to demonstrate that the earliest megaliths originated in northwest France and spread along the sea routes of the Mediterranean and Atlantic coasts in three successive principal phases.

Writing in Science, science journalist Michael Price explained more about Schulz Paulson’s work: What she did was sift through radiocarbon dating data from 2,410 ancient sites across Europe to reconstruct a prehistoric archaeological timeline. The radiocarbon dates came mostly from human remains buried within the sites. The study looked not just at megaliths, but also at so-called premegalithic graves that featured elaborate, earthen tombs but no huge stones. Schulz Paulsson also factored in information on the sites’ architecture, tool use, and burial customs to further narrow the dates.>>
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Re: APOD: Star Trails and the Equinox Sunrise (2019 Mar 21)

Post by BDanielMayfield » Thu Mar 21, 2019 5:59 pm

Among the many star trail images that have been featured as APODs over the years, this one is outstanding.

Thanks to the photographer and to the editors for presenting it.


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Re: APOD: Star Trails and the Equinox Sunrise (2019 Mar 21)

Post by Nitpicker » Thu Mar 21, 2019 10:30 pm

Ann wrote:
Thu Mar 21, 2019 5:43 am
Nice. I like it. The heavily dimmed Sun rising in a straight line is quite striking.

Ann
The straight line of the sun path is by virtue of the direction the camera was pointed, rather than the equinox. Had the sun been less centred in the frame, the path would have been curved methinks.

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Re: APOD: Star Trails and the Equinox Sunrise (2019 Mar 21)

Post by neufer » Fri Mar 22, 2019 2:05 am

Nitpicker wrote:
Thu Mar 21, 2019 10:30 pm
Ann wrote:
Thu Mar 21, 2019 5:43 am

Nice. I like it. The heavily dimmed Sun rising in a straight line is quite striking.
The straight line of the sun path is by virtue of the direction the camera was pointed, rather than the equinox. Had the sun been less centred in the frame, the path would have been curved methinks.
.
.
With a normal (pseudo-pinhole/non-fish eye) type camera lens
both the Milky Way & the sun's path at equinox appear as straight lines.
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Re: APOD: Star Trails and the Equinox Sunrise (2019 Mar 21)

Post by Ann » Fri Mar 22, 2019 5:29 am

BDanielMayfield wrote:
Thu Mar 21, 2019 5:59 pm
Among the many star trail images that have been featured as APODs over the years, this one is outstanding.

Thanks to the photographer and to the editors for presenting it.


Bruce
I wholeheartedly agree!

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Re: APOD: Star Trails and the Equinox Sunrise (2019 Mar 21)

Post by Nitpicker » Fri Mar 22, 2019 6:52 am

neufer wrote:
Fri Mar 22, 2019 2:05 am
Nitpicker wrote:
Thu Mar 21, 2019 10:30 pm
Ann wrote:
Thu Mar 21, 2019 5:43 am

Nice. I like it. The heavily dimmed Sun rising in a straight line is quite striking.
The straight line of the sun path is by virtue of the direction the camera was pointed, rather than the equinox. Had the sun been less centred in the frame, the path would have been curved methinks.
With a normal (pseudo-pinhole/non-fish eye) type camera lens
both the Milky Way & the sun's path at equinox appear as straight lines.
Not so sure about that. Even in the APOD, if you look closely, the trails just to the left (north) of the sun, at the top edge of frame, are curving ever so slightly to the right (south). I think this is because the lens axis is not pointed perfectly eastward (which is not a criticism of the APOD, which is beautiful).

Edit: Of course, this APOD was taken a day or so before the equinox, so maybe that explains it. (Now I want to test to see if the celestial equator always appears straight in my images.)

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Re: APOD: Star Trails and the Equinox Sunrise (2019 Mar 21)

Post by neufer » Fri Mar 22, 2019 2:29 pm

Nitpicker wrote:
Fri Mar 22, 2019 6:52 am
neufer wrote:
Fri Mar 22, 2019 2:05 am
Nitpicker wrote:
Thu Mar 21, 2019 10:30 pm

The straight line of the sun path is by virtue of the direction the camera was pointed, rather than the equinox. Had the sun been less centred in the frame, the path would have been curved methinks.
With a normal (pseudo-pinhole/non-fish eye) type camera lens
both the Milky Way & the sun's path at equinox appear as straight lines.
Not so sure about that. Even in the APOD, if you look closely, the trails just to the left (north) of the sun, at the top edge of frame, are curving ever so slightly to the right (south). I think this is because the lens axis is not pointed perfectly eastward (which is not a criticism of the APOD, which is beautiful).

Edit: Of course, this APOD was taken a day or so before the equinox, so maybe that explains it. (Now I want to test to see if the celestial equator always appears straight in my images.)
The sun, itself, is "moving" from the southern to the northern hemisphere at the rate of about 1 arcminute per hour.
  • (Ergo: the straight line path is not due East to West.)
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Re: APOD: Star Trails and the Equinox Sunrise (2019 Mar 21)

Post by alter-ego » Fri Mar 22, 2019 7:36 pm

Nitpicker wrote:
Fri Mar 22, 2019 6:52 am
neufer wrote:
Fri Mar 22, 2019 2:05 am
Nitpicker wrote:
Thu Mar 21, 2019 10:30 pm


The straight line of the sun path is by virtue of the direction the camera was pointed, rather than the equinox. Had the sun been less centred in the frame, the path would have been curved methinks.
With a normal (pseudo-pinhole/non-fish eye) type camera lens
both the Milky Way & the sun's path at equinox appear as straight lines.
Not so sure about that. Even in the APOD, if you look closely, the trails just to the left (north) of the sun, at the top edge of frame, are curving ever so slightly to the right (south). I think this is because the lens axis is not pointed perfectly eastward (which is not a criticism of the APOD, which is beautiful).

