Although today's anniversary links orbit with space travel, lexical punners know orbit has made the rounds. The Latin term orbita first gave birth, in the 15th century, to the orbit that refers to the bony socket of the eye. By the early 18th century, poets had plucked the term from the strictly anatomical realm and were using it as a colorful synonym for eye. Not content to stop there, orbit rolled on, and by the end of the 18th century it had a zoological definition naming the border or region surrounding the eye of a bird or insect.

The twentieth century witnessed the emergence of both the mathematical application of orbit (whose definition dizzies us) and that term's metaphoric meaning, which refers to "a range or sphere of activity or influence." We've circled around it long enough: your challenge is to name the century in which orbit developed the sense that started us off today: "a path described by one body in its revolution about another."

It turns out the astronomical sense dates to the 17th century, when Johannes Kepler and Isaac Newton discovered basic laws governing orbits.