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Black Drop Effect
Initially seen with the Transit of Venus, but actually present in many other optical situations!
The Black Drop Effect is an optical effect which has historically been associated with the transit of the planet Venus. When the planet Venus goes across the disc of the sun (which only happens twice every hundred and so-many years), astronomers are keen to observe it. In 1769 some intrepid explorers went to Tahiti to observe the transit of Venus and to make precise measurements of the timing of it. By this means it was possible to use parallax (with other observations in other parts of the world) to calculate how far away Venus was, and therefore how big the Solar System was.
This was an amazing achievement for the time, but it was marred slightly by a mysterious effect, the Black Drop Effect, which caused the timings to have inaccuracies.
What the Black Drop Effect is:
When the tiny black disc of Venus is very close to the edge of the inside of the large luminous disc of the sun, the circular shape of the planet's disc is oddly distorted into a black teardrop. It's as if shadow encroaches across short gaps against a brightly lit backdrop.
Early explanations included the idea that the planet Venus had a thick atmosphere which muddled up the edge. I am shocked to find in 2012 this sort of explanation is still around even on scientific websites about astronomy! Why shocked? Because a transit of Mercury also has Black Drop Effect even though the planet is without a thick atmosphere, but moreover, your thumb and forefinger also has the Black Drop Effect when put close together with a bright backdrop. As your thumb and forefinger do not have a thick planetary atmosphere, the idea it's anything to do with planetary atmospheres really does need to be gone.
What the Black Drop Effect is Really:
When any dark objects have a close proximity and there is a bright backdrop, the effect is as if the shadows encroach across the gap. A shadow bridge is created, leaving a dark region inbetween the objects. In the experiment where the objects are your own forefinger and thumb, you know they are not touching because you can feel it, and yet your eyes can see darkness across the gap. If you vary the gap distance, you can see shadows seem to gravitate to one-another, as if the effect is sticky. You don't need to take my word for it; you can try it yourself and observe the phenomenon yourself.
As if that wasn't odd enough and you might be wondering if you're about to find out how deep the rabbit hole goes, it gets worse, because you can take photographs of the effect! There is clearly something going on, and it's something observable directly and it is also evident by digital imaging. This tend to put-paid to the sceptics saying it's all in the imagination.
What causes the Black Drop Effect:
First of all, let's establish what the Black Drop Effect is not. There have been a variety of "explain it away" explanations of the form "Oh it's just...", but for one thing a lot of these are wrong, and also the idea of explaining away a phenomenon is bad for progress. Semiconductors could have been "explained away" by saying it was just bad contacts and the conduction being imperfect!
So now let's go over a set of things which have been suggested, or even stated for certain, and which are now known to be inadequate explanations for the Black Drop:
The black drop effect is not planetary atmosphere effects of distant worlds, as it can be observed with small objects such as human digits, shaped pieces of Blu-Tack in variable spanners, and carefully cut pieces of cardboard, with lightbulbs in rooms.
The Black Drop Effect is also not quantum mechanical effects, nor diffraction, nor microscopic effects to do with the wavelength of light. These can be dismissed from the Black Drop Effect theorising because the effect has been observed with large objects such as planets.
Even Wikipedia 2012 got it wrong with saying it was the Earth's atmosphere.
It is also not purely a biological subjective sensor optical illusion, as tangible digital evidence can be acquired by taking photos of it.
Another suggestion is that it's "faults in the viewing apparatus". Well, if that means "your eye", it sounds plausible. Except that the effect can be photographed with a digital camera, and that's not a fault in your eye. Admittedly the camera could have a fault, but it seems odd that it's the same fault as the eye.
I have made some observations and done some experiments myself and have noticed some interesting things:
* The effect of shadows encroaching across gaps is much more intense if the backdrop is more bright. This suggests it is an effect associated with contrast. However, the effect can also be observed in low light conditions.
* As a gap narrows, shadows reach across the gap, altering the visual shape of the outline, counter to known geometry. Visually the gap is often closed before the objects meet.
* Pinholes and narrow slits with light shining through against a dark background do not exhibit this phenomenon. Almost the opposite occurs, where they appear brighter and more extended than their physical dimensions suggest.
* A thin cable passed in front of a light source seems to have part of it disappear momentarily.
* Sources of lighting in observations of the black drop effect are typically spread over a surface. This could lead to the idea that it might be "penumbra", although I've yet to experiment into that. I've noticed that with a lightbulb 3ft away, the black drop is very fat, whereas with a lightbulb 50ft away, there's hardly any black drop at all. For further examination of contrast-effect, see close-up of a black-drop-effect image
* A pair of small coins Blu-tack'd to the jaws of a variable spanner exhibit surprisingly small amounts of black drop effect, whereas a pair of spherical snooker balls have a much more pronounced black drop effect even when the gap between them is small.
Experimentation continues on this. Sorry I haven't yet got a complete answer to the black drop question yet, but at least it's a start!
If you don't believe it, take a closer look at this picture