Edmond Halley had envisaged a method in which observers could agree on the timing of a transit to the precise second.
The most crucial moment was “internal contact”, when Venus should have been a perfect circle touching the edge of the Sun. Instead, it often appeared elongated and distorted. How could a precise instant of contact be established when Venus was afflicted with this problem of the “black drop”?
Even if an expeditions observers could reconcile their own results, there was the further problem of comparing them with others from around the world.
Working with a mass of data required not just mathematical proficiency and stamina, but the ability to select. Precision astronomy here depended ultimately on individual judgement.