As we have long been reminded, the new millennium is approaching. In less than 365 days we shall be party to the beginning of the next thousand years of Earth's life, and the BBC is marking the countdown with Time Season, a series of programmes exploring the philosophy, history and uses of time. Who has heard of John Harrison? Airline pilots, probably. And captains of cruise liners.While promoting Longitude, a Horizon special, actor Patrick Malahide even met astronaut Joseph Conrad, veteran of two Gemini missions and Apollo XII, who told him that he was a Harrison devotee.

"Anybody who knows anything about navigation will know of him," says Malahide. John Harrison solved possibly the greatest scientific challenge of the 18th century: measuring longitude accurately. Before his success, sailors had known their latitude, from the angle of the sun, but not how far east or west they had travelled. Setting sail back then," says Malahide, "would be like taking a jumbo jet up today without an altimeter. They were absolutely busking it."

In 1707 a British fleet returning from war misjudged its position and foundered on rocks, sacrificing 2,000 lives.It prompted the government to offer a prize of £20,000 equivalent to £6million today to anyone accurately measuring longitude. Astronomers pompously believed that they alone would succeed, but it took Harrison, a carpenter-turned-clockmaker from remotest Lincolnshire, to realise that time was the key and to build a clock reliable enough.

The Horizon special tells Harrison's story as popularised in the scientific bestseller Longitude, by Dava Sobel using interviews with experts and a dramatisation starring Malahide as the unlikely hero.

Another programme in the season answers such questions as why tooth pain is lowest after lunch and why we urinate less at night. It's all to do with our internal body clock, or Circadian Rhythm, the subject of What Makes Us Tick (Saturday 9 January). Our body clocks run on 24.6-hour cycles and are reset by daylight (60 per cent of registered blind people have serious sleeping problems). And trying to fight against our body clock can have serious consequences: there is a peak of car and air crashes at 3am, and shift workers live, on average, ten years less than other workers.

BBC2's documentary follows scientists trying to pinpoint the area in our brains responsible for Circadian Rhythm. They needed mutant animals whose body clocks were malfunctioning, to isolate the tissue responsible and, by chance, they found a family from California, three generations worth, who all woke up at 3am and went to bed before 9pm, their body clocks 4.5 hours out from the rest us.

Scientific developments in this field could mean we can avoid jetlag, aid the blind, swat flies and win wars. Flies' brains, it transpires, work in fast-forward, so that everything we do appears in slow motion to them, a perception which, if harnessed by soldiers, could make all the difference in war. Then, on Wednesday , The Flow of Time looks at our perceptions of time. While we think of the past, present and future as distinct entities, some scientists tell us otherwise. Events that took place a millennium ago or in the future are actually happening "now" but on a different time scale known scientifically as space-time Doctor Who fans will be familiar with the concept and the documentary indeed suggests that we will one day master time travel.
Nick Griffith

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