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From: Doc Johnson on 9 Jun 2010 12:17 On Wed, 9 Jun 2010 13:59:10 +0100, "whisky-dave" <whisky-dave(a)final.front.ear> wrote: >Earth is flat because the Earths surfacwe curves less than the 1/4 >wavelengh. IIRC A bad mix of apples and oranges but a fun concept if the surface were perfectly smooth, and then only applicable to one small portion of the earth at a time. Though I know one section of earth where it's even flatter, than that bad-analogy. A section of North Dakota. I surmise it has to do with the continental plains slowly rising to meet the Rocky Mountains further out west--this slow geological rise negating the curvature of the earth. When traveling through N. Dakota in 1979 to go photograph the total eclipse, I was driving all night just the night before--to catch up with the event next morning. I had been noticing that when I would first spot a car's headlights in the distance on the flat and straight roads (at 2-3 am there's so little traffic too), that it seemed like an astoundingly long time before our cars would pass each other. The duration being so long is what first caught my attention. When driving at night and you see another car's headlights you instinctively expect it to pass in an acceptable amount of time, even if it was sitting still on the side of the road in front of you. You can almost judge if a car is standing still or traveling toward you just by how long it's taking to meet those oncoming lights. This expected time delay becomes almost instinctual, not worth thinking about. Something was highly amiss that night. I decided to time just how long it took to meet up with some oncoming headlights (prompted by the environmentally induced boredom too). I was speeding along that night at 70-75 mph in order to be there on time for the solar spectacle. From the moment I first noticed the tiny glint of another oncoming car's headlights on the distant horizon (and looked down at the clock on the dashboard to note the time), until the time we passed each other, it took (drum roll please) ... 35 MINUTES. There was no break in seeing those headlights during this time so I know they were coming from the same vehicle. If the other car was traveling toward me at a similar rate of speed this means that I spotted another car about 20.4 to 21.8 miles away. Keeping in mind those headlights are only a couple feet from the ground and being seen from the viewing height in the average car-seat. Knowing that the distance of the horizon on a calm ocean as seen by a 6 ft. tall person from a standing height is only 3 miles away, it puts that section of N. Dakota's flatness into perspective. If viewing the ocean's horizon from a seated position knock off about another half mile or so, it's about 2 to 2.5 miles away. If wanting to see the ocean's horizon 21.5 miles away you'd have to do so from a viewing height of about 308 ft. That one stretch of road to Minot, N. Dakota should be in a record-book somewhere as the flattest spot on earth. (Until another area can be found to best it.) p.s. The eclipse photographs turned out spectacular. Using my OM2n I got some nice images of bright crimson prominences, among many other unique images. Localized events like crescents of light projected on snow-drifts by pin-holes formed between shadows of hands and fingers, etc. (I engaged some nearby kids on that impromptu viewer's-hill-gathering to help, and to help entertain and educate them.) Though I had no idea that it would get so dark that changing a roll of film during totality would be as difficult as it was, my most frantic film-change ever. A light overcast sky that morning (which at first concerned me but only added to the event) also afforded some specialized solar-eclipse effects. The light high-altitude haze acting as a mild rear-projection screen had relayed waves of rainbow colors in the sky just moments before totality approached. Then during totality the whole sky seemingly inverted in an instant, as if seeing the universe in negative. The darkest spot in the sky was the sun. It was astounding, right to the very core of your DNA. Afterward I stopped off at a large nearby truck-stop for an early lunch. About 300-400 others were there. (Minot had become an eclipse destination that morning.) You could have heard a toothpick drop in that large dining-hall. Nobody was talking during their whole meal. Only hushed and solemn whispers when placing orders or paying at the register. As if they were all put in their place for once in their small lives. Now knowing with absolute certainty for the first time in their lives that they cannot be in control of everything. They were all genuinely awe-struck by what had just occurred, as was I. I can only imagine how fearful someone would have been if they didn't know what had just taken place. Here it is 31 years later and I still get goose-bumps reminiscing about it. It's as if all throughout the evolution of life on earth that DNA has never quite been able to adapt to the solar eclipse experience. Your DNA doesn't know how to react to it. I understand the "eclipse chaser". Experiencing totality is a mental, emotional, and physical rush, unlike any other. That one in Minot that morning, with the high-altitude rear-projection screen unveiling another awe-inducing layer of its effects, even more so.
