The black drop effect is not an atmospheric phenomenon
The so-called black drop effect is an optical phenomenon which can be seen during transits of Mercury or Venus in front of the sun. It can only be observed through a telescope protected against the bright sunlight. When the planet begins to cover the sun, it seems as if the silhouette of the planet would form a kind of black drop when it detaches from the rim of the solar disk. The same effect appears again when the silhouette touches the rim of the solar disk at the end of the transit. It looks as if the planet merges with the rim of the solar disk like two converging drops of water.
Originally astronomers thought that this phenomenon was caused by different refraction of light in the atmospheres of the planets. But today we know that the phenomenon is caused by the limited resolving capacity of the telescopes used. In this context experts often refer to an experiment which everybody can realize using his own fingers:
Just form a ring with your thumb and your trigger finger, but exactly so that the fingers just do not touch each other. Hold this narrow gap in font of your eyes, so near that they cannot focus it. A “shadow bridge” appears between the fingers, especially when the fingers are held in a different distance from the eyes and you start closing the gap by changing the perspective. The shadow bridge then moves from the finger which is further away from your eyes to the closer one.
Important for the successful execution of this experiment is that your eyes are defocused. If you move the fingers away from your eyes so that they can focus them, the shadow bridge completely disappears.
I slightly modified and analyzed this simple experiment. Instead of two fingers, I only used one, but in front of a pattern of blue and white stripes.
With this method I observed two sources of fuzziness , which are the silhouette of the dark brown finger and that of the stripes. In front of the dark stripes, the area of fuzziness of the finger appears more tangent than in font of the white ones. This gives the impression of the finger being as double as wide in front of the blue stripes compared to the white ones. In reality, however, the fuzziness of the finger is always the same as I tried to keep it parallel to the background and perpendicular to the line of sight. The most interesting area is that where the fuzziness of the finger meets the fuzziness of a blue line. There it also causes a deeper tinting of the blue area. As a consequence, a kind of dark “mound” forms in the zone of fuzziness of the blue area which points in direction to the finger tip. Moving the finger so, that its silhouette touches the outer rim of the fuzziness between the blue and the white line makes a shadow bridge appear.
Using this knowledge, you can easily simulate a transiting planet yourself. The experiment is very simple. Just draw a white circle with a black background on your computer and print it. Then die-cut a circle out of a sheet of black paper using a hole puncher. You only need the chad to represent the planet. Put this on a clear CD-cover and put this onto your printed solar disk, so that the planet lies as near to the rim of the solar disk as possible. Here are an animation and two photos illustrating the black drop effect, an exactly focused one with no black drop effect, and another, defocused one, in which the black drop effect appears.
Author: Reinhard Nitze, Barsinghausen, Germany