London, Apr 21: UK's Queen Elizabeth II turned 89 today and she celebrated her birthday privately at her Windsor Castle residence.
She has been staying at the castle in Berkshire, south-east England, for the past month and has no official engagements listed for her big day.
In her honour, the King's Troop Royal Horse Artillery staged a 41-gun royal salute. The salute follows a selection of celebratory music from the Band of the Royal Artillery close to the firing position in Hyde Park, central London.
The King's Troop in full dress uniform rode their horses and gun carriages past Buckingham Palace en route to Hyde Park for their gun salute, using six 13-pounder Field Guns dating from around the time of World War I.
The Honourable Artillery Company also fired a 62-gun royal salute from the banks of river Thames near the Tower of London.
Gun salutes will also take place at Edinburgh Castle in Scotland, Cardiff Castle in Wales and Hillsborough Castle in County Down.
The basic Royal salute is 21 rounds. A further 20 are fired at royal parks and royal palaces and fortresses and another 21 for being in the City of London.
Though the Queen's actual birthday is April 21, her official birthday is on June 12. The tradition of two birthdays dates back to the time when, if monarchs were born in winter, the weather was deemed unsuitable for outdoor events.
The birthday comes as people begin to gather outside St Mary's Hospital in London in anticipation of the birth of the Queen's newest great-grandchild.
The Duchess of Cambridge, Kate Middleton, is expected to give birth to her and Prince William's second child there any time now.
Kensington Palace said Prince William was now on unpaid leave until June after completing the first phase of his in-work air ambulance helicopter pilot training sooner than expected.
A palace spokesperson said he would return to work after a period of pre-planned official royal engagements in late May.
Prince Harry will also briefly return to the UK this weekend, following the Gallipoli commemorations in Turkey, to attend the London Marathon as patron of the London Marathon Charitable Trust.