Drop phenomenon
The drop phenomenon or Black drops ( English black dropEffect ) is an optical phenomenon during a Mercury or Venus transit . Shortly after the second and shortly before the third contact during a transit, a narrow black “tear drop” appears, which seems to connect the disk of the planet with the edge of the solar disk.
This phenomenon made it impossible for astronomers in the 19th century to determine the exact time of the second or third Venus contact, which resulted in the distance between the earth and the sun being a few parts per thousand too small .
At the time, this phenomenon was considered evidence of the dense and thick atmosphere of the planet Venus . Bessel and Argelander were able to show as early as 1832 that the atmosphere-free Mercury can also deliver this phenomenon and that its occurrence depends on the instrument. Today it is known that the effect is caused by the limited optical resolution of the telescopes used: Due to the wave properties of light, every telescope has a limited resolution. The larger the lens opening of a telescope, the better its resolution and the better details can be reproduced. If Venus moves away from the sun's edge during the second contact, it is not possible - depending on the resolution - to distinguish whether a detail belongs to Venus or to the dark edge of the sun. A kind of dark bridge forms - the drop. The same thing happens when Venus approaches the sun's edge again during the third contact.
Observations of the transit of Venus on June 8, 2004 showed that the drop effect hardly occurred in telescopes with a large lens opening and good optics, while it was clearly pronounced in smaller devices. The effect could also be demonstrated in the transits of the atmospheric Mercury in 1999 and 2003.
The phenomenon exists independently of the perspective through instruments; For example, it is sufficient to bring two shadows closer to one another: the shadow "bulges" shortly before touching it. The same phenomenon can be observed when the thumb and forefinger are brought closer against a light background; shortly before the immediate contact, the space between them turns black.
Web links
- Explanation of the drop phenomenon at Mercury and Venus ( Memento from November 3, 2007 in the Internet Archive )
- The black drop effect (English; offline, use e.g. wayback machine)
- Sky and Telescope : Where was the black drop? (English; article not available at this address)