It's a birthday the world can share in. The peals of London's favorite clock are carried globally by BBC radio, and its 315-foot tower, roughly 16 stories, is the city's most famous landmark.
But getting inside and seeing Big Ben, the sonorous main bell that gives its name to the whole contraption, isn't easy. Security measures mean few are granted admission, and there's no elevator, so those who are escorted in must climb 334 winding limestone stairs.
Catherine Moss, who took journalists on a pre-anniversary tour, said that in one year as a Big Ben guide, she had climbed the height of Everest three times over.
"It's my own private step machine," the trim-looking 51-year-old called down from the top of the tower.
No special events are planned, aside from an exhibition opening Sept. 19 in the nearby parliamentary offices.
Although the tan-colored tower above the Houses of Parliament is covered in a riot of gilt crowns, sculpted masonry and coats of arms, the interior looks functional. The 4.2 meter- (14 foot-) long minute hand casts a faint shadow over the pale white glass of the dial. The 5-ton (5.6-U.S. ton) clock mechanism, like a giant wristwatch, is wound three times a week. In the age of atomic clocks, its near-perfect time is regulated by heavy old pennies laid on or removed from the pendulum.
The chimes, supposedly based on four notes from Handel's "Messiah," ring out every quarter hour from the intricately ornamented belfry. The bongs of Big Ben itself are heard every hour.
It is rare - and a matter of citywide consternation - for the clock to go mute. But wars and accidents happen. Initial construction was one disaster after another, and in 1916 the chimes were stopped for two years lest they guide German bomber zeppelins to the parliament building.
Carried on BBC radio since 1924, the chimes took on added significance in World War II. Every night Britons observed a minute's silence as the clock struck nine. It was called the Big Ben Minute. Even as German bombs fell and air raid sirens howled, Big Ben's voice was heard.
The solemn chimes were a metaphor for Britons unflappable under fire, says Tam Dalyell, 76, a former lawmaker.
"It's defiant," Dalyell said in a telephone interview from his home in Scotland. "It's a sign of confidence and undaunted spirit."
On the night of May 10, 1941, an air raid wrecked the parliament building, sending up flames as high as the belfry. A small explosive shattered the clock's south dial and damaged stonework, but the clock didn't skip a second.
Its durability was "as great a boost to the morale of the British people as the speeches of Winston Churchill," according to Peter MacDonald, the author of "Big Ben: The Bell, the Clock and the Tower."
Moss, the tour guide, said she sees it in the faces of Blitz survivors who visit the tower.
She recalls a woman who lost her father in the war and was crying. "The sound of the bell means everything to her. And to stand up here, over 80 years of age, and to see it, she just ... tears. And she's not the only one; a lot of people cry when they hear the bell."
But the clock and tower got off to a bad start. Construction was marred by delays, budget overruns and bureaucratic squabbling. The tower proved too small for the clock machinery. The 16.25-ton (18.2-U.S. ton) copper and tin main bell had to be scrapped when it cracked during testing. It was forged anew, slimmed down, and painstakingly hoisted to the top of the tower. Meanwhile, the minute hands had to be replaced - twice - because the first two sets were so heavy they couldn't circle the tower's dials.
On May 31, 1859 the clock officially started keeping time. Big Ben - the origin of the name is disputed - started tolling a few days later, but within months it cracked again and didn't resume service until 1862.
At first lawmakers complained that it was too loud, while the Times of London lamented that the blunders surrounding the project were "a disgrace to all concerned in it."
But as the bad memories faded and London's collective ear became used to the bong of the great bell, the tower became a cherished part of the city's landscape - and the country's self-image.
Honoring Big Ben's centenary in 1959, the then speaker of Parliament, William Morrison said the clock was like Britain's legislature - "born in controversy and wrangling, yet it keeps excellent time and serves us admirably."
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(This version CORRECTS Corrects height of Big Ben, minor edit. AP Video.)