On June 5, the planet Venus will appear to pass in front of the Sun as seen from the surface of the Earth. This phenomenon, called a transit of Venus, happens only a few times in 250 years. The next one will be in 2117 AD. Here’s a guide to watching the once-in-a-lifetime event. But why does it matter?

It provides interplanetary perspective.

Transits of Venus were scientific gold for early astronomers, who used them to derive an accurate measurement of the size of the solar system. By noting the time each planet took to go around the sun, and then crunching that data via methods developed by 17th century mathematician Johannes Kepler, these telescope-equipped boffins could determine each planet’s relative distance from the sun, as measured in terms of astronomical units (the distance from the Earth to the Sun). Collecting such data during a transit was the reason Captain Cook was able to travel halfway around the world from London to Tahiti in 1769.

via Why the Venus Transit Is One of the Most Important Events in Scientific History.