Transit of Venus

In 1716, Sir Edmond Halley realized that the predicted future transits of Venus in 1761 & 1769 could be used to solve a great problem (he never lived long enough to witness the transit himself). Your mission is to explain what problem could be solved and the "complications" that arose as a result of this quest. Hint: Part of the "complication" is depicted in the image above. Please answer the question using your own words (I hate copy/paste answers) ... and list your sources. You should be able to paraphrase the answer in a paragraph or two.

The big problem was that nobody knew the size of the solar system.  That is, the value of an astronomical unit (in miles or kilometers) was unknown.  All we knew (from Copernicus) was a scale model but only relative to our orbit (1 AU from the sun).  Halley realized that an upcoming transit of Venus could produce an answer.  Using a complicated algorithm, all one needed to do was observe the duration of the transit from two observers (whose location was well established) and do the math.  Halley never lived to see this experiment carried out but in 1761, it was first attempted.

In 1761, great lengths were taken to observe and measure the transit all over the world.  In fact, warring countries made a truce to allow safe passage to observing sites.  Hopes were dashed when it was discovered that all the observations were flawed by an optical disturbance known as the "black drop effect".  The image above shows a blurring that renders the exact times of the transit uncertain.  You can observe this effect first hand by slowly moving your thumb and index finger together until they almost touch.  You should observe a small "black drop" appear just before physical contact.  In addition, the exact longitude of the observers were in error which further flawed the data.

In 1769, the transit was again observed, but this time, a method of parallax would be used to determine the size of an astronomical unit.  Essentially, two observers triangulate to get the required distance.  The exact method is shown here.  The results yielded a distance of 95 million miles (fairly close to the modern value of 92.9 million miles).

This page goes into great detail about this famous experiment.