When asked for your age, it’s likely you won’t slip (with the exception of a recent birthday mistake). But for the sprawling sphere we call home, age is a much trickier matter.

This week, Cherry Lewis of the University of Bristol presented a talk about the history of dating the Earth as part of the BA Festival of Science in York, England.

Before so-called radiometric dating, Earth’s age was anybody’s guess. Our planet was pegged at a youthful few thousand years old by Bible readers (by counting all the “begats” since Adam) as late as the end of the 19th century, with physicist Lord Kelvin providing another nascent estimate of 100 million years. Kelvin defended this calculation throughout his life, even disputing Darwin’s explanations of evolution as impossible in that time period.

In 1898, Marie Curie discovered the phenomenon of radioactivity, in which unstable atoms lose energy, or decay, by emitting radiation in the form of particles or electromagnetic waves. By 1904 physicist Ernest Rutherford showed how this decay process could act as a clock for dating old rocks.