James Ussher, was Archbishop of Armagh, Northern Ireland. In 1650, he calculated when was the world created based on his studies of the Old Testament and some ancient Egyptian and Hebrew texts. He came up with a date. He concluded that the world was created on the evening before October 23, 4004 BC. Another scholar named John Lightfoot made a similar research based on the Bible, Egyptian and Jewish chronologies and came up with a more precise calculation: the world was created on October 23, 4004 BC at exactly 9:00 a.m. Very precise, indeed! Or is it?
The scientific community thought that the earth is much, much older but until the early part of the 20th century they lack a more reliable tool to measure the age of the earth. In 1862, Lord Kelvin, the inventor of the international system of absolute temperature that bears his name, calculated how long Earth might have taken to cool from its original molten state. He concluded that the Earth was born 20 to 400 million years ago. Today’s scientists believe that that answer is incorrect, but Kelvin’s calculations were scientific being based on logical thinking and mathematical calculation.
The discovery of isotopes and radioactivity in the late 19th century and the realization that it can be used to measure eons of time gave scientists the right tool to begin tinkering on objects around them. Armed with this toolkit, scientists have spent decades scouring the planet for its oldest rocks. Some of them are in our own backyard. The oldest known materials on Earth are the zircon crystals in the Jack Hills of Western Australia that formed 4.4 billion years ago, not long after the formation of Earth itself. In 1956 Clair Cameron Patterson determined the age of the earth to be 4.55 billion years plus or minus 70 million using uranium-lead isotope dating on some rocks.
But there’s a problem: we know that rocks are continuously recycling, being formed, reformed and destroyed on an epic geological timescale, thanks to patterns of volcanism and erosion on our dynamic, atmosphere-shrouded planet. So how can we be sure that our oldest rocks really are our oldest rocks? How do we know that an even older rock hasn’t simply ground down to dust somewhere, or been subducted beneath a neighboring continent?
To get around this problem, scientists have also dated lumps of space rock-specifically meteorites
including the moon rocks that Neil Armstrong and company brought home to Earth. The results are strikingly aligned; our neighborly space debris can be dated to the same 4.5 billion-year window. We can date Earth with such a degree of precision because we have multiple lines of evidence pointing to the same window of time.
