The transit of Venus
1874 & 1882: The contribution of photography
In the 19th Century there was considerable progress in observation techniques, the invention of photography and its use in astronomy, as well as more thorough analysis of previous transits, were decisive factors that allowed improvement of the estimation of solar parallax.
The transits in the 19th century would again give rise to numerous expeditions in both hemispheres. And, once again, the results of each would be variable. Astronomers had placed a lot of faith in the use of photography, but most pictures from the transit of 1874 were blurry!
Astronomers were aware that this technique could be improved between the two transits, but began to wonder if it was useful to spend so much time and money for the next alignment.
However, these events are so rare that it was unthinkable to just give up trying to observe them, and new expedition plans were made.
In the United States , Simon Newcomb (1835-1909) took part in an expedition to Africa and managed to determine a remarkably accurate value for the Sun-Earth distance: 149.59 million kilometres (±0.31 million). He did this by combining data obtained from the 4 previous transits. Astronomers were disappointed with the uncertainty associated with this measurement but the scientific community decided that to improve the value, they would have to wait for future transits.
This was noted by Camille Flammarion (1842-1925) in an article he wrote for the 'Astronomy' journal in 1882 :