From: Doug McDonald on 9 Jun 2010 15:10 On 6/9/2010 11:17 AM, Doc Johnson wrote: > > Though I know one section of earth where it's even flatter, than that > bad-analogy. A section of North Dakota. I surmise it has to do with the > continental plains slowly rising to meet the Rocky Mountains further out > west--this slow geological rise negating the curvature of the earth. When > traveling through N. Dakota in 1979 to go photograph the total eclipse, I've only seen one total eclipse of the sun, in Costa Rica in, I believe, 1991. This eclipse was teh beginning of the eclipse craze; the travel company I went with was oblivious to it happening before I asked them if they could do a pre-existing trip "add-on" ... which they did, getting one other customer. After that, they have done lots of eclipse trips. This was awesome ... it lasted 7 minutes. I had a 600mm mirror lens to photograph it. This was on a camera on a small mini-tripod about a foot high, which turned out to be quite adequate. The thing of note was that the feet of the tripod were slightly awash in the Pacific Ocean. The beach was packed with eclipse-gazers (this beach, Manuel Antonio, is the most popular in Costa Rica.) Doug McDonald
From: Wolfgang Weisselberg on 9 Jun 2010 18:46 DanP <dan.petre(a)gmail.com> wrote: > On 8 June, 02:53, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com> > wrote: >> DanP <dan.pe...(a)hotmail.com> wrote: >> >> I guess you could do a thought experiment. >> >> If 2 lenses gather the same amount of parallel rays along >> >> their lens axis, they must have the same diameter. Go from >> >> there. >> > Right, at maximum lens aperture bigger lenses gather more light. >> > Close the bigger lenses aperture to match the small ones and you get >> > the same amount of light. >> >> Please clarify, what exactly do you mean "to match the small >> ones"? f/stop? Diameter? Area? What exactly? >> >> -Wolfgang > f/stop. > Big lens: 42mm diameter, 50mm focal length, f/1.2 aperture. > Small lens: 18mm diameter, 50mm focal length, f/2.8 aperture. > Set them both to f/2.8 or smaller and you get the same light. So use a 200mm as the big lens and a 50mm as the small lens and answer again. -Wolfgang
From: DanP on 10 Jun 2010 15:25 On Jun 9, 11:46 pm, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com> wrote: > DanP <dan.pe...(a)gmail.com> wrote: > > On 8 June, 02:53, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com> > > wrote: > >> DanP <dan.pe...(a)hotmail.com> wrote: > >> >> I guess you could do a thought experiment. > >> >> If 2 lenses gather the same amount of parallel rays along > >> >> their lens axis, they must have the same diameter. Go from > >> >> there. > >> > Right, at maximum lens aperture bigger lenses gather more light. > >> > Close the bigger lenses aperture to match the small ones and you get > >> > the same amount of light. > > >> Please clarify, what exactly do you mean "to match the small > >> ones"? f/stop? Diameter? Area? What exactly? > > >> -Wolfgang > > f/stop. > > Big lens: 42mm diameter, 50mm focal length, f/1.2 aperture. > > Small lens: 18mm diameter, 50mm focal length, f/2.8 aperture. > > Set them both to f/2.8 or smaller and you get the same light. > > So use a 200mm as the big lens and a 50mm as the small lens > and answer again. > > -Wolfgang 200mm? I am talking about lens diameter not focal length. DanP
From: Paul Furman on 11 Jun 2010 03:23
On 6/10/2010 12:25 PM, DanP wrote: > On Jun 9, 11:46 pm, Wolfgang Weisselberg<ozcvgt...(a)sneakemail.com> > wrote: >> DanP<dan.pe...(a)gmail.com> wrote: >>> On 8 June, 02:53, Wolfgang Weisselberg<ozcvgt...(a)sneakemail.com> >>> wrote: >>>> DanP<dan.pe...(a)hotmail.com> wrote: >>>>>> I guess you could do a thought experiment. >>>>>> If 2 lenses gather the same amount of parallel rays along >>>>>> their lens axis, they must have the same diameter. Go from >>>>>> there. >>>>> Right, at maximum lens aperture bigger lenses gather more light. >>>>> Close the bigger lenses aperture to match the small ones and you get >>>>> the same amount of light. >> >>>> Please clarify, what exactly do you mean "to match the small >>>> ones"? f/stop? Diameter? Area? What exactly? >> >>>> -Wolfgang >>> f/stop. >>> Big lens: 42mm diameter, 50mm focal length, f/1.2 aperture. >>> Small lens: 18mm diameter, 50mm focal length, f/2.8 aperture. >>> Set them both to f/2.8 or smaller and you get the same light. >> >> So use a 200mm as the big lens and a 50mm as the small lens >> and answer again. > > 200mm? > I am talking about lens diameter not focal length. He's talking about both. 200mm with 42mm aperture f/4.5 50mm with 18mm aperture f/2.8 or 200mm with 18mm aperture f/11 50mm with 18mm aperture f/2.8 |