Back in the 18th century, observing the Transit of Venus took a ridiculous amount of effort, involving ships, draft animals, wagons with wooden wheels, and telescopes made by the best optics engineer in the world. Today — say it with me — there's an app for that.
In 1716, Edmond Halley (yes, the comet guy) asked the world scientific community to mount massive expeditions in 1761 and 1769 to watch Venus cross in front of the Sun. He expected that by comparing the observations from different points around the globe (called parallax), astronomers would be able to calculate the Earth's distance from the Sun.
As detailed in historian Andrea Wulf's recent book, Chasing Venus: The Race to Measure the Heavens (Knopf), European nations (and the American colonies) took Halley up on his proposal in 1761 and again in 1769, sending astronomers to the far reaches of the planet.
The expeditions took years, and assembling the results and making the calculations took even longer. It wasn't until 1771 that the British Royal Society was ready to declare a result: 93,726,900 miles. That's less than one percent different than the present calculation of 92,960,000 miles.
We don't need your help measuring anymore, but if you want to attempt to re-enact just the observational challenges (for long, dangerous journeys, you're on your own), visit transitofvenus.org and download the free app, for iPhone and Android. It'll give you some simulated runs so you can perfect your timing, and be ready to go. (See below for your local observatory's viewing activity.)
You should take this opportunity — it's the last chance you'll have to see the Transit of Venus. (You caught the last one, in 2004, right? Yeah, neither did we.) The last pair happened in December 1874 and December 1882, and the next will be in December 2117 and December 2125. So mark your calendars.
Transit of Venus | dome show + viewing in the field (weather
permitting) | Southworth Planetarium, 96
Falmouth St , Portland
Transit of Venus | live telecast | Museum of Natural History 1000 Elmwood Ave , Providence
Transit of Venus | viewing + live telecast | Harvard-Smithsonian Center
for Astrophysics, 60 Garden St Cambridge Spectacle Island (ferries from Long
Wharf ), Boston Harbor