Comments on: Pinhole astronomy http://blogs.discovermagazine.com/badastronomy/2008/10/29/pinhole-astronomy/ I am an astronomer, writer, and skeptic. I likes reality the way it is, and I aims to keep it that way. My real name is Phil Plait, and I run the Bad Astronomy blog. Mon, 09 Nov 2009 08:08:51 -0600 http://wordpress.org/?v=2.8.4 hourly 1 By: Ian O'Neill http://blogs.discovermagazine.com/badastronomy/2008/10/29/pinhole-astronomy/comment-page-1/#comment-130491 Ian O'Neill Sat, 01 Nov 2008 10:44:09 +0000 http://blogs.discovermagazine.com/badastronomy/2008/10/29/pinhole-astronomy/#comment-130491 Woohoo! That's my hometown! Most interesting view of Clifton Suspension Bridge ever! Loving it :-) (My dad lives just to the left of this image, I'll be sending him this, he'll love it!) Cheers Phil! Ian :D Woohoo! That’s my hometown! Most interesting view of Clifton Suspension Bridge ever! Loving it :-)

(My dad lives just to the left of this image, I’ll be sending him this, he’ll love it!)

Cheers Phil!

Ian :D

]]> By: Kelly http://blogs.discovermagazine.com/badastronomy/2008/10/29/pinhole-astronomy/comment-page-1/#comment-130211 Kelly Fri, 31 Oct 2008 07:59:54 +0000 http://blogs.discovermagazine.com/badastronomy/2008/10/29/pinhole-astronomy/#comment-130211 That is just an awesome image! Thanks for posting it! Love this blog, BTW! That is just an awesome image! Thanks for posting it! Love this blog, BTW!

]]> By: Tom Miller http://blogs.discovermagazine.com/badastronomy/2008/10/29/pinhole-astronomy/comment-page-1/#comment-130172 Tom Miller Fri, 31 Oct 2008 04:13:26 +0000 http://blogs.discovermagazine.com/badastronomy/2008/10/29/pinhole-astronomy/#comment-130172 The "screwy geometry" of the sun's path is a result of the image being recorced on a curved film plane. Typically these cameras are made from 35mm film canisters. Imagine looking down at the film canister. The pinhole is at the front of the circle, with the photographic paper wrapped nearly completely around the circle. Rather than imagine, there are drawings and step-by-step instructions on the www.solargraphy.com website. Look for the "how to" link. It will make sense then. The “screwy geometry” of the sun’s path is a result of the image being recorced on a curved film plane. Typically these cameras are made from 35mm film canisters. Imagine looking down at the film canister. The pinhole is at the front of the circle, with the photographic paper wrapped nearly completely around the circle. Rather than imagine, there are drawings and step-by-step instructions on the http://www.solargraphy.com website. Look for the “how to” link. It will make sense then.

]]> By: John Lerch http://blogs.discovermagazine.com/badastronomy/2008/10/29/pinhole-astronomy/comment-page-1/#comment-130061 John Lerch Thu, 30 Oct 2008 17:08:33 +0000 http://blogs.discovermagazine.com/badastronomy/2008/10/29/pinhole-astronomy/#comment-130061 There are only about 30 tracks, so roughly speaking he took the camera in and then out again about a week later (presumably on a day the weather should be good. The weather has roughly a 7 day period.) But he seems to have had some strange algorithm for how he was going to line it up (he didn't line it up on the bridge--the bridge was exposed only on one day or so, maybe a few days, there is a gap above the bridge that might be the bridge obscuring part of one track; he seems to have obscured the horizon most days). He seems to have lined it up according to the rising of the sun so that the sun always rose at the same spot. Then in the summer when the sun skims the horizon in northern latitudes, the arc curved toward the horizon in the sun's setting sequence. A pity he didn't have a translucent paper with an image of the bridge on which to align or better yet on some object due south of his location. BTW re overexposure--film solarizes so white becomes black and approximately vv. There are only about 30 tracks, so roughly speaking he took the camera in and then out again about a week later (presumably on a day the weather should be good. The weather has roughly a 7 day period.) But he seems to have had some strange algorithm for how he was going to line it up (he didn’t line it up on the bridge–the bridge was exposed only on one day or so, maybe a few days, there is a gap above the bridge that might be the bridge obscuring part of one track; he seems to have obscured the horizon most days). He seems to have lined it up according to the rising of the sun so that the sun always rose at the same spot. Then in the summer when the sun skims the horizon in northern latitudes, the arc curved toward the horizon in the sun’s setting sequence. A pity he didn’t have a translucent paper with an image of the bridge on which to align or better yet on some object due south of his location.
BTW re overexposure–film solarizes so white becomes black and approximately vv.

