Absent Prince Philip Casts Shadow over Queen's Jubilee Tim Ireland / WPA Pool / Getty Images Queen Elizabeth II, after a service of thanksgiving to mark her Diamond Jubilee at St. Paul's Cathedral in London on June 5, 2012 Grace Jones marched onstage with a red hula hoop. Annie Lennox pranced around in white angel wings. And Elton John tickled the ivories sporting a pink sequined jacket. But amid all the color of the Diamond Jubilee concert, held outside Buckingham Palace on Monday night, there was one glaring dark spot. Halfway through the concert Queen Elizabeth II entered the royal box wearing black. Just hours before the concert began, an ambulance had rushed Prince Philip, her husband of more than 64 years, to a hospital to treat a suspected bladder infection. His sudden illness meant that he would miss out on the final two days of celebrations marking the Queen's 60th year on the throne. Her Majesty felt it: not even royal stoicism could camouflage her remarkably dour expression as the concert unfolded without him. At the climax of the evening, shortly before fireworks exploded overhead, Prince Charles gave voice to his mother's concern. "The only sad thing about this evening is that my father couldn't be here with us, because unfortunately he was taken unwell," he said before a crowd of 10,000 well-wishers. But ladies and gentleman, if we shout loud enough, he might just hear us in hospital and get better. The Duke of Edinburgh, who turns 91 on June 10, will remain under observation at London's King Edward VII Hospital for several days. Buckingham Palace described it as a "precautionary measure" in a statement released on Monday. Although his condition is not life-threatening, the incident brings to the fore fears - more felt than spoken - that Prince Philip is slipping away. On Tuesday afternoon, about 20 photographers and journalists stood watch outside the hospital where he is being treated, and six broadcasting vans with antennas sat ready to beam out updates on the Duke's condition. Two guards watched the door of the hospital. Several more stood on corners watching the media scrum. See TIME's coverage of Queen Elizabeth's Diamond Jubilee. Britain's tabloid press - which has remained on its best behavior throughout the Diamond Jubilee - isn't working overtime merely to get a scoop. The media's concern stems from very real developments with Prince Philip's health that remain on the public's mind. In the fall of 2010, Prince Philip relinquished his commitments with several charities and organizations like the Royal Agricultural Society and his chancellorship of Cambridge and Edinburgh universities. Speaking to the BBC for a documentary timed to his 90th birthday last June, he said rather bluntly that "it's better to get out before you reach the sell-by date." The presenter protested, but Prince Philip remained adamant about the personal limitations that come with age. "There is an ageism in this country, as everywhere, and quite rightly so, because I think you go downhill - physically, mentally and everything," he said. I reckon I've done my bit. I want to enjoy myself for a bit now. With less responsibility, less rushing about, less preparation, less trying to think of something to say ... On top of that, your memory's going, I can't remember names. Yes, I'm just sort of winding down. Prince Philip's version of winding down, however, hasn't exactly involved leading a more secluded life where he sets his own schedule. Despite undergoing heart surgery just five months ago, he has traveled up and down the country with the Queen as part of her Diamond Jubilee tour, attending fashion shows, sitting through lengthy speeches and standing upright for hours at a time as the Queen cut ribbons and unveiled plaques. Regardless of age, the perks and privileges of a royal life still come wrapped in self-denial and sacrifice - a reality made all the more evident by the Thames River pageant on June 3. Throughout the four-hour spectacle - during which 1,000 boats passed the royal barge at the leisurely pace of 4 knots - Prince Philip and the Queen did not appear to sit down. Sharp winds blew into their faces, temperatures dipped into the mid-40s, and rain blighted much of the afternoon. Neither wore a coat, and Philip's ceremonial uniform appeared particularly heavy and uncomfortable given the series of medals that accompany the ceremonial regalia. It's unclear if either of them had an opportunity to use the restroom. Commentators have already wondered aloud whether the Prince's bladder infection resulted partly from his duties onboard the royal barge. Muscles inside the bladder weaken with age, which makes it more difficult to empty the bladder fully. That boosts the chances of bacteria growing in the residual urine. Not using the restroom for an extended period of time gives that bacteria even more time to multiply inside the body. Prince Philip's heart surgery last December could also have played a role in his sudden illness. Doctors inserted a coronary stent - essentially a metal tube - into his heart to release arterial blockage. As part of that procedure, doctors sometimes prescribe drugs to suppress the immune system afterward. The fear is that the body will attempt to reject the stent, which it reads as a foreign object, and that in turn could increase the likelihood of another blockage forming. Such medications make it more difficult to fight off infections. But an explanation of the cause will come later. On Tuesday, the Queen had still more ceremonies to attend - albeit without her beloved Prince. The Palace did its best to make her comfortable. As a car ferried her to a service of thanksgiving at St. Paul's Cathedral on Tuesday morning, her lady-in-waiting accompanied her in Prince Philip's place. Inside the cathedral, Prince Charles occupied the red velvet seat next to his mother - a seat Prince Philip has occupied for so many years. It's no wonder, then, that during a ceremony meant to honor the Queen, the Archbishop of Canterbury Rowan Williams took a moment to honor her husband. "She has made her public happy, and all the signs are that she is herself happy, fulfilled and at home at these encounters," he said. The same, of course, can manifestly be said of Prince Philip, and our prayers and thoughts are very much with him this morning. With reporting by Anoosh Chakelian / London Next The Stamp of Her Influence