Britain's Queen Elizabeth II is celebrating her 91st birthday today. The world’s oldest and longest reigning monarch will celebrate her birthday in a low-key fashion privately at her Windsor Castle home near London. Artillery gun salutes in Hyde Park and at the Tower of London will be the only formal ceremony to mark her birthday. Buckingham Palace have shared a special photo of her to mark the occasion. The queen has also an official birthday in June which is publicly marked with a large parade of soldiers through central London, known as Trooping the Colour.