Millions around the world tracked the transit of Venus, a once-in-a-lifetime—if not once-in-two-lifetimes—event. The second planet in our solar system paced across the face of the sun in six hours and 40 minutes. "People got excited watching a little black dot for a couple of hours," said Shawn Laatsch, planetarium manager of the Imiloa Astronomy Center of Hawaii. As Yahoo! noted in May, this "celestial rarity once guided adventurous astronomers in their quest to determine the size of the solar system and yielded the first-ever global scientific collaboration." In 1769, astronomers traveled for days, across continents, and into war zones to collect data, ultimately yielding a very good approximation of the distance between Earth and the sun. Nearly 243 years later, the transit of Venus allowed scientists to track the optical changes in the planet and atmosphere. The next transit of Venus won't happen again until 2117.