It is only when the elderly white-haired bloke slips in his monocle that he transforms, before my eyes, into Sir Patrick Moore, TV astronomer extraordinaire. His face, rather unremarkable when naked, takes on a purposeful, Churchillian quality, as his right cheekbone and brow clamp on to the glass disc that for decades has been a hallmark of one of the nation's best-known scientists.

The transformation doesn't last long. The eyeglass slips on to his chest and he hunts for it wearily, a man whose mind is still sharp betrayed by a body that is letting him down. As if to defy it, referring to the legacy of a war wound that now leaves him unable to walk, he barks: "I was playing cricket not so long ago... before this problem with my spine. I was a great leg-spinner, you know."

His friends from the local cricket club who call round attest to this and to his hardiness in younger days: the spry physique defying bitter cold as he stood, shirt unbuttoned, in the slips. They speak fondly of good times after the games, when a well-refreshed Moore would cycle home in the dark, whatever the weather.

The hall clock chimes that it's quarter after the hour. "Ah well, gin and tonic, I suppose," his says, with a wry chuckle. I disappoint my genial host by opting for tea. We settle and talk. The trademark clipped, Pathé News diction is not what it was. Sentences dissolve into a slur after the first couple of words and I have trouble understanding him at times.

His mind, however, remains crystal clear. In March, the 87-year-old will record the 700th installment of The Sky at Night. It is the longest-running programme with the same presenter in the history of television. He has been doing it since 1957, missing only one recording in 2004 when salmonella poisoning from a goose egg nearly killed him. It seems only fitting that the BBC throw him a lavish bash to mark the event, in the back garden of his West Sussex home.

To scotch any lingering doubts about whether he should remain in the role, he has just completed one of his weightiest books. Ten years in the writing, The Data Book of Astronomy, published next month, is a compendium of just about everything anyone with an interest in the cosmos could want to know: a summation of his life's work. The book could possibly be his last. "I can't go on for ever: I may not bring out another book. In my career, you could say it's the summary of my life so far.

"So much has happened in the past 10 years," he says. "There's the probing of the planets; unmanned probes that have gone well beyond the solar system; new instrumentation and new advances in astronomy."

His eyes brighten as he talks about his project with his friend, a fellow astronomer and rock guitarist, Brian May. The Queen songwriter often visits Moore's home.

The octogenarian is rather proud of other famous names he has rubbed shoulders with over the years. He's very impressed with the physicist and former D:Ream band member Dr Brian Cox, and has invited him over to the house.

On the dining room wall hangs a black and white photograph of a young Moore seated at a piano, Albert Einstein playing a violin. "That's a fake," he confesses. "But we did meet." The pair met in America and duetted on Camille Saint-Saëns' "The Swan".

"I've never had a music lesson," he says. Indeed, he seems to have had relatively few lessons of any kind, making his prominence in the academic world all the more remarkable.

He was born in Pinner, north-west London, in 1923. His father, an army captain was awarded the Military Cross in the First World War. Patrick's path should have been conventional enough for a lad from a well-to-do family: prep school, then Eton. But ill-health through heart problems meant he was tutored at home.

Captivated by astronomy at six, he joined the British Astronomical Association at the age of 11. Exactly 50 years later, he would become its president. The Second World War thwarted his plans to study at Cambridge.

His father, gassed in the First World War, never fully recovered. But the biggest blow in Patrick's life came when his fiancée, Lorna, a nurse, was killed by a bomb. "She was in the wrong place when a German bomb fell. There was no one else for me after that. I knew then I'd never marry."

To this day, he remains a Eurosceptic: "In the war, the Germans tried to beat us, the French did nothing and the Italians made good ice-cream. Out of Europe!" And his views on women are similarly unreconstructed. He is only a little sheepish about suggesting recently that women have ruined the BBC. "I think that might be true... You never see a male newsreader nowadays, they're all women. There are two things the BBC doesn't like: I'm male and I'm white."

