The black drop effect is an optical phenomenon visible during a transit of Venus and, to a lesser extent, a transit of Mercury. == Description == Just after second contact, and again just before third contact during the transit, a small black "teardrop" appears to connect Venus's disc to the limb of the Sun, making it impossible to time the exact moment of second or third contact accurately.
The black drop effect is an optical phenomenon visible during a transit of Venus and, to a lesser extent, a transit of Mercury. == Description == Just after second contact, and again just before third contact during the transit, a small black "teardrop" appears to connect Venus's disc to the limb of the Sun, making it impossible to time the exact moment of second or third contact accurately.
Source: Wikipedia "Black drop effect" · CC BY-SA 4.0
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