Pope Benedict XVI, in the Year of Our Lord 2009, will have a remarkable opportunity to shape further the archiepiscopal landscape of the Catholic Church around the world. As Britons wonder and worry about who the Pope will pick to succeed the nation's top Catholic leader in Westminster, Catholics in several other cities also wait to see who will replace their own ageing cardinal-archbishops. They include New York, Seoul, Cologne, Prague, Guadalajara and Manila - to name but a few.

Cardinal Cormac Murphy-O'Connor, at 76, is not the only cardinal beyond the canonical retirement age of 75 who is still in office. Twelve others are even further along in years, including Cardinal Michael Kitbunchu of Bangkok, who will be 80 later this month, and Cardinal Adam Maida of Detroit, who will be 79 in March. They are among the 21 cardinals over 75 who, at the end of 2008, were still governing Latin Rite archdioceses around the