Cambridge held open a place for him while he had a stint being "banged about" in the RAF. But he wanted to stand on his own two feet. He set about forging a career in astronomy. In 1957 his first book, about the Moon, was published. When the BBC decided it wanted to produce a monthly astronomy programme, aimed at enthusiasts, Moore's reputation and knowledge, combined with his diction, made him a perfect choice. On 26 April 1957, the first episode of The Sky at Night was broadcast. Why has it lasted? "Partly because it's cheap! Ha, ha, ha... But lots of people have an interest in astronomy. I've tried to make it as interesting as possible for people. I've done my bit."

His lack of formal training and career in popular science TV might have led to sneers from academics. "Quite the opposite. Although I'm an amateur, I'm on Nasa committees and committees on Moon matters. I think they're quite taken with me." Being made a Fellow of the Royal Society makes him the most proud. He still appears astonished a decade later.

Perhaps unsurprisingly, given his own education, he is against the teaching of astronomy as a school subject: "If it's taught badly, as so many things can be, interest can be killed. Anyone who has an inkling of interest will come to it naturally."

He reminisces about giving a talk many years ago at Torquay Boys' School: "An 11-year-old started talking to me. And I thought to myself, 'I'd better keep an eye on this boy'." Chris Lintott is now an astrophysicist and Sky at Night co-presenter.

Even after seven decades, he claims to have no idea what started it all, why his interest in space developed. "I can't tell you!" he says. "It just did."

Relenting, he offers a clue. His inspiration sits behind me on a shelf in his study, among the flotsam of 50 years. "Behind the dragon," he barks, as I search. In passing, I spot the Bafta he was awarded in 2000 for best presenter. "Behind the brass dragon!" I stumble on something called the Golden Egg award. "In front of you, man!"

Perched at the end of a series of seven small tomes, I finally find an 1898 copy of G F Chambers's The Story of the Solar System. It is in immaculate condition. Its blue-bound cover and hand-drawn illustrations of planets captured the heart and mind of the six-year-old Moore. It is a wonder I managed to the find the book at all. The house is packed with mementos, models and pictures. The ceilings are festooned with tankards.

He lived here after returning from the war, with his mother, Gertrude, until her death in 1981. There are reminders of her all over the house in the form of curious little water-colours. "She did those when she was 82," he says. "She had the music and the art. I got the music, but not the art."

Music became a feature of his TV appearances: he played the xylophone on Morecambe and Wise. He composed on the piano: operettas and scores for local theatre productions. "My last composition was called 'Out of the Sky', for a marching band... but I can't do it any more." He pauses and looks down at his arthritic hands: the hands that, until recently, bashed out books on his beloved 1908 Woodstock typewriter. The most recent was composed on a considerably less challenging computer keyboard.

He has little time for the creep of advancing years. The monocle drops again. He scrabbles for it, finds it, and looks disgruntled. Time is not on his side. But then comes the defiant, clipped diction. Clear this time: "There are other things I want to do!"

'The Data Book of Astronomy' by Patrick Moore is published on 28 February

Curriculum vitae

1923 Born in Pinner, north-west London to Captain Charles Traschel Caldwell-Moore and Gertrude. Father dies in 1947, his mother lives at his home in West Sussex until her death in 1981, aged 94.

1934 Joins the British Astronomical Association (BAA).

1940 Joins RAF after lying about his age. Aged 16 when he signs up, he serves as navigator in RAF Bomber Command, reaching the rank of Flight Lieutenant.

1945 Elected a Fellow of the Royal Astronomical Society.

1957 The BAA invites him to write a book about the Moon. Later presents first episode of The Sky at Night, a BBC TV programme for astronomy enthusiasts.

1965 Appointed director of the new Armagh Planetarium.

1968 Made an OBE. Consultant on Kubrick's 2001: A Space Odyssey.

1982 Asteroid 2602 is named after him.

1992 Plays the role of Gamesmaster in the Channel 4 computer games show.

1998 Appears on Have I Got News for You and accompanies the show's closing theme tune on the xylophone.

2000 Wins Bafta award for services to television. Is knighted the next year for "services to the popularisation of science and to broadcasting".

2004 Suffers near-fatal food poisoning; misses The Sky at Night for first time.

2005 Publishes his autobiography.

2007 Sparks fury when in an interview with the Radio Times, he says BBC was being "ruined by women".