What is time?" Saint Augustine wondered. "I know if nobody asks me, but when I try to explain it...I don't know." Most of us can sympathise. We know about daytime and nighttime, we can read a clock and decide when to go to school, to work, or to turn on Eastenders. We may often feel that our lives are governed by time. But when we try to say what this imperious, inescapable entity is, we find that it is also elusive and inexplicable.
We have other experiences of time. Birthdays and feast days recur at regular intervals, as do the seasons. We are subject to the same processes as all life, in which "biological clocks" determine growth, change and decay. On different occasions we may feel the minutes rushing past, or trudging by with leaden feet.
Great deeds and works of art live on beyond their creators and challenge the limits of time; the aspiration to immortality, the search for lost time, the desire to understand change are fundamental to our knowledge of the human condition and are the starting-point for some of the highest achievements of art, science, literature and philosophy.
Language itself has had to devise sophisticated ways of situating the moments at which events occur - in Indo-European languages usually through verb tenses (the word "tense" derives, via Old French, from the Latin tempus, "time"), to indicate whether actions are past, present or future, continuing or completed.
How did we acquire this acute sense of time? The simple division between day and night regulated the lives of the earliest humans and may continue to do so in some isolated rural communities. The first settled farming communities needed to be aware of seasonal change so that they would know when to plant crops. Those among them who understood that such changes might be related to movement in the heavens possessed knowledge that was vital to their survival. No wonder then that many of the monuments of ancient peoples seem to have had both an astronomical and a religious function.
A civilisation cannot develop without some sense of its own history. Around 5,000 years ago the Sumerians made the first calendar, dividing the year into 12 months according to the phases of the moon, and the month into 30 days. The problem is that a lunar year, such as this, does not synchronise with one based on the earth's movement around the sun, so there have been increasing disparities. Reforms, such as those introduced by Julius Caesar in 45 BC, were necessary to put them right.
The way that we construct time reflects our view of the world. The Aztec and Mayan solar calendars, remarkably accurate, were based on a concept of time as essentially cyclical, while the Babylonians were most interested in astrology as a means for predicting favourable and unfavourable periods for future action. Christianity introduced the concept of history as a linear progression from the Creation, via the Fall, to the coming of Jesus Christ.
Modern cosmology gives us a quite different account of the past, present and future of our universe. As we gaze at the stars, we not only see them across vast distances: we are also looking back into the past - and the instruments of modern astrology can peer back almost as far as the beginning of time itself.
Modern science is generally agreed that the universe came into being some 12-15 billion years ago, when both space and time originated in a cataclysmic "Big Bang". Before that there was no time; all the matter in the universe was compressed into an infinitely small space (called a "singularity").
Eventually, the energy in the universe may be used up, and when this happens, there will be no further "events": time will cease. Alternatively, the universe may stop expanding and start to collapse; time will end as it began, in a singularity, perhaps as a prelude to another cycle, initiated by a further Big Bang, and so on...