A 17th-century Irish prelate reached the heights of scientific sophistication in estimating Earth's age, writes Mary Mulvihill
In 1650 the archbishop of Armagh, James Ussher, began counting all the "begats" in the Old Testament. He also studied ancient Egyptian and Hebrew texts, analysed how the ancient calendars were calculated and came up with a date for the Creation.
The world, he concluded, had begun one weekend in 4004 BC - specifically, on the evening before October 23rd.
These days most people laugh at the Irish clergyman's work. Yet in 1650 it was the height of scientific sophistication, and many other erudite scholars were computing similar sums.
There was even heated academic debate about whether time would have begun on the Saturday evening or the Sunday morning.
Modern geologists can use complex dating techniques to assess the age of a piece of rock. But transport them back to 1650 and they'd find that the only way to calculate an age for Earth was to follow Ussher's technique, treating the Bible, "God's truth", as an accurate historical record.
Ussher chose October 23rd for his moment of Creation as, under the old calendar, it was the autumn equinox, a traditional start to the year.
He believed the 23rd would have been a Sunday, as time would surely have begun on the first day of the week, and he specified the previous evening, as traditionally this was when each day began. Many scholars agreed with Ussher that Earth was about 5,650 years old.
The Venerable Bede, for example, believed the Creation had happened in 3952 BC; Isaac Newton plumped for 3998 BC. The date was still hotly disputed, however; John Lightfoot, an eminent Hebrew scholar at Cambridge, believed the Creation took place at 9 a.m. on the day of the equinox and not, as Ussher suggested, the previous evening.
When it came to printing English Bibles, however, and adding a chronology in the margins, Ussher's calculation was the one chosen, and in time his work was accorded the same respect as the scriptures themselves.
The work was still widely accepted in the late 19th century, but by then scientists were exploring other ways of calculating Earth's age, based on the amount of salt that had accumulated in the oceans, for example, or the time Earth took to cool from a molten mass to a solid planet.
Now, thanks to radioactive techniques, we know Earth is 4.6 billion years old, an age that leaves plenty of time for the relentless processes of geology and evolution to take place. What would James Ussher have made of such a timescale?
Ussher, who was born in Dublin in 1581, was from a merchant family whose name survives in the city's Ussher's Island.
He was one of the first students to attend Trinity College in Dublin, starting at the age of 13, and his book collection later formed the nucleus of the college's library.
He was a renowned scholar, a professor of theology at Trinity and a keen astronomer who used a telescope to verify for himself the theories of Galileo, Kepler and Copernicus.
He was in England in 1642 when civil war broke out and remained there, managing to be both a royalist (he attended Charles I at the scaffold) and a friend to Oliver Cromwell.
When Ussher died, in 1656, he was buried, at Cromwell's request, in Westminster Cathedral.
• Ussher is one of the scientific pioneers featured in Mary Mulvihill's award-winning Ingenious Ireland (TownHouse)