One of the bedrooms which visitors will see is Diana's favorite bedroom, the King William III bedroom. She liked its feminine touch and the view from the window across the deer park.
Another room is called the Great Room, which houses some historical paintings. One is a portrait of Lady Jane Grey, who in Tudor times was queen of England for only nine days. It is believed to be the only portrait of her that was painted during her lifetime.
The Spencer family originally came to Althorp as sheep farmers, building a house there in 1508. Charles Viscount Althorp became the ninth Earl Spencer on the death of his father in 1992.
But he's better known across the world as Princess Diana's brother and it's because of Diana that many people will come to Althorp this summer. They will come to see the estate and the museum and to pay their respects at the island on the lake, her final resting place.
It hardly seems possible that it was nearly four years ago, on August 31, that Princess Diana was killed in a Paris car accident, along with her boyfriend, Dodi al Fayed, and driver Henri Paul.
"When Diana died, I was confronted with a major problem, really," explains Earl Spencer. "By rights, she should have been buried in the local church in the family tombs, which have looked after my family for 20 generations. But it just didn't seem right. The village would have been swamped, and I wouldn't have been able to cope, I don't think, with sightseers."
Also, he wanted Diana's sons to be able to visit their mother's grave in privacy. The ideal setting seemed to be right there at Althorp: On an island in the middle of a lake. Diana had always loved water; it was her element.
"I find that when I speak to the pubic when they're going round (the lake), they all bring their own tragedies, their own disappointments in life," Earl Spencer says. "And they sit on these benches around here for hours, and... you can see they're miles away, thinking about their own things."
His theory is that Diana's death is an emotional touchstone. People easily remember their grief over the passing of the princess, and that serves as "a springboard for their own worries, their own disappointments."
Charles' father, the late Earl Spencer, was a keen photographer and in the exhibition at the stables, visitors can see his home movies of Diana as a baby: her christning in 1961, her first birthday party in August 1962, even her first steps, and playing with her younger brother in the garden.
"I particularly like the footage of Diana dancing," says Earl Spencer. "She's a great dancer and liked to play up to the camera. It's very moving to see this carefree girl enjoying herself, before the sequence closes with a shot of her looking so very happy, young and innocent. It really brings home the tragedy of her premature death 30 years later."
The museum, a half dozen rooms in the stables, highlights different moments from Diana's life. These include a record of a dinner party where Diana first properly met Prince Charles in 1977.
"My father kept records of all his entertaining, and here are a record of the guests who were at that dinner and what they ate and drank," says her brother. "So what was the record of a family event subsequently took on a meaning that was slightly more significant."
A very significant moment in history. Four years later, Diana married the Prince of Wales at Westminster Abbey and went on to become possibly the most famous woman in the world.
Yet, when he is asked what it was like to grow up with such a person, Earl Spencer acknowledges that it was an extraordinary thing to watch his sister transform from a shy little girl into an international superstar. "But there again...I have no other experience," he adds. "That is the person I grew up with. I've got nothing to compare it with."
By the time of Diana's death, her younger brother had inherited not just Althorp and the title "Earl Spencer" but also other responsibilities, including the delivery of his sister's eulogy, a speech so steeped in emotion that it made headlines around the world.
"I remember finding the last 30 seconds or minute of the speech intensely difficult to get out," he recalls. "The last bit, when I was talking to the boys across their mother's coffin, was about as crucifying as you can get, in terms of agony, and I could barely get it out. So the reaction afterwards, I never, just honestly never noticed it. I just dragged myself back to the pew we were in and just collapsed, really."
What it was that made her unique?
"She had a unique charisma, but also a way of never being distant from people," says Earl Spencer. "People felt very connected to her when they saw her, when they spoke to her. Whenever I saw her in action in her public role, it was beyond anything else I've ever seen."
Did he see that as a brother?
"A brother, especially a younger brother, is probably the most critical audience anyone can have," he says. "But I was always amazed by the presence she had in a room... After a charity event, say, she would have an orchestra standing around her who she was thanking for having performed, and they were enthralled by her. They weren't just interested by her. It wasn't just a simple star worship at all. It was something much more, and I don't know what it was, but it was somethng unique to her. I've never seen it with anyone else."
It's that unique quality that made her sudden death so difficult for people to comprehend. Earl Spencer says that she seemed probably the least likely person to die, and as her brother, he's still trying to come to terms with that.
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