Queen Elizabeth II
Queen Elizabeth II visits Goodenough College, the leading residential community for British and international postgraduate students studying in London, during a visit in London, Dec. 1, 2016. Reuters/Stuart C. Wilson

Queen Elizabeth II will celebrate her Sapphire Anniversary on Monday, marking 65 years on the British throne. The longest-living monarch took the throne in 1952 after King George VI's death, who didn't have any sons.

In recent days, concerns about the health of the 90-year-old monarch have risen, especially since she skipped annual royal holiday celebrations, including Christmas and New Year services, due to a "heavy cold." However, the queen has recovered since then, attended several events and resumed her royal duties.

While it's time to remember the queen's achievements as she becomes the first British monarch to celebrate the Sapphire Anniversary, there may not be any grand events as the day also marks the anniversary of the death of her father George VI.

In Britain the crown is normally passed on from the monarch to the eldest son. Since King George VI did not have any son, Queen Elizabeth II was crowned on Feb. 6, 1952. But, in 2011, the Act of Settlement was changed before Prince George’s birth to ensure succession would not be affected by gender of the child.

The British Line of Succession:

If the queen gives up her throne, retires or dies, the next in line to sit on the throne will be Prince Charles, the eldest son. However, even if Charles, who has the title of Prince of Wales as well as the Duke of Rothesay and the Duke of Cornwall, becomes king, it is unclear if his wife Camilla Parker Bowles will be named queen. Charles married Camilla after their extramarital affair while he was still married to the late Princess Diana.

After Charles takes the throne, Prince William will become the new Prince of Wales. The eldest son of Prince Charles and the late Princess Diana, is the second in line to the British throne. When William takes the throne, his wife Kate Middleton, the Duchess of Cambridge, is likely to hold the ceremonial title of queen without having any claim to the throne.

William’s oldest child, George Alexander Louis, is third in line, followed by his daughter, Princess Charlotte.

Charles’ younger son Prince Harry is the fifth in line to the British throne. This could change in the future if William and Middleton decide to have more children.