]]> By: John Lerch http://blogs.discovermagazine.com/badastronomy/2008/10/29/pinhole-astronomy/comment-page-1/#comment-130048 John Lerch Thu, 30 Oct 2008 16:28:21 +0000 http://blogs.discovermagazine.com/badastronomy/2008/10/29/pinhole-astronomy/#comment-130048 Is the screwy geometry of the sun's path some manifestation of projective geometry? I'm not very fond of projective geometry, but if no one posts an answer I guess I'll have to investigate. I can understand the point of highest elevation being an effect of projective geometry; but I can't see any reason for the sun coming up straight at sunrise (and from the same point!) and grazing the horizon at sunset. Is the screwy geometry of the sun’s path some manifestation of projective geometry? I’m not very fond of projective geometry, but if no one posts an answer I guess I’ll have to investigate.
I can understand the point of highest elevation being an effect of projective geometry; but I can’t see any reason for the sun coming up straight at sunrise (and from the same point!) and grazing the horizon at sunset.

]]> By: Lars http://blogs.discovermagazine.com/badastronomy/2008/10/29/pinhole-astronomy/comment-page-1/#comment-130046 Lars Thu, 30 Oct 2008 16:23:37 +0000 http://blogs.discovermagazine.com/badastronomy/2008/10/29/pinhole-astronomy/#comment-130046 I agree with mighty favog. All rays entering any camera from a distance are nearly parallel, so that's irrelevant. Theoretically, if all light from the landscape were to enter the camera through the very same point (the size of one photon?), the image on the film side would be in perfect symmetry with the real world, and so the picture would be infinitely sharp. But then again, (almost?) no light would make it through this pointhole, so I suppose that exposure time would have to approach infinite, too. Or maybe there are some weird quantum effects that I'm completely ignoring. I agree with mighty favog. All rays entering any camera from a distance are nearly parallel, so that’s irrelevant.

Theoretically, if all light from the landscape were to enter the camera through the very same point (the size of one photon?), the image on the film side would be in perfect symmetry with the real world, and so the picture would be infinitely sharp.

But then again, (almost?) no light would make it through this pointhole, so I suppose that exposure time would have to approach infinite, too. Or maybe there are some weird quantum effects that I’m completely ignoring.

]]> By: John Lerch http://blogs.discovermagazine.com/badastronomy/2008/10/29/pinhole-astronomy/comment-page-1/#comment-130023 John Lerch Thu, 30 Oct 2008 15:40:34 +0000 http://blogs.discovermagazine.com/badastronomy/2008/10/29/pinhole-astronomy/#comment-130023 Ok. I give up. I know the earth goes around the sun in an ellipse so the solar day is not 24 hrs long (shorter near periphelion) and longer at aphelion.) But WHY in this picture does the sun rise at the same point day after day and set at different points? And it sets skimming the horizon, whereas it comes straight up. I may well have submitted this; I was racing outside to see the second sonic boom so I don't know if this went through. I am resubmitting in order to ask the second question. Why is the point of maximal elevation of the sun changing? Due south changes as the year progresses? Unless my understanding of the spherical trig is wrong (it shouldn't be--I was the only one getting the spherical trig questions about the sun right in trig class), there is some fundamental fudge in this picture--like the non smudged bridge maybe? IOW did he move the camera? Ok. I give up. I know the earth goes around the sun in an ellipse so the solar day is not 24 hrs long (shorter near periphelion) and longer at aphelion.) But WHY in this picture does the sun rise at the same point day after day and set at different points? And it sets skimming the horizon, whereas it comes straight up.
I may well have submitted this; I was racing outside to see the second sonic boom so I don’t know if this went through.
I am resubmitting in order to ask the second question. Why is the point of maximal elevation of the sun changing? Due south changes as the year progresses?
Unless my understanding of the spherical trig is wrong (it shouldn’t be–I was the only one getting the spherical trig questions about the sun right in trig class), there is some fundamental fudge in this picture–like the non smudged bridge maybe? IOW did he move the camera?

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