NavList:

George Huxtable, you asked:
"What does "usually" mean, in this context?"

Bad choice of a word on my part. Sorry. I should have said something like "one reasonable choice in the absence of further evidence".

And you wrote:
"Why must the scale distortion be the same in one direction (largely circumferential, about the centre of the image) as it is radial? Far from it."

Well, no, not "far from it". That could easily be the case. There's nothing wrong with your analysis as a purely theoretical model. Trouble is, cameras and other optical systems are designed with various sorts of distortions built in which go around the theoretical argument you've proposed. Mike Burkes described some of the issues (which he added to his post "Ocean Nav Problem" in case anyone missed it). Please understand, I'm not saying your pinhole camera analysis is wrong --only that it is a special case which would certainly apply to some optical systems, but we just don't know here.

We've got very little evidence to work with in this midnight sun photo, and my numbers for the azimuths and altitudes away from the center of the photo were little more than a consistency check. Does it all make sense? As it turns out, the numbers ARE reasonably consistent with the apparent diameter of the Sun near the center of the image and the apparent altitude we might find around August 10, 1947. We can't do any more than that because we don't have any other photos to calibrate the camera's optics, and all we know from the description is that the photos were taken "about" twenty minutes apart. If we had a few simple photos for calibration, it would be easy enough to do much more detailed analysis work on this photo (though there's hardly any reason to!). This is standard stuff in photogrammetry and astrometry.

Meanwhile, for those who have been to Mystic Seaport, there's a connection. Isn't there always? :-) This midnight sun photo was taken during one of the last Arctic cruises of the schooner Bowdoin. Bowdoin made over a dozen such cruises starting in the 1920s. It's a schooner built on the typical lines of a New England fishing schooner and would not have looked out of place on the Grand Banks at the time. But it was built specially armored for work in the ice floes up north. It survived many groundings and collisions with ice. In 1959, a few years after Adm. MacMillan's last Arctic cruise, Bowdoin was sold to Mystic Seaport where it was on exhibit for nearly a decade. She rotted pretty quickly while there, and in late 1967 (I know this indirectly since I was only four years old at the time), there was a bit of an uproar when MacMillan's widow publicly denounced Mystic Seaport for allowing Bowdoin to deteriorate. The museum at that time had an uncertain vision and owned more historic vessels than it could maintain, so I'm sure that the criticism was at least partly correct. A few months later, the Bowdoin was sold to an organization in Maine committed to restoring the vessel, and it is still sailing today, now owned by the Maine Maritime Academy.

The arctic voyage of the Bowdoin under MacMillan in 1947 was described in a feature article in the October, 1951 issue of National Geographic magazine. A version of the midnight sun photo was printed (I believe "colorized" as was still common in magazine production back then) and also a nice photo of the Bowdoin very near the location where that midnight sun time-exposure was taken. I am attaching the latter. You'll note that those cliffs are fairly high so the dip correction may be a bit larger than I imagined! From the photo I had guessed that the photographer was well above sea level based on some perspective clues. I assumed 50 feet for height of eye, but 150 feet might be more like it. That's only an extra 5' or so for dip. It wouldn't really affect the date of the image much. By the way, the accompanying article, despite the typical "data-free" style of National Geographic articles does seem to make it clear that this image was taken in late summer, ruling out the alternative date in May.

The linked image is taken from a CD version of NG released c.1999. For reference, here's the caption for the midnight sun photo as printed:
"Midsummer Night's Dream at Refuge Harbor: Multiple Suns Light the Polar Sky. This is the midnight sun. To make the picture, the camera was placed on firm ground overlooking this north Greenland bay (about 78d 30' N). The shutter was snapped every twenty minutes; the sun in the center was shot exactly at midnight. ..."

And the caption for the photo of Bowdoin:
"Rust-colored Cliffs Overhang Bowdoin in a Fjord near Etah, Greenland. Sails and Diesels Are at Rest. The 88-foot schooner, veteran of more than 30 years of Arctic exploration, is one of the smallest ships to venture this far north, but also one of the toughest. Her oaken hull wears a girdle of ironwood; 22 tons of cement reinforce her bottom. Steel-armored bow smashes through heavy ice."

-FER

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