stoneonstone wrote:I have a 5 mile non-locked canal separating me from the mainland. And I can see one end to the other, no problem. Markers, walls etc. Doing the math (something I don't do, apart from basics at tax time), I realize that there should be a 16 and a half foot drop from one end to the other, so I should not be able to see what I do. Sure, some can say that the atmosphere is a corrective lens on this, and 'nothing to see here'.
Maybe next year I'll have to arrange my own Bedford Level test here.
The Lake Ontario shipping lanes are out, 10 to 25 miles out from here as well. Maybe my math is wrong, but doesn't that mean over a 66 foot difference, to a 416 foot difference for those distances out? Why can I see the lakers then plying those lanes...hull at waterline up, and not just a telecom mast or something?
The distance to the visible ocean horizon depends on a number of things and can vary massively. Good page about it here:
http://www-rohan.sdsu.edu/~aty/explain/atmos_refr/horizon.html So the nice-looking formulae for calculating “the distance to the horizon” are really only rough approximations to the truth. You can consider them accurate to a few per cent, most of the time. But, occasionally, they will be wildly off, particularly if mirages are visible. Then it's common to see much farther than usual — a condition known as looming.
How far can you see?
Still, even with those caveats, it's of interest to consider how far the eye can see under different conditions. Usually the visibility is limited by scattered light in the lower atmosphere; see
Craig F. Bohren and Alistair B. Fraser
“At what altitude does the horizon cease to be visible?”
American Journal of Physics 54, Issue 3 (March, 1986) p. 222
for details. Even under extremely clear conditions, it's unusual to see more than a couple of hundred kilometers.
However, there is one situation in which objects can be made out at great distances: when they are silhouetted against a bright background, such as the setting Sun, or (just after sunset) a bright twilit sky.
Here in San Diego, we rarely can see San Clemente Island, about 125 km offshore. The top of the island should just be visible above our horizon with normal refraction, but it's concealed by “airlight” during the day. Even in the clear air of a “Santa Ana,” which causes looming and raises more of the island above the apparent horizon, it's often hard to make out.
But just after sunset, the island is often visible, if you know where to look. The air between you and the island is only dimly illuminated after sunset, but the sky behind the island — i.e., the air beyond the horizon that is still in direct sunlight — is still fairly bright. Then the silhouette of the island is striking, even if it had been invisible a few minutes before sunset.
The Sun itself can be seen through a long duct when it is several degrees below the astronomical horizon; however, its image is then so distorted that any intervening terrestrial object (such as an island, a mountain, or even a cloud) would probably also be so distorted that its silhouette against the Sun would be unrecognizable. Some extreme claims can surely be discounted, such as Jessen's 1914 illusion. (Jessen claimed to have seen a mountain nearly 900 km away, but he certainly did not; Korzenewsky (1923), who refers to this report in a footnote, somehow inflated that to 1177 km.)
What's the record for visibility without help from the silhouetting effect? I think that might belong to the report of the expedition led by Korzenewsky (1923), who reported seeing snow-capped peaks of a mountain range 750 km away. Conditions were perfect: the lower atmosphere was in shadow at sunset; the peaks were quite high (4650 meters, or over 15,000 feet); they were covered with white snow, increasing their visibility; and there must also have been considerable looming to bring these distant features above the observers' horizon. As the observation was made on June 1, near the peak of superior-mirage season, the looming is not improbable, though the amount required is hard to believe. The observers themselves were in the deserts of Turkestan [now southeastern Kazakhstan] at a height of nearly a kilometer, where the dryness of the air favored extreme clarity, and looking across a broad, sandy depression. And, of course, much of the air path was in thinner air well above ground level, because of the mountains' height.
When I worked on boats, the rule of thumb was 9 to 12 miles to see the top of another vessel appear on the horizon -- which I did many times, watching the full height of the ship emerge vertically as it sailed toward us over the
curve of the Earth's surface. It's eerie through binoculars.
Once I was camping in the mountains -- true story -- and watched two sunsets on the same evening. From a lake at the bottom of an almost vertical mountainside, I watched a gorgeous sunset. Then I ran as fast as I could up the switchback trail to my camp, gaining a couple hundred feet in elevation, and there was the full disc of the sun on the horizon. I watched it once again slowly disappear under the horizon.
This proves that there are two suns. 