"We are the Ancients of the Earth," said Tennyson: "These times are the ancient times, when the world is ancient, and not those which we count ancient ordine retrogrado, by a computation backwards from ourselves."
When trying to decide exactly how ancient these times might be, the Bible was the natural starting point in days gone by, and provided an answer of about 6,000 years.
Then, in the 18th century it was noticed that certain natural processes, like the cutting of river channels, operated on a very long time-scale. In order for observed phenomena to have a satisfactory explanation, they must have been in the making for millions, rather than thousands, of years. The age of the Earth was revised to about 500 million years.
Another way of estimating the age of the Earth was by the accumulation of salt in the oceans. Rivers steadily wash salt into the sea, and since only fresh water leaves it by evaporation, the salt concentration rises with time.
By the mid-19th century it was estimated that the time required for rivers to endow the oceans with their present 3 per cent concentration of salt was around a billion years. And finally, the discovery of radioactivity in 1896, allowing scientists to calculate the age of rocks from the quantity of uranium and lead they contained, provided the now generally accepted figure of about 4.6 billion years for the Earth in its present solid form.
So how do we keep track of this near-approximation to eternity? Geologists divide it into four eras: the Precambrian, the Paleozoic, the Mesozoic and Cenozoic, and each of these eras is divided into "periods". The two most recent periods - which together form the current Cenozoic Era - are the Quaternary and the Tertiary.
Then there are further subdivisions. Each period is divided into a number of "epochs". Again staying in the very recent past, the current Quaternary Period consists of two epochs, the Holocene and the Pleistocene. More confusingly, the Tertiary Period is subdivided into no fewer than five epochs, the Pliocene, Miocene, Oligocene, Eocene and Paleocene epochs.
As we noted yesterday, one way of making sense out of this unpronounceable tangle of chronological complexity is to relate it to the conventional calendar. If you think of an "era" as a year, then a "period" is analogous to a month, an "epoch" to a week, and an "age" to a single day. And if even this is too much to digest in these dreamy doldrum days, just remember that this is the Holocene epoch, in the Quaternary period of the Cenozoic era.