It's three quarters of a century since Sir Patrick Moore was seized by an obsession which saw him become a world expert in his field, even though he was an amateur. Now he is determined to reach an unbeatable milestone. Nick Morrison reports.

HIS mind is as sharp as ever, but Sir Patrick Moore's body is letting him down. A bad hand, the legacy of a Second World War injury, means he has had to give up two of his passions.

"Five years ago I was playing cricket. Now I can't. I can't play the piano. Those are my great griefs, but I have had a long run," he says sadly. "But they said my hand would last me until I was 30 and it lasted me until I was 77."

But the third of his passions, and perhaps the greatest, is keeping him going. He is 81 now and has been presenting the Sky at Night since 1957, making him the longest-serving television presenter in the world, and he has his eye on the half century.

"If I survive for two more years I will come to my 50th anniversary. That is a record that will never be beaten. I have only missed one," he says.

That one was earlier this year, when a bout of salmonella kept him out of the studio, but it is still a remarkable record and, as he says, unlikely ever to be matched.

His short-burst rapid-fire delivery - he once timed himself at 300 words a minute - and unconventional appearance, complete with monocle and slightly mad stare, made him one of the most instantly recognisable and often imitated presenters, and ensured his place in the pantheon of great British eccentrics.

It all started when he was six, and he picked up a book, The Story of the Solar System, by GF Chambers, published in 1898, priced sixpence - "I can still see it now". What was it about astronomy which attracted him?

"How can you tell what appeals to who?", he asks in reply, but whatever it was, it captured his imagination.

It may have helped that a bad heart kept him at home throughout almost all of his childhood, and he could indulge his fascination with the stars and the planets at will. "I didn't have a normal boyhood, no-one's fault. From six to 15 I was ill," he says, in a stiff-upper-lip way. "The idea was first Eton then Cambridge and I didn't make it, then the war broke out.

"I got all my school exams, I had my Cambridge entrance, I had my place at Cambridge waiting, the war broke out and I decided I would go and fight."

He lied about his age and enlisted in the RAF at 16. By the time his superiors discovered his deception he was old enough anyway, and allowed to stay, and it was from his stint as a navigator in Bomber Command that he received the hand injury that plagues him now, as well as a knee injury.

The war also claimed the life of the girl he planned to marry, killed in an air raid. There was never anyone else, and he has lived alone in Sussex since the death of his mother in 1981.

In 1945, while many other ex-servicemen took up their university places from before the war, the then 22-year-old decided not to. It was for reasons of pride, although it's still a source of sadness almost 60 years on.

"I would have had to take a government grant and that didn't appeal to me, so I wrote a book and I never looked back. I was sorry I missed university. I didn't mind missing school so much.

"I wish I had gone. I would have loved Cambridge. Sadly, it was not to be. I preferred to stand on my own feet. My parents couldn't do it for me, so I thought I will do some writing," he says, matter-of-factly.

The book laid the foundations for what was to become an astonishing career. Always an amateur astronomer, he has nevertheless been accepted into the science's professional body, the International Astronomy Union, a source of great pride to him, and he helped map the moon for Nasa in advance of the first moon landings.

"The moon was my subject. It may have been the first thing I ever saw through a telescope. I remember being fascinated.

'But the only thing I have ever done with some success is to interest other people, either by writing or lecturing or being on television," he adds.

He has lived through a time of astonishing developments in astronomy, new discoveries every month. He thinks the most exciting time is still to come, as man looks to other planets, but a major breakthrough in the search for life on other worlds could be close at hand. Does he think we're alone in the universe?

"No I don't," he says firmly. "A hundred thousand million stars in our galaxy, and we know a thousand million galaxies. The total number of stars is colossal, and we know many of those have planets going around them.

"There are millions of worlds where life could appear. What we're not sure of is, if life can appear, will it?

"The key may be Mars, if there is any trace of life at all. It is the only world within range that could have life. If there is any trace of life on Mars, it will show that life can exist."

The first evidence of intelligent life on other planets is likely to come from radio waves, but of course this proves only that life did exist once, when the radio waves embarked on their journey through space, not that it does now.

Travel to other inhabited planets is in the realm of science fiction, but how should we view the possible arrival of extraterrestrials to our world? Should we welcome them? What if they come with conquering intent? Sir Patrick's not worried.

"Any being who can come here will be far more advanced than we are and they will have too much sense to go to war," he says. Then again, maybe none of them will get that far. "It may be that every civilisation gets to the stage where we are, and they wipe themselves out."

This brings him onto the invasion of Iraq. It may be due to his own experiences, but he's virulently anti-war, and is horrified at what has happened in the Middle East.

"Unless someone stops George W Bush we could be in for another war. He is a war criminal. He is terrifying. If he goes for North Korea or Iran we will be in trouble. I hope he won't do it."

So how does he feel about our own prospects? Will we be around long enough to make contact with other worlds? Or, having got to a certain stage, will we wipe ourselves out? His answer is not entirely reassuring.

"It depends on how we choose our world leaders. The way we choose them now, it inspires me with a feeling of no confidence at all. I hope people in other worlds are doing better," he chuckles.

2005 Yearbook of Astronomy, edited by Patrick Moore (Macmillan) £14.99.