Black drop effect

The drop phenomenon (english black drop effect) is an optical phenomenon during a Mercury or Venus transit. Shortly after the second and just before the third contact during a transit, a thin black " tear-drop ", which seems to connect the disk of the planet to the edge of the sun appears.

This phenomenon made ​​it impossible to astronomers in the 19th century, the exact time of the second or third Venus contact to determine what is preventing the accurate determination of the Earth-Sun distance failed.

At the time this phenomenon has been viewed as evidence of dense and thick atmosphere of the planet Venus. Today it is known that the effect of the limited optical resolution of the telescopes used caused: Due to the wave properties of light each telescope has a limited resolving power. The larger the lens aperture of a telescope is, the better its resolution and the better details can be imaged. Removes the Venus during the second contact from the solar limb, can - depending on the resolution - not possible to distinguish whether a detail to Venus or to the dark edge of the sun is one. It is formed from a kind of dark bridge - the drops. The same happens when Venus approaches the edge of the sun again during the third contact.

Observations of the transit of Venus on 8 June 2004 had shown that in telescopes with large aperture lens and good optical effect of the drops hardly occurred while he was clearly pronounced in smaller devices. The effect could also be demonstrated at the transits of airless Mercury in 1999 and 2003.

The phenomenon exists independently of the view through instruments; it is sufficient, for example, two shadows to approach each other, " bulges " Just before touching the shadow off. The same phenomenon can be observed when one approaches the thumb and index finger against a bright background; shortly before the direct contact of the space is black.