Black Drop

An appearance seen at the end of second contact and at the start of third contact of a transit of Venus. As the planet moves across the Sun’s disc it seems to draw a strip of blackness after it.  It makes measurement of the exact time of these contacts difficult to measure accurately.

The black drop has been known since telescopic observation of the transits of Venus and Mercury began with several explanations put forward. It now appears, according to the AAS, that is is a combination of the Sun’s limb darkening and the telescope that produces this effect.


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