Christmas, looked at with a suitably inquiring mind, is a scientific wonderland. Its customs and origins can be analysed psychologically and astronomically, dissected ethnically and sociologically, investigated biologically, and even examined for their meteorology. Christmas, indeed, is a veritable treasure house of rich material for any theme-less, would-be Ph.D.

Take, for example, the psychology of Christmas cards. The custom of sending Christmas cards started, it seems, in the middle of the last century, when Henry Cole - a lively entrepreneur whose other achievements included founding the Victoria and Albert Museum, and organising the Great Exhibition - decided that the burden of conveying Christmas greetings could now be alleviated with the advent of the penny post. In 1843 he produced the first printed Christmas card, and during the succeeding decades the habit became so widespread that by 1883, The Times was able to proclaim that "this wholesome custom has become a way to end strife, mend relationships and strengthen family ties".

But Christmas cards, the psychologists tell us, do even more than this: they also provide an insight into the Christmas psyche. People to whom you send cards, but from whom you never receive one in return, for example, are likely to be above you on the social ladder; conversely, those from whom you receive cards without reciprocating are almost certainly below you in the social pecking order. And what you write on the card may also be revealing: a long, rambling message indicates someone trying to impress; a joke indicates guilt at contact overdue; a simple signature upon a Christmas card says that the sender is much too busy and important to bother with lengthy pleasantries with the likes of you. And a privately printed card reeks of wealth and self-importance, an obvious indication that the sender has insufficient time in his schedule to sign individually the enormous number of cards he has to send.

The type of card may also tell a story. A card printed on recycled paper not only establishes that the sender's environmental credentials are impeccable, but also subtly suggests to the recipient that he or she should feel a twinge of guilt for laying waste the Earth's resources to acquire the glittery commercial card already dispatched the day before. The charity card, of course, advertises the great generosity and social conscience of the sender.

Any astronomical analysis of Christmas inevitably focuses on the Christmas star, the Star of Bethlehem. The story is well known: "In the days of Herod the king, there came wise men from the east to Jerusalem, saying: Where is he that is born King of the Jews?; for we have seen his star in the east, and have come to worship him. And, lo, the star went before them, till it came and stood over where the young child was."

Finding an astronomical explanation for the Star of Bethlehem is not a simple matter of calculating how the sky might have looked on December 25th in AD1. For one thing, there is considerable uncertainty about whether or not the first Christmas actually occurred on Christmas Day. The date of December 25th coincides with old pagan festivals that marked the very darkest period of the year. Ever since ancient times people have lit candles, bonfires and yule logs to help nourish the sun god when he was at his weakest, in the hope that in due course he would recover and drive away the winter hardships.

The Roman festival of Saturnalia in mid-December was one such example, and even in the early Christian epoch the Roman Emperor Aurelian proclaimed December 25th to be Natalis Solis Invicti, the festival of the "birth of the invincible sun", which was marked by chariot races and widespread decorations with branches of small trees and other foliage. The ancient Britons and Scandinavians also held midwinter sun festivals, which they called yule or jol - giving us, incidentally, the words "yule" and "jolly". These pagan mid-winter festivals remained popular centuries after Christ was born, and when it came to establishing one of the most important feast days of the Church, it seemed wiser to the early fathers merely to use a date already there, than establish new festivities from scratch.

But even the year of Christ's birth is quite uncertain. The AD-BC system was devised in AD 525, and it is widely accepted that it may well be several years in error. The best that scholars can suggest is that, by our present Calendar, Christ could have been born anywhere between 8 and 1 BC, thereby defining a fairly lengthy period during which we must try to identify the Christmas star.

It could, of course, have been a comet, but then none of the really spectacular comets, like Hale-Bopp or Halley's, would have put in an appearance near that time. On the other hand, in 7 BC a very rare astronomical event took place. Every 139 years the Earth, Jupiter and Saturn lie in a straight line - a phenomenon known as a "triple conjunction" of these planets. Around the time when this happens, Jupiter and Saturn appear to trace out loops in the night sky against the background of the stars. Moreover the triple conjunction of 7 BC occurred when the planets were in the constellation Pisces, and this coincidence occurs only once every 900 years.

Now the Magi are believed to have come from what is now Iraq, then Babylon, and modern thinking is that they are more likely to have been astrologers than kings. The rarity of this event, therefore, would not be lost on them. Moreover, Pisces was a constellation associated in astrology with the Jewish people; Jupiter was associated with royalty and brought good fortune; and Saturn was the protector of the Israelites who would herald a new age when the time was right. No wonder, certain scholars think, that the Magi were excited!

THE Biblical account implies that the Star was not an impressive spectacle, obvious to everyone. Could it be that it was not the single brilliant object that we think about, but rather an astrological signal in the sky which would be of importance to those familiar with such things - but go unnoticed by the general population? Was it the triple conjunction that the Magi followed as a pointer to their destination? - "till it came and stood over where the young child was; and when they opened their treasures they presented to him gifts: gold, frankincense and myrrh".

Much of the meteorological significance of Christmas centres around the El Nino phenomenon of which we heard so much earlier this year. Near the end of each calendar year, a weak ocean current starts to flow slowly southwards along the east coast of South America. Long ago, the inhabitants of these coastal regions gave this warm current a name; they called it El Nino, "The Boy Child", because it came along during the Christmas season and caused great disruption to their fishing. In recent times the name has been borrowed by meteorologists to describe a much more widespread phenomenon, an extensive and very marked warming of the surface of the tropical Pacific which occurs at irregular intervals of between four and seven years, and which causes oscillations in the world's climate. These semi-regular swings of the climatic pendulum are rivalled in their magnitude only by those which triggered the advance and retreat of the great ice sheets over the millennia.

If you enjoy titbits of useless information like some of the foregoing, and a little adventurous but harmless speculation, I can do no better than recommend to you for seasonal reading a delightful little book now in the bookshops called Can Reindeers Fly?: the Science of Christmas. It is written by the veteran science journalist Roger Highfield, and leaves not a Yuletide stone unturned in its attempts to explain our curious behaviour at this time of year. Highfield finds scientific explanations for many of the strange things that we believe.

Did you know, for example, why Rudolph has a red nose? The commonly held view, of course, is that its colour is due to the cold; alternatively it has been suggested that while the paunchy Santa has been eating the mince pies so thoughtfully left out for him, Rudolph has for centuries been helping himself to the sherry. But according to Highfield, reindeer noses are a particularly welcoming environment for parasites, and many are discoloured to a shade approximating the traditional hue by a simple parasitic infection of the respiratory system.

And there is another odd thing about Rudolph. Male reindeers, it seems, shed their antlers during the winter months and grow new ones for the rutting season. Only if Rudolph's physical entirety, if you get my delicate drift, had been seriously and very permanently interfered with, could he appear as an antlered male at Christmas-time to pull Santa's sleigh. Or so says Highfield, anyway.

There is much food for thought in Can Reindeers Fly? You will be shocked, no doubt - and so you should be - by some of the tendered explanations for the Virgin Birth, but there is a chapter on the alcoholic associations of the Christmas season that will warm the cockles of your heart. But let me not spoil it for you: this aspect of the Christmas spirit I will leave you to savour for yourself.

Can Reindeer Fly: the Science of Christmas by Roger Highfield (Metro, £12.99 in UK).