On rare occasions, the planet Venus reaches a point in its (and Earth's) orbit around the sun such that we can see Venus as a dark spot moving across the sun. Known as the transit of Venus, it's a bit like an eclipse, except Venus is so far away that it shows up as just a dot, rather than blocking out much of the sun (which is what our nearby moon can do).

In 1627, Johannes Kepler successfully predicted the transit of Venus coming in 1631, though unfortunately it wasn't visible in most of Europe. But because transit events occur in pairs, eight years apart, the next transit in 1639 was observed by several scientists, which helped establish the size of Venus and make a rough estimate of the distance from the Earth to the sun (as astronomical unit, or AU). This set expectations for the next set of transits, which would happen in 1761 and 1769, the latter of which saw scientists attempt to view the transit from various points around the world.

In this video, Brady Haran and James Hennessy examine an orrery (model of the solar system) from the 1760s that explained to the layperson how the event would occur, in an extremely miniaturized and simplified form. This orrery is stored at the Royal Society in London. Amazingly, the 250+ year-old orrery still works, though it's fragile. Have a look, and learn a little more about the history of these events:

There were further transits in 1874, 1882, 2004, and 2014. If you missed the recent ones, you're out of luck, as the next transit will not occur until 2117. You might catch Halley's Comet in 2062, though.