Meanwhile, Arthur Holmes (1890-1964) was finishing up a geology degree at the Imperial College of Science in London where he developed the technique of dating rocks using the uranium-lead method.By applying the technique to his oldest rock, Holmes proposed that the Earth was at least 1.6 billion years old.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 a report of his findings published in 1913 in the journal , Holmes expressed the less-than-ecstatic reception his findings received: "The geologist who ten years ago was embarrassed by the shortness of time allowed to him for the evolution of the Earth’s crust , is still more embarrassed with the superabundance with which he is now confronted." The Earth's age continued to be hotly debated for decades afterward. In the 1920s, Earth's age crept up toward 3 billion years, making it for a time even older than the universe, which was then estimated to be about 1.8 billion years old.
The uranium-235 to lead-207 decay series is marked by a half-life of 704 million years.
These differing rates of decay help make uranium-lead dating one of the most reliable methods of radiometric dating because they provide two different decay clocks.
This provides a built-in cross-check to more accurately determine the age of the sample.
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.