Practical use of the transit of Venus

I didn't watch the event this past week - I'll plan to catch it next time in 2117.  The best summary I've seen of the scientific importance of the phenomenon is at Brain Pickings, which explains that in the past the transit of Venus was used to measure the size of the solar system:

In 1716, sixty-year old Sir Edmund Halley called on astronomers all over the world to leave their cozy observatories, travel to the edges of the known world, set up their telescopes, and turn their eyes toward the sunrise on the morning of June 6th, 1761, when the first Transit of Venus of the scientific age would march across the face of the sun.

In the eighteenth century, the solar system had a shape but not a size. By timing the entrance and the exit of Venus across the sun from latitudes all over the world, Halley explained, astronomers could roughly calculate the distance between the Earth and the Sun— a “celestial yardstick” for measuring the universe, as Andrea Wulf calls it in her excellent book Chasing Venus: The Race to Measure the Heavens...

Months before the 1761 transit, a fleet of international astronomers was dispatched to South Africa, India, Siberia, Mauritius, eastern Finland, Newfoundland, and the remote island of St. Helena with instructions to set up their massive telescopes in some of the most inhospitable places on Earth...

Eight years later, in 1769...[o]ver four hundred viewings were scheduled, including locations in Lapland and Baja California. The brightest minds of the Enlightenment rallied around the cause: Benjamin Franklin spearheaded the calculations in the colonies, and Captain James Cook shuttled a fleet of scientists and naturalists to Tahiti, where they viewed the transit with cloudless blue skies.  

Cook's diagram is the top embed.  More info at Brain Pickings, via BoingBoing.