Cook speculated that this phenomenon, which became known as the black-drop effect, was caused by "an atmosphere or dusky cloud round the body of the planet."

这个现象后来被称为「黑滴效应」，库克猜测是由「金星周围的大气或尘埃云」造成的。

youdao

There is also historical research to be done: for instance, what causes that meddlesome black-drop effect?

ECONOMIST: Transits of Venus

This was because they saw the black-drop effect in a transit of Mercury, a planet that has no atmosphere.

ECONOMIST: Transits of Venus