visual range Q

[JWintjes]

Ok, so since I'm not very good at physics here's what's probably a very simple question.

Assuming you have a ship with a freeboard of, say, 3m. What would be a realistic range at which someone on deck could detect and make sense of another ship's movement with an unaided eye? For the sake of the example let's assume the sea is calm and visibility is good.

Jorit, on whom it is slowly dawning that the telescope may be one of the most underrated inventions in naval history...

[Timmy C]

How large is the target being observed?

PS: Chuck posted a formula for this in the Sydney&Komoran thread in main.

[chuck]

Ok, so since I'm not very good at physics here's what's probably a very simple question.

Assuming you have a ship with a freeboard of, say, 3m. What would be a realistic range at which someone on deck could detect and make sense of another ship's movement with an unaided eye? For the sake of the example let's assume the sea is calm and visibility is good.

Jorit, on whom it is slowly dawning that the telescope may be one of the most underrated inventions in naval history...

That would depend on the size, shape, and design of the other ship. The movement of a surfaced submarine whose island is more or less symmetrical in outline front and back probably would be hard to make out until one are close enough to see the wake. On the other hand a large sailing ship with sky sails set would probably betray her movement to an observer far away not only by having a large visible sail high up, but also by showing how the sail is drawing in the general prevailing wind.

[Werner]

I also get the idea that determining the rate of course change at range and with smoke, splash and flash intervening was a highly subjective skill, almost an art. According to Friedman, the Japanese added a special new instrument together with a trained operator the only purpose of which was to estimate the course changes and rate of change of the target. This information was added to the WW.I-style plotting table by a separate automatic pen, and transmitted as a separate input to the gunlayers in each turret.

One can imagine that at 20-30Km these estimates are done by very subjectively analyzing the way superstructure parts are presented in the spotting scope from moment to moment.

[chuck]

You are confusing "detecting ship's movement by eye from deck level" with "measuring ship's movement with fire control optics".

[HMAS]

There's got to be one.
Do we work on whether it is a flat earth or the other silly idea it is round?

[JWintjes]

That would depend on the size, shape, and design of the other ship.

Talking about the average Quinquereme here - 30-40m long, perhaps 7m wide, 3-4m freeboard. Probably no sails.

Jorit

[Andy G]

Jorit, on whom it is slowly dawning that the telescope may be one of the most underrated inventions in naval history...

...But when was it first used?

Traditionally, Hans Lippershey (1570-1619) of Holland is credited - but he just popularised the idea. How difficult is it to put two convex lenses together in a tube?

I find this fascinating:

Democritus stated that the band of the Milky Way consisted of many stars which were faint because they were so far away.

You can't see the Milky Way as individual stars without a telescope - though even a poor one would do. But perhaps he was just extrapolating from his experience of naked-eye observing. However, Robert Grosseteste, writing in the 13th century in De Iride says:

Haec namque pars Perspectivae perfecte cognita ostendit nobis modum, quo res longissime distantes faciamus apparere propinquissime positas ... et quo res longe positas parvas faciamus apparere quantum volumus magnas, ita ut possibile sit nobis ex incredibili distantia litteras minimas legere ...

This part of Optics, when well understood, shows us how things a very long distance away can be made to appear as if placed very close ... and how small things positioned at a distance can be made to appear any size we want, so that it is possible for us to read at incredible distances the smallest letters ...

Grosseteste goes on to repeat Democritus' comment (whether or not he was aware of it) on the stars of the Milky Way. More compelling evidence: Leonardo de Vinci revealed in Codex Atlanticus (1490) that he was:

...making glasses to see the Moon enlarged.

A particulary natural home for telescopes is on board ships. Did Magellan sail with one? Columbus?

I think it's more than likely.

Andy

[JWintjes]

Jorit, on whom it is slowly dawning that the telescope may be one of the most underrated inventions in naval history...

...But when was it first used?

Traditionally, Hans Lippershey (1570-1619) of Holland is credited - but he just popularised the idea. How difficult is it to put two convex lenses together in a tube?

I find this fascinating:

Democritus stated that the band of the Milky Way consisted of many stars which were faint because they were so far away.

You can't see the Milky Way as individual stars without a telescope - though even a poor one would do. But perhaps he was just extrapolating from his experience of naked-eye observing. However, Robert Grosseteste, writing in the 13th century in De Iride says:

Haec namque pars Perspectivae perfecte cognita ostendit nobis modum, quo res longissime distantes faciamus apparere propinquissime positas ... et quo res longe positas parvas faciamus apparere quantum volumus magnas, ita ut possibile sit nobis ex incredibili distantia litteras minimas legere ...

This part of Optics, when well understood, shows us how things a very long distance away can be made to appear as if placed very close ... and how small things positioned at a distance can be made to appear any size we want, so that it is possible for us to read at incredible distances the smallest letters ...

Grosseteste goes on to repeat Democritus' comment (whether or not he was aware of it) on the stars of the Milky Way. More compelling evidence: Leonardo de Vinci revealed in Codex Atlanticus (1490) that he was:

...making glasses to see the Moon enlarged.

A particulary natural home for telescopes is on board ships. Did Magellan sail with one? Columbus?

I think it's more than likely.

Andy

I'll readily agree - actually, I have tried to use the Grosseteste text once to show that the ancient world may well have known about telescopes. The trouble is, they are not only totally absent from the archaeological record, but also almost totally from the ancient texts.

Hm. Perhaps this needs revisiting.

