Wednesday Book: Why one day is like any other

there is a famous cartoon showing two hippopotamuses wallowing in a huge expanse of water with nothing else in sight. One of the hippos is saying to the other: "I keep thinking it's Tuesday".

After reading David Ewing Duncan's captivating account of the history of man's attempts to compute time, my respect for the hippo increases. For the tale of the calendrists of the past 6,000 years confirms that nobody has ever really had much idea of whether it is Tuesday or not.

Duncan's book begins in the 13th century, when a sickly but truculent English friar named Roger Bacon wrote to Pope Clement IV to inform him that the Christian church had its calendar all wrong, and told him what to do to put it right. It took the Roman church another 300 years before they began to realise that Bacon had been right, and Britain got the message only a century and a half later. But, as Duncan demonstrates, such delays have not been unusual in the elaborate interplay between church, science and state that underlies the slow development of our calendar.

The two greatest intellectual achievements of early man are both connected with the passage of time: the realisation that the seasons repeat themselves according to a predictable cycle, and the discovery that having sex causes babies.

These revelations, more than anything else, showed that the world obeyed rules affecting matters months or years ahead. They showed us that the world was predictable and that, to some extent, our futures were controllable.

And that, Duncan lucidly explains, is why the measurement of time and the accurate prediction of the calendar became so important to the Church, from the Council of Nicaea in 324, when the Julian calendar was adopted, to Pope Gregory's adjustment of it in 1582.

"The history of science in the Middle Ages would have been very different if the bishops of Nicaea had decided simply to name a fixed date for Easter," he writes. "Instead, in the wake of Nicaea, Christians developed a complex equation to determine the proper day, forcing time-reckoners to return to something Caesar had dispensed with centuries earlier: a dependence on the moon."

The problem was that the date of the Crucifixion - the most important Christian feast - was known only by reference to the Jewish lunar calendar: the first Sunday after the first full moon after the spring equinox. The trouble here is that 12 lunar months add up to just over 354 days - 11 short of a solar year. Fiddling with this discrepancy played an important part in the history of medieval Christianity. Duncan attributes the Julian (solar) calendar to Caesar's love for Cleopatra, who apparently told him about the secret of leap years (which the Egyptians had known for centuries) during a dull moment in their affair. So Julius Caesar recalibrated the Roman calendar, and gave us an extra day every fourth February.

Compared with the inexact measurements of everything else in the Middle Ages, the correctness of the calendar - so important to the Christian Church as an indication of the mind of God - was astounding. Caesar's figure of 365.25 days was less than .01 of a day out. What other measurement of the time was correct to four significant figures? And when a slippage was noticed in the predictions of the equinoxes and the full moons on which Easter was based, Pope Gregory made his amendment specifying that century years would not be leap, except for every fourth one. (So 1600 and 2000 are leap years according to the new system, but 1700, 1800 and 1900 were not). That adjustment gave a year so accurate that it will not have slipped out by a whole day until the year 4909.

But to come back to the hippo's problem: was yesterday really Tuesday? The seven-day week was bequeathed by the Babylonians, who assigned the days to planet-gods. If they had known that there were nine planets in the Solar System, we'd now have a nine-day week. One of the many fascinating facts I learnt from this book was why Sunday follows Saturday:

"The order of the day names themselves comes from ancient Mesopotamian astrologers' attaching a planet-god to preside over each hour of the day, arranged according to their correct cosmological order. For instance, Saturn controlled the first hour of Saturn's day (Saturday), followed in its second hour by Jupiter, then by Mars, the Sun, Venus, Mercury and the moon. In the eighth hour the cycle started again with Saturn and the progression repeated until the 24th hour of the day, which happened to fall to Mars. Because the next hour in the cycle - the first hour of the new day - belonged to the sun god, the day after Saturday was called Sunday."

So it might as well be Tuesday after all. Forget Stephen Hawking: The calendar is a brief history of time that we can all understand and enjoy.