`LONGITUDE' IS A REAL CLASSIC, BY ANY MEASURE.

Byline: Rob Lowman Entertainment Editor

While the airwaves and films are populated with plenty of whodunits, the most compelling mysteries are cerebral. These stories, however, rarely make for compelling viewing.

But if you can tear yourself away from the reality-based show du jour, ``Longitude'' on A&E is a handsomely mounted and absorbing true story of the man who, after a lifetime struggle, solved one of the greatest mysteries in history, creating a clock that allowed sailors to know their position at sea. And while ``Longitude'' is four hours (it will be repeated), it's well worth the time.

In 1707, after a maritime disaster caused by an incorrect calculation of a fleet's position (``guess'' is more appropriate, as navigators had no way of really knowing then, using dead reckoning and an unreliable method of computing knots), the British Parliament offered a sum of 20,000 pounds (about $5 million today) to anyone who could solve the problem of finding longitude at sea.

More than saving lives was at stake, though. Any nation that could find an accurate method of navigation could rule the economy of the world.

Taking up the challenge in 1730 was a Yorkshire carpenter named John Harrison (Michael Gambon), a man with no formal education and who had never been out to sea. Harrison, though, already had fashioned a wooden clock that kept better time than the one in the Royal Observatory.

Of course, a mere carpenter trying to solve a problem that had stumped the best minds of the land didn't sit well with the academic establishment at the time. While he did receive encouragement from Astronomer Royal Edmund Halley (John Wood), the Board of Longitude was prejudiced against the problem being solved by a mechanical method.

So even when Harrison almost succeeds his first time out with a test at sea with his first device, he's met with resistance. Eventually, though, he receives enough backing to continue his work. Harrison, however, knows there is a flaw in his design.

What occurs then is a nearly 50-year quest by the clockmaker, during which he has to overcome both naysayers and his own doubts. For a man obsessed with time, Harrison's own time meant little to him. All of this came at a personal cost. He had hoped to win the prize so his son, William, could grow up to be a gentleman. Instead, Harrison watched his son grow into a disappointed middle-age man.

``Longitude'' takes one great liberty with Dava Sobel's critically acclaimed book in order to give the filmed version a more dramatic reading. Rupert Gould, who is a minor character in her book, becomes something of a parallel story in this version. Gould is a World War I British naval officer who becomes obsessed with restoring Harrison's four clocks, which have fallen into disrepair in the cellar of the Royal Observatory. There he asks Frank Dyson, the current Astronomer Royal (Alec McCowen), if he can put the clocks back in working order. Dyson agrees, but there is no money involved and Gould must do it in his spare time.

With Gould's mental health in an already fragile state and with his wife unable to understand, Gould's obsession, like Harrison's, takes its toll.

The more interesting story, though, is Harrison's, whose obstacles, including facing more rigorous class distinctions and just plain ignorance, are external. Gould's, in contrast, take the form of inner demons. Harrison's stem more from his fears, entirely unfounded, that someone would steal his plans and cheat him out of his reward.

The story of ``Longitude'' is not an easy one to tell. Taking place over a 50-year period, as one might expect, characters come and go. And some of the more esoteric aspects of the tale are smoothed over. But like Harrison's quest, the show ultimately triumphs.

Gambon is superb as Harrison, and Oscar winner Irons has made a living playing vulnerable Englishmen who must keep a stiff upper lip. The cast is peppered with marvelous actors such as Wood, McCowan, Barbara Leigh-Hunt as Gould's mother and Frank Finlay as Adm. Wager.

But the most credit should go to writer-director Charles Sturridge, who get this unwieldy undertaking on course, making reality (that's what history is, after all) entertaining.

The facts

--The show: ``Longitude.''

--What: the story of the man who invented a timepiece that allowed a ships's longitude to be determined accurately.

--The stars: Jeremy Irons, Michael Gambon, Ian Hart.

--Where: A&E.

--When: 8 and midnight tonight. Repeating at 8 p.m. Thursday, 1 and 8 p.m. Saturday, 3 p.m. July 16.

--Our rating: Three and one half stars.

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Photo:

Michael Gambon stars as John Harrison, a carpenter with no seafaring experience who created a clock that allows sailors to determine their position at sea in ``Longitude.''