The importance of Venus transits starts with the famed astronomer Johannes Kepler, who, in the early 1600s, was the first person to figure out the shape and properties of planetary orbits. His three laws of planetary motion allowed Kepler to figure out the relative size of the solar system. If we call the average distance between the Earth and the Sun as 1 Astronomical Unit, then Kepler knew the distances to other planets in terms of this unit. For example, he knew that Venus was about 0.7 Astronomical Units. The problem was, Kepler didn't know what an astronomical unit was in terms of familiar units like miles or kilometers.
In 1687, Sir Isaac Newton published his book Philosophae Naturalis Principia Mathematica (Mathematical Principals of Natural Philosophy). In this book, Newton introduced his now famous laws of motion and law of gravity, and he found that Kepler's Laws were a natural consequence of his laws of motion and gravity. In particular, Newton knew that the force of gravity between two objects depended on the mass of each object (how much "stuff" it is made out of) and the distance between the objects. So, if the distances were known, the masses of the objects could be figured out.
Thirty years later, British astronomer Edmund Halley (of Halley's Comet fame) realized that careful observations of a transit of Venus across the sun would allow scientists to determine the size of an astronomical unit, which would in turn allow scientists to calculate the mass of the Sun (as well as the actual sizes of all the known planets). The reason for this is geometry. People at different points on the Earth would see Venus take a slightly different path across the sun because they were viewing Venus from a slightly different angle. Because the size of the earth is known, a little geometry would give us the size of the astronomical unit in miles or kilometers (see picture below). The next transit of Venus wasn't until 1761, which gave expeditions nearly 50 years to prepare.
|A not-to-scale image showing how observers on different parts of the Earth would see Venus cross the sun at slightly different places. In reality, the Earth and Venus are the same size, the sun is much, much larger and much, much further away, and the difference in paths much smaller. Image credit: Wikipedia Commons|
|An actual photograph of the black drop effect from the 2004 transit of Venus across the sun. The "bridge" that appears to connect Venus with the edge of the sun is an illusion caused by Earth's atmosphere and limitations of telescope optics. Image Credit: Stanislava Simberova, Ondrejov Observatory, and ESO|
These days, we don't need a planetary transit to measure distances in the solar system. We can use laser and radar ranging to determine distances. But the 1761 and 1769 transits of Venus were part of the transformation of astronomy from a descriptive science to the mathematical and physics-based science it is today.