So ... happy birthday, world!
How did Bishop Usher get such a precise date? Well, he didn't just count up all the "begats" and generations in the bible (although he did take Nebuchadnezar and the day the sun stood still when Joshua fit the battle of Jericho into account). Instead he relied on a complex set of astronomical calculations that sought to reconcile the Julian and Gregorian calendars. It's complicated. Here's how Wikipedia explains it in its article on the "Julian day," a related astronomical calculation that dates back to the 1500s and is still used to time orbits in space and to compare dates in different calendars. And here's how Bishop Ussher explained his final calculation in a book called The Annals of the World, published in 1658:
… from thence I gathered the creation of the world did fall out upon the 710 year of the Julian Period, by placing its beginning in autumn: but for as much as the first day of the world began with the evening of the first day of the week, I have observed that the Sunday, which in the year 710 aforesaid came nearest the Autumnal Æquinox, by astronomical tables (notwithstanding the stay of the sun in the dayes of Joshua, and the going back of it in the dayes c Ezekiah) happened upon the 23 day of the Julian October; from thence concluded that from the evening preceding that first day of the Julian year, both the first day of the creation and the first motion of time are to be deduced.Ussher is quoted in more detail on a webpage by Donald Simanek of Lock Haven University. And Ussher's methodology is discussed at http://en.wikipedia.org/wiki/Ussher_chronology. Included is the quote, which strikes me as both generous and wise, by evolutionary biologist Stephen Jay Gould:
I shall be defending Ussher's chronology as an honorable effort for its time and arguing that our usual ridicule only records a lamentable small-mindedness based on mistaken use of present criteria to judge a distant and different past. ... Ussher represented the best of scholarship in his time. He was part of a substantial research tradition, a large community of intellectuals working toward a common goal under an accepted methodology[.]