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What is time? If you were to put that question to the ordinary person in the shopping centre you might get a look of kindly resignation, masking a mild annoyance at the idiots in immigration who allow people to come into the country without being able to speak the Queen's English. The answer would be given slowly and with exaggerated diction. ''IT-IS-TEN-PAST-THREE.''

At this stage, you realise that market research is not a good way to find an answer to your question so you do what academics call a literature mapping. According to St Thomas Aquinas, a clever priest who lived about 700 years ago, time is the measure of things that change. So, if there is a small change - the child stops crying, for example - it is a small amount of time. If there is a bigger change - the husband finally comes home roaring drunk from ye olde alehouse - that is a longer period of time. Then, when the child finally begins to grow up and search for interesting internet sites, that is a much longer period of time, what we might today call 11 years.

Although it is not very precise, that definition of time was successful for many centuries. The church was happy with it, since it enabled them to take a large number of theologians off the Angels-on-the-Head-of-a-Pin Project and put them to work on refining what became known as the Thomistic definition, descriptio Thomae.

Inevitably, when science came along, they wanted a more exact formulation, so they made up a unit called the second, which was defined as a very small fraction of the Mean Solar Day. Unfortunately, not many people understood the concept of a mean solar day.

For example, somewhere out in the Gambia in mid-summer, you had people meeting each other at the shopping centre and complaining about the weather in the way people do: ''It's a mean solar day, brother.'' ''It sure is. A good day for the cotton, though.''

There would be some further pleasantries, perhaps a brief mention of a trading barque which had pulled into a quiet cove some nights earlier, manned by people with cutlasses and muskets. 17,388 seconds later, our two friends would be in chains on board one of those boats, headed for a new life as part of the civilised world.

The mean solar day was soon regarded as not accurate enough for the clever scientists with too much time on their hands because their houses were cleaned and their meals prepared and their gardens tended by descendants of people from the Gambia. The main problem with the definition was that people could not agree which city to use as the place to measure the mean solar day - cities have been built on perfectly good sheep stations for lesser disputes.

Besides, the second was defined as a fraction of something, and schools at this time had stopped teaching fractions because they were too difficult and gave pupils an experience of failure, which is something they would never meet in the real world. Or as Barnaby Joyce put it in these pages last week writing about something else altogether, ''we have to be over-considerate of feelings ahead of competency''. So scientists came up with the following, much clearer, definition: one second is the duration of 9,192,631,770 cycles of the radiation associated with a specified transition of the caesium-133 atom.

That is the current definition. It has the great advantage of being a multiple of something rather than a fraction, so that children can easily use their calculators to work out 10 seconds or 30 minutes or whatever. What the definition means is that those small batteries which power our watches have an even smaller being which sits there day after day with a caesium-133 atom on its lap, counting the number of cycles of radiation each second.

It has to be exactly 9,192,631,770 - one more or less will not be close enough. This requires great patience and great speed in counting.

I calculated that if you or I were to do the count, at exactly one number per second, day and night non-stop, it would take more than 291 years to count each second.

Clearly, this is impossible, for reasons which I won't go into here, and so we should be grateful to the Swiss who succeeded in getting these tiny beings to do the counting for the rest of us. The beings are called angels and the clever Swiss horologists succeeded in enticing them off the head of a pin to do some useful work. They - the angels, not the Swiss - had to work at a temperature of 0 degrees, a situation currently being investigated by local OH&S. The net effect is to put a number of otherwise gainfully-employed theologians out of work.

Anyway, that's what I think.

Frank O'Shea is a Canberra writer.