Queen Elizabeth II's coffin left Buckingham Palace at exactly 2:22 p.m. Here's why.

Queen Elizabeth II left Buckingham Palace for the final time on Wednesday as her coffin was escorted through Central London to lie in state at Westminster Hall. The queen's coffin left the palace precisely at 2:22 p.m. local time – and for a very specific reason. 

Biographer, historian and Wall Street Journal columnist Amanda Foreman told CBS News that the procession left the palace at that time so that the coffin would arrive at Westminster Hall, where the queen will lie in state until her funeral on Monday, at 3 p.m. 

"They timed it using a drum which beats 75 beats per minute, which enabled them to arrive at 3 p.m.," Foreman said. "The time of 3 p.m. wasn't itself significant, but leaving at 2:22 was the kicker." 

And while the time itself wasn't significant, the steps to accomplish such a task were. 

"The horses required incredible training to be able to walk that slowly," Foreman said. "It's not a natural gait for a horse, let alone a human being. So that symbolized the extraordinariness of the occasion. Everyone is doing something different." 

The roughly 40-minute walk brought together the most senior members of the royal family, including Britain's new monarch and the queen's son, King Charles III, and his three siblings. The queen's grandchildren, William, Prince of Wales, and Harry, Duke of Sussex, also marched side-by-side behind the coffin. It was the first time they have walked together behind a coffin in 25 years, when they had to do so for their mother, Princess Diana. 

As the queen lies in state, members of the public will be permitted to visit her coffin 23 hours every day until her funeral on Monday. CBS News' Norah O'Donnell reported that at least one million people are expected to say their goodbyes to the queen, and that by the time her coffin reached Westminster Hall on Wednesday, there was already a line miles long.

The funeral will take place at Westminster Abbey. The queen's coffin will be transported from Westminster Hall to the abbey by a gun carriage, with senior royals following behind.

After the funeral service, there will be a private service at Windsor Castle later that day, before she is laid to rest at the King George VI memorial chapel with her parents and sister. The coffin of her husband, Prince Philip, will be moved to rest alongside hers. 

In:
• King Charles III
• Prince William Duke of Cambridge
• Prince Harry Duke of Sussex
• King Charles
• Queen Elizabeth II
• The Royal Family
• United Kingdom
• London