How was the transit of Venus used to determine the distance from Earth to the Sun?
Astronomers made measurements of the path of the transit from many different stations across the globe. Because of the parallax effect shown in the figure below, observers that were far apart (A and B) would see the path of Venus shifted in position on the face of the Sun. This parallax shift, together with the distance between the observers, lets astronomers determine how far away from Earth the planet Venus was. Because this number is known very precisely in terms of the Astronomical Unit (about 0.35 AU), once astronomers had determined the planet’s parallax distance in miles, they could now determine how large the Astronomical Unit was and so the distance from the Sun to Earth.
