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* Trial will take place according to a 19th century code
* Butler who leaked documents will be tried by 3-judge panel
* Said he wanted to root out corruption, help pope
By Philip Pullella
VATICAN CITY, Sept 27 (Reuters) - The Vatican has certainly seen more sensational trials in its long history. The Inquisition ordered Galileo to recant his theory that the earth revolves around the sun, and philosopher Giordano Bruno was burned at the stake for heresy.
But even those cases, both in the 17th century, did not involve a breach of trust by a papal aide - the issue at the core of this Saturday's trial of papal butler Paolo Gabriele for stealing and leaking the pontiff's personal papers.
One of the worst crises in Pope Benedict's papacy will play out in a small Vatican tribunal, where a three-judge panel will decide the fate of the 46-year-old Gabriele, whom the pope used to call "Paoletto" (little Paul) and who is now described in Vatican documents as "the defendant".
The case will put the inner workings of the tiny Vatican, the world's smallest state, in the type of media spotlight it usually strives to avoid.
The man who served Pope Benedict his meals and helped him dress is charged with aggravated theft for leaking private papers in a self-styled attempt to clean up what he saw as evil and corruption in the headquarters of the Roman Catholic Church.
The documents pointed to a power struggle at the church's highest levels.
Gabriele, who said he saw himself as a whistle-blowing "agent of the Holy Spirit", risks up to four years in jail if convicted, which is widely expected to be the outcome of the case because he has confessed.
Since the Vatican is a monarchy where the pope reigns supreme, the trial will start when the president of the tribunal, standing in front of a crucifix, says, "In the name of His Holiness, Pope Benedict XVI ...".
The trial procedures will be based on a 19th century Italian penal code.
The wood-panelled courtroom, which can hold only several dozen people and has an ornate papal crest at the centre of its ceiling, is the venue for about 30 trials a year, usually for petty crimes such as theft in St Peter's Square, according to Prof. Giovanni Giacobbe, an expert on Vatican law who briefed reporters.
It is not clear how long the trial might last.
Gabriele, a father of three living a simple but comfortable life in the city-state, told investigators after his arrest in May that he believed a shock "could be a healthy thing to bring the Church back on the right track".
The trusted manservant said he wanted to help root out the corruption, "because the pope was not sufficiently informed", according to details made public when Gabriele was indicted in August.
"The Pope cannot tell the judge what verdict to reach, but he can intervene at any time if he wants to, and he can also grant a pardon," Giacobbe said.
Since the papal state has no prison, Gabriele would serve time in an Italian jail if he is convicted and the pope does not pardon him.
Either side at the trial can call witnesses, but the president of the court will decide on each request. The prosecution and the defence cannot directly question the defendant or witnesses but must do so through the judge, Giacobbe said.
The trial will be covered by a pool of eight reporters. Television cameras and recording devices will be not be allowed in the courtroom, but the Vatican will release a short, silent video clip of the opening of each session of the trial.
Gabriele's arrest capped nearly five months of intrigue and suspense as a string of documents and private letters found their way into the Italian media.
The most notorious of the letters were written to the pope by Archbishop Carlo Maria Vigano, currently the Vatican's ambassador to Washington, who was deputy governor of Vatican City at the time.
In one, Vigano complains that when he took office in 2009, he discovered corruption, nepotism and cronyism linked to the awarding of contracts to outside companies at inflated prices.
Vigano later wrote to the pope about a smear campaign against him by other Vatican officials who were upset that he had taken drastic steps to clean up the purchasing procedures.
Despite begging not to be moved away from the Vatican, Vigano was later transferred to Washington by Secretary of State Cardinal Tarcisio Bertone, the Vatican's number two.
Other leaked letters concerned the Vatican's bank, which has been at the centre of several scandals in the last few decades.
The Vatican has described the revelations as a "brutal" attack on the pope. Benedict himself has merely alluded to personal pain and criticised a media portrayal of the Vatican that "does not correspond to reality".
Gabriele will go on trial alongside Claudio Sciarpelletti, a Vatican computer expert who is charged with aiding and abetting a crime. Sciarpelletti risks up to one year in jail. (editing by Jane Baird)