The Year of Our Lord 2000, as we know, has almost come upon us. It has acquired this number because of the widespread acceptance throughout the modern world of the Christian calendar, a chronological system in which the years are assigned the cardinal numbers before or after the putative year of the birth of Jesus Christ.
But then, as we also know, our planet has been in existence for a much longer time than can usefully be measured in millennia. In a world which is billions of years old, it is sometimes more convenient to measure time by some other scale than Anno Domini: we use geologic time, defined in periods, eras, ages and epochs, instead of months and years.
When the system was first devised, geologic time was divided simply into four "periods", which with indisputable logic were called one, two, three and four. Only the last two of these, however, are still used: number three - or the Tertiary Period - lasted from 65 million to two million years ago; the fourth, or Quaternary Period, was that from two million years ago until the present day.
Fresh discoveries, however, brought a new complexity. Most geological periods are named after a feature - sometimes a very esoteric feature - of the geographical area in which they were first studied.
A 19th-century English geologist called Alan Sedgwick, for example, spent much of his career examining a certain sequence of rock strata in northern Wales: he suggested that this rock system be named Cambrian - Cambria being a Latin name for Wales. Then, based on the age of this rock, the period from 570 million to 500 million years ago became the Cambrian period.
Another geologist was even more obscure. He chose the name Silurian for his rock strata, after discovering that an ancient tribe called the Silures had inhabited the area around the time of the Roman conquest. The Silurian period became that from 440 to 400 million years ago.
Next comes the Devonian a period called rather ingenuously after Devonshire; then the Carboniferous - from the abundance of coal - was followed by the Permian period, which was called after a Russian province known as Perm.
The Triassic, apparently recalling some long-forgotten threefold subdivision in Germany, preceded the Jurassic period, named after the Jura mountains in the east of France.
And finally the Cretaceous period - "abounding in chalk" - brings us up to the beginning of the Tertiary period some 65 million years ago.