Jorit

[Andy G]

I'm currently looking at writing on a well-lit white board about 4m away from me, and I'd say I'd need a hull to cover about 15mm on the board (1/5th of a degree) before I would be reasonably confident about its heading and intentions.

Maximum human visual acuity is about 1/60th degree, so I'm a magnitude above this cut-off point, which feels about right.

At sea, the maximum sighting range due to the curvature of the Earth (neglecting refraction and assuming a calm surface) is about 14 kilometres for two objects 4m high - that height at that range is equivalent to maximum acuity, so it would be very easy to lose sight of the object in any sort of swell or heat haze.

At 10km, a 35m hull subtends my 1/5th of a degree requirement for confidence. Part of the hull is still below the horizon, but the length is good enough for a solid contact.

10km is around forty minutes' rowing - twenty if both ships turn towards each other.

Andy

[JWintjes]

Thanks Andy, that are some interesting figures! 20 minutes is not that much if your command and control relies on cumbersome methods of communication.

Interesting indeed...

Jorit

[chuck]

You can't see the Milky Way as individual stars without a telescope - though even a poor one would do. But perhaps he was just extrapolating from his experience of naked-eye observing. However, Robert Grosseteste, writing in the 13th century in De Iride says:

Andy

Well, You might not be able to see that every part of the smudge that makes up Milkyway consists of stars, but you can certainly see a higher density of faith stars in the Milkyway band, even from relatively light polluted places. Then it's merely a question of whether you are willing to make the leap to conclude that in fact the entire band consist of faint stars.

People should not necessarily credit ancients who made somewhat dodgy speculation that so happen to turn out to be correct with either spectacular insight or possession of amazing instruments unknown to us. The speculation that Milky way was made of stars was a dodgy one, but not an entirely groundless one, that so happen to beat the odds and turn out to be correct.

[JWintjes]

Well, You might not be able to see that every part of the smudge that makes up Milkyway consists of stars, but you can certainly see a higher density of faith stars in the Milkyway band, even from relatively light polluted places. Then it's merely a question of whether you are willing to make the leap to conclude that in fact the entire band consist of faint stars.

People should not necessarily credit ancients who made somewhat dodgy speculation that so happen to turn out to be correct with either spectacular insight or possession of amazing instruments unknown to us. The speculation that Milky way was made of stars was a dodgy one, but not an entirely groundless one, that so happen to beat the odds and turn out to be correct.

Chuck,

it's not Democritus that's strange - it's the de iride. One would assume that optical knowledge available in the 13th century was even more there in antiquity. Which begs the question whether they made any attempts at actually using it in any meaningful practical way.

Jorit

[chuck]

At 10500m, 3.1 m of the 4 meter hull will be visible above the horizon, and that 3.1 m will subtend exactly 1 minute of arc. So 10500 meter is the real absolute maximum range at which human eye can conceivably resolve a 4 meter freeboard hull under absolute ideal line of sight conditions.

Surface roughness, heat haze, lack of target contrast with background and other impairments will start taking their toll from there. If the quinreme's optical size is at the threshold of visual detection, than its tonal level had better stand out from the background for it to gain any notice.

My guess is at high noon, when contrast between the quinreme, the sky and and the sunlit sea will be at its least, you probably won't make out a quinreme moving without sails or banners until 6-7kms. Perhaps the visibility would be better when the sun is low behind the observer, when the quinreme will be billiantly lit while the sea dark.

A sail would, of course, make a huge difference.

[HMAS]

Gentlemen
Can anyone get hold of a boat small pleasure craft size where the radar dome would be about 4m above the water line?
going with a radar for an accurate distance reading will help a lot, esp during the yachting season,you could pick a boat about the size you require then track it visually until it dissapears & read off the radar. I no longer have the privilege of hopping a tugboat for a quick squirt out, to bring in or see a ship off these days. I cannot help.

RE stars etc, maybe in "olden times" the sky view was clearer less dust? I remember a colleague telling me about "ALDRIN?" or one of the astronauts anyway, as he made passes over the Brooklyn bridge he was able to see individual cars on the bridge. Of course ground control disbelieved him as the "eye" is not that good! However at a prearranged time next orbit or two, a chap was positioned to count the cars on the bridge as the astronaut passed over head, apparently the results tallied!
looking down through the atmosphere the eye could see better than across the atmosphere.

[Werner]

Atmospheric dust is at an all-time low, except in the Western Hemisphere in the latitude of the Sahara.

[Andy G]

Looking down (or up) through the Earth's atmosphere is the equivalent of looking through 11km of surface pressure gas - but it's obviously not as dusty as a horizontal section.

I don't know if the story attributed to Aldrin is true - but some early Gemini flights had astronauts observing airplanes and ships at the head of contrails and wakes. The linear form of the trail helped locate and define the point source at the head of it. A bit like being able to see the cable stays on a distant transmitter mast when the thickness of the stay places it under the minimal acuity.

For the 30-40m vessel at 10km, when the acuity cut-off is 4m, the hull's length could conceivably help detection.

Andy

[chuck]

Yes, indeed. Cars on the Brooklyn Bridge.

Apollo astronauts also allegedly claimed that the Great Wall of China is visible to the naked eye from the surface of the moon. Back of the envelope calculations show that, if the Great Wall is 5 meters wide, then a resolving power 200 times greater than those possessed by the Hubble telescope would be required to pick it out from the distance of the moon. The power of vision possessed by those astronauts must have been truly, how shall I put it, visionary?