The Duke of Edinburgh is celebrated with gun salutes in London as he spends the day "privately," a Buckingham Palace source tells PEOPLE

By Simon Perry
Updated June 10, 2015 12:40 PM
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Credit: Samir Hussein/WireImage

As he turns 94 on Wednesday, Prince Philip is still the one who can elicit the best smiles from his famously stiff-upper-lipped wife.

The pair put their sweet bond on display Tuesday, when they joined other members of the family, including Prince Harry, at a celebration to mark the 200th anniversary of the Nepalese Gurkha regiment’s service to the crown.

And he’ll be with her in Germany later this month, continuing a vow he made when he married her in 1947 to be her consort.

“For nearly 70 years he has been a few paces behind the Queen and been at her side and supported her,” says his longtime biographer, Phil Dampier. “That’s what he vowed to do when he gave up his career in the Navy to support her, and no one could have done a better job.”

Philip is determined to spend Wednesday “privately,” a Buckingham Palace spokesperson tells PEOPLE.

But he might not get much peace: With a 41-gun salute in Green Park by the King’s Troop Royal Horse Artillery trotting past the palace and another 62-gun salute fired across the River Thames by the Honorable Artillery Company, central London was a noisy place by lunchtime.

The Duke of Edinburgh (to use his proper title) has had health setbacks in recent years, but he “keeps young by being active. He can’t stand sitting around,” says Dampier.

The royal husband lives by a strict routine of going to bed by midnight and rising at 6 a.m. He likes only the occasional beer, and he’s exceedingly careful about what he eats and keeps quantities small.

It’s his humor that keeps a smile on the Queen’s face and provides her critical support, says Dampier. “When you see them at the end of an engagement, they get back in their car, or carriage, put a blanket over their legs, and you always see them laughing,” he notes. “You see that all the time, especially on ceremonial occasions. He’s obviously making some quip.”

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“I’m sure that s been a great factor in her success, the fact that his humor has kept her going when sometimes situations may have been difficult,” adds Dampier, who has coauthored two books compiling more than 500 of the prince’s quips and occasional foot-in-mouth moments: Prince Philip: Wise Words and Golden Gaffes and Duke of Hazard: Wit and Wisdom of Prince Philip. “They keep each other going.”

And despite the royal family’s stoic reputation, Prince Philip has a surprising side when it comes to PDAs.

When Prince Harry, 30, returned from New Zealand and opened his special charity garden at Chelsea Flower Show last month, Philip arrived to congratulate him. He walked up to his grandson and greeted him with a smooch on each cheek.

“He comes from a Danish-German background, and the male members of the royal family always kiss each other,” says Dampier.

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