A transit of Venus is similar to a solar eclipse, occurring when the planet comes between the Earth and the Sun. Because the planet is so much further away than the Moon, it appears as only a tiny circle on the bright disc of the Sun.
Just as with an eclipse, where you are on the Earth determines what you can see and when. For two widely spaced observers, there are measurable differences in the exact timing and length of Venus path across the sun. With the separation of the observers taken as a base line, the distance to the sun can be calculated.
Edmond Halley was the first to develop a method for making the complex
calculations. He published his most important paper on the transit of Venus
in 1716, knowing he would not live long enough to witness the next transit