Shadows on the ground

The HT lines are probably at a greater height so the shadow will spread out more. Nothing synister here. They are often grouped in fours per phase.

Has it got anything to do with the proximity of the cable to the ground and the resulting penumbra wedge shape?

That's technical.

Vorsprung durch Technik.

Both of these will not be visable to the human eye but could cause a shadow in the sun.

The cable does not move perceptibly at 60 Hz (you can resolve fine details in the cable). Would you not be using the naked eye to view the shadow too?

You are wrong I have seen shadows from over head power lines on many an occasion. It just depends on the angle of the sun an how bright it is.

I wasn't suggesting that you could not see shadows. If I am not mistaken, you would see the power lines and the shadow with the same eyes, true?

This...

Both of these will not be visable to the human eye but could cause a shadow in the sun.

... was a quote from the original poster.

HT cables are usually placed on much higher poles / towers, therefore projecting a larger shadow on the ground than thicker cables that are closer to it.

The shadow could not be bigger no matter how high.

It is so far form the sun that light travels in parallel lines and should be the same size.

Is not like a lamp which will cast larger shadows the closer the object but that is because of light travlelling conically compared to the distance

Correct! Comparing with 150 million km or the sun-earth distance, transmission tower height is next to nothing, by no means making line shadow any bigger.

I think Brainwave made a good point that HT lines are often grouped in fours per phase. The cluster shadow bigger than a single line.

And also I guess it is partly because the turbulent air surronding the HT line, caused by the heat out of high current passing through the HT line, may cast vague shadow on the ground, which makes line shadow look a little bit bigger.

The image of the sun in the sky has a dimension of several arc-seconds across. That parallax is what causes partial vs total lunar eclipses. The "fuzziness" of shadows from greater distances is the result of this. Try to find the shadow of a needle two feet high. It won't show.

The higher HT lines are a "softer" shadow showing wider due to the parallax.

richh

During a total eclipse of the sun the patch of totality on the surface of the earth is much smaller than the diameter of the moon. In fact it is of the order of just a 100 km. However the area over which a partial eclipse can be seen is larger than the moon's diameter. This is because the sun is about half a degree across on the sky and a completely dark shadow is possible only when the object in the sky is half a degree or more from the perspective of an ant on the ground. Otherwise you get a partial shadow or "penumbra"; which will be larger than the object.

Thanks everyone for your thoughts on this, I forgot about the heat thing, that is a very good suggestion. I don't agree with the height of the HT wires as the difference is only a matter of feet on the same pole and the source of light (sun) is so far away.

As far as seeing the shadow on the ground and not seeing the wires move, If you point your remote at a video camera you will be able to see the infra-red LED glowing. However, it is not visable with the naked eye.

Let me clarify what I meant re seeing vibrating things. An object oscillating at a speed greater than 30 Hz will appear blurred. For this reason, you cannot see the teeth on a running circular saw, you just see a blur. The shadow of such teeth is also a blur, and less dark than that of the the central disc of the blade. If the power line were moving side-to-side enough to spread the shadow noticeably, then the wire would look blurry and wide when you look up at it. But I can look up at a power line and clearly see the ridges formed from the wrapping of individual wire bundles, etc. If a shadow of an object is blurry because that object is moving, then the same effect makes it blurry when you look at it directly. You've probably seen aircraft propellers.

Human eyes see pulsed (30 Hz and up) light as continuously ON. (This "flaw" in vision is what makes movies possible.) E.g., fluorescent lights are pulsed, but they look like they are on continuously. They can be dangerous around moving equipment, for this reason. They can act as strobe lights, appearing to freeze moving equipment. (In fact, getting back to the saw blade, fluorescent lights have "frozen" saw blades, leading to accidents.)

So, the reason the camera sees the pulsed IR is not because the camera sees pulses and combines them into what appears to be coherent light (or eye naturally does that). Instead, it is because the ccd image sensor is much more sensitive to IR light than our eye is. (The CCD produces a usable electrical signal in response to IR. Our eye does not.) The camera can then produce a visible image on its screen. In normal mode, all cameras should not respond to IR -- in other words, you normally don't want hot things to appear brighter than cool things in ordinary video. But most video cameras and digital still cameras will display a visible indication of IR, simply because the sensor picks up IR. My still camera is quite good at this, and makes the light from an IR remote seem very bright. Night vision goggles, binocs, etc use this technique.

So, I think PWSlack and others are right re penumbra. We are taught that the sun's rays are parallel, and for climatological purposes, they are. But for producing sharp shadows, they are not. The sun is .52 degrees wide in the sky. If a cable is 10 meters high, the shadow spread, due to penumbra effect, is .087 M, so you'd expect shadows from high objects to be diffuse, and wider than the object. If the sun were even wider in the sky (like a photographer's umbrella) we'd completely loose distinct shadows, as we do on an overcast day.

Maybe each "Guest" could assign themselves a number or a key word so that we could recognize who each guest is in a thread. let's say a prompt asking guest for a thread name, only for that thread. I'm sorry, that would make too much sense. Guest 27609

RichH

You're right, that makes far too much sense! Even signing with a first name would help. Often there can be two guests appearing to be one... and then they appear to drift apart into two (based on content of the discussion), etc. It can make a coherent discussion a little difficult.