Edit: Of course, this APOD was taken a day or so before the equinox, so maybe that explains it. (Now I want to test to see if the celestial equator always appears straight in my images.)
Created by a normal camera lens, this image is essentially a stereographic projection in which all great circles (e.g. galactic equator, Earth's equator, and ecliptic) can be imaged as straight lines. However, camera pointing does matter. If the camera is centered on the intersection of any two great circles, say the ideal horizon and equator, those great circles will appear as straight lines. This case is often encountered because we often want undistorted (flat) horizons. So, as it is here, this condition is had when looking due east/west on the horizon. If you point the camera away in any direction, the great circles will appear to curve, increasing the distortion the further away the camera is pointed. In the case of the ecliptic and galactic equator, they can be viewed as straight lines independent of the horizon as long as the camera is centered at the intersection point.

So, the detail of the camera not pointing due east will cause the equator (rising sun path) to curve a little. It should readily be visible in this image, but is not. However, it is possible to generate a "straight" equator this image at the cost of horizon and star-trail curvature distortions which is what I think is going on in this image. The real terrain geography is in fact sloped similarly to the image, and the star trails are curve close to the due east view. Consequently it is not obvious, nor trivial, to see the difference in the two stereographic projections. I'm assuming the photographer did not digitally modify the projection to generate the straight equator (rising sun path), although it is possible to do that.
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Post by neufer » Fri Mar 22, 2019 8:43 pm

alter-ego wrote:
Fri Mar 22, 2019 7:36 pm

Created by a normal camera lens, this image is essentially a stereographic projection in which all great circles (e.g. galactic equator, Earth's equator, and ecliptic) can be imaged as straight lines.
https://en.wikipedia.org/wiki/Rectilinear_lens wrote: <<In photography, a rectilinear lens is a photographic lens that yields images where straight features, such as the walls of buildings, appear with straight lines, as opposed to being curved. In other words, it is a lens with little or no barrel or pincushion distortion. At particularly wide angles, however, the rectilinear perspective will cause objects to appear increasingly stretched and enlarged as they near the edge of the frame. These types of lenses are often used to create forced perspective effects.

The most famous example is the Rapid Rectilinear Lens developed by John Henry Dallmeyer in 1866. It allowed distortionless photos to be taken quickly for the first time, and was a standard lens design for 60 years.

The vast majority of video and still cameras use lenses that produce nearly rectilinear images. A popular alternative type of lens is a fisheye lens which produces a distinctly curvilinear, wide-angled result.>>
https://en.wikipedia.org/wiki/Gnomonic_projection wrote: <<A gnomonic map projection displays all great circles as straight lines, resulting in any straight line segment on a gnomonic map showing a geodesic, the shortest route between the segment's two endpoints. This is achieved by casting surface points of the sphere onto a tangent plane, each landing where a ray from the center of the sphere passes through the point on the surface and then on to the plane. No distortion occurs at the tangent point, but distortion increases rapidly away from it. Less than half of the sphere can be projected onto a finite map. Consequently, a rectilinear photographic lens, which is based on the gnomonic principle, cannot image more than 180 degrees. A stereographic projection projects the entire sphere onto a plane, except at one point: the projection point. It is conformal, meaning that it preserves angles at which curves meet.

The gnomonic projection is said to be the oldest map projection, developed by Thales in the 6th century BC. The path of the shadow-tip or light-spot in a nodus-based sundial traces out the same hyperbolae formed by parallels on a gnomonic map.>>
Last edited by neufer on Fri Mar 22, 2019 10:51 pm, edited 1 time in total.
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Re: APOD: Star Trails and the Equinox Sunrise (2019 Mar 21)

Post by Nitpicker » Fri Mar 22, 2019 9:27 pm

Thank you for the detailed answer alter-ego. I read all the same Wikipedia articles as neufer, and managed to start confusing and doubting myself. But then I checked a few of my wide angle sky images that included the equator by coincidence, and it was always slightly curved. This is because these images were never directed precisely at the intersection of two great circles.

There are a number of articles on the web that state words to the effect that the straight trail in a star trail image is always the equator. But it isn't in general. Not exactly.

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Re: APOD: Star Trails and the Equinox Sunrise (2019 Mar 21)

Post by Nitpicker » Sat Mar 23, 2019 6:59 am

Upon reading more, a lens with no distortions from a perfectly rectilinear/gnomonic projection should indeed make all great circles appear straight in an image. This includes the horizon, celestial and galactic equators, ecliptic, and all lines of constant azimuth and RA.

The "trouble" with my own wide-angle images (apart from the fact that they are rarely good enough to interest anyone but me) is that I use a zoom lens to get my widest angle with an 18 mm focal length (still only a ~66 degree wide FOV). Zoom lenses typically do show a small range of distortion from rectilinear, as the focal length is varied, and my particular zoom lens is no exception. So I do need to take care with my aim, if I want a particular great circle to appear straight (not that likely). Or I could adjust it in post processing (even less likely).

My humblest apologies. (And now I want a lens that can give me a stereographic projection, too.)