By Eric Marrapodi, CNN Belief Blog Co-Editor

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(CNN)– Michelangelo's fresco on the ceiling of the Sistine Chapel, one of the world's most iconic pieces of art, celebrated its 500th anniversary on Wednesday in Vatican City. Pope Benedict XVI marked the occasion with the celebration of Vespers in the chapel on Wednesday evening.

Nine centered panels in the ceiling fresco show stories from the book of Genesis, fanning out from the center of the ceiling with the iconic "Creation of Adam" that shows God reaching down from heaven and touching the finger of Adam. The vaulted ceiling also features images of biblical prophets and ancestors of Jesus.

Work on the ceiling began in 1508 when Pope Julius II della Rovere decided to make some changes to the room including the ceiling alteration. He commissioned Michelangelo Buonarroti to paint the ceiling and the lunettes, which are the upper parts of the room. According to the Vatican, Julius dedicated the newly decorated space with a Mass on the Feast of All Saints Day, which falls on November 1.

Michelangelo painted over a starry sky scene put on the ceiling between 1477 and 1480 according to Vatican records.

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British Art critic Andrew Graham-Dixon writes in his book "Michelanglo and the Sistine Chapel," that the artist did not want to paint the 12,000-square-foot ceiling, which is 68 feet high, because he thought it was a ruse by his enemies to get him to fail on a grand stage. "As they well knew he was a sculptor, not a painter, and would be bound to make a fool of himself," Graham-Dixon wrote.

“He kept turning it down saying 'I’m not a painter; I’m a sculptor,'” said Diane Apostolos-Cappadona, a Georgetown University professor who has studied the work extensively. She said Michelangelo had come to Rome to sculpt the tomb of Julius II, which he thought would be his masterpiece.

Originally the idea from the papal commission was to have Michelangelo paint the 12 apostles on the ceiling, but somewhere along the way the plans changed. “He was up on the ceiling, and the concept just was not working for him," Apostolos-Cappadona said.

Apostolos-Cappadona said correspondence at the time suggest the subject shifted to Genesis after conversations related to the writings of Saint Augustine between the artist and a prominent theologian, Egidio da Viterbo.

It took a full four years to complete the sweeping painting which stretches out above the chapel's 40.23 meters in length by 13.40 meters in width footprint, dimensions which are thought to mirror Solomon's Temple as described in the book of Kings.

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“He created paintings, that in many ways, the figures seem three dimensional. That’s something he does that other painters of the day did not do. From the ceiling you need the mass and volume,” Apostolos-Cappadona said.

Pope John Paul II called the Sistine chapel, "the shrine of the theology of the human body," in a homily following restorations for the space in 1994.

"This is a priceless cultural and universal heritage," the late Pope John Paul II said at the time. "This is confirmed by the countless pilgrims from every nation in the world who come to admire the work of the supreme masters and to recognize in this Chapel a sort of wonderful synthesis of painting."

Visitors today can still see the Michelangelo's fresco as the crown jewel of the Vatican Museum tour. The chapel space is still used by the Vatican for worship services, which museum staff loudly remind chatty patrons to keep quiet and not take pictures.

The chapel is also used for the secret conclave to elect the new pope. The College Cardinals, whose job it is to elect the new pope, gather there to cast ballots for the man who would become the next pope.

As cardinals cast their secret ballots they walk under the painted ceiling through the steps of the creation story and put their ballot in an urn on the altar. Above the altar is another gigantic Michelangelo fresco, "The Last Judgement," which takes up the entire wall and was finished in 1541.

"The ceiling is almost a background for that extraordinary picture of "The Last Judgement," Cardinal Theodore McCarrick, the archbishop emeritus of Washington, told CNN. McCarrick was a member of the conclave that elected Pope Bennedict the XVI in April 2005.

"The ceiling sort of prepares you for that with the creation of man and all that. When you come to the altar, and you look right behind the altar is this extraordinary painting of "The Last Judgement," he said.

Standing in front of the painting and casting a ballot for the next pope he said, "ceases to become an election and becomes more of a discernment, a discernment of what God wants you to do. A discernment of what you think God wants you to do, to pick the man God needs for the church today."

"All those things go through your head when you're in that extraordinary building, that extraordinary chapel. You are about to do an extraordinary thing," he said.

The Vatican Museum director Antonio Paolucci, writing in L'Osservatore Romano, noted in the Wednesday edition, "five million visitors a year inside the Sistine Chapel, 20,000 per day at peak periods, certainly bring about a difficult problem."

"The anthropic pressure with dust, the humidity which bodies bring with them inside, the carbon dioxide produced by perspiration involves discomfort for visitors and damages to the painting in the long run," he added.

Paolucci said there are no plans to limit access to the work in the short term or medium term, rather he said, "we have been graced with technology which allows us, if used correctly, to preserve Michelangelo's work which history has given us under the best conditions, for the longest possible time."

For those who cannot get to Rome to see it, the Vatican website has a virtual model that lets you move through the room and see all the artwork, including the ceiling and "The Last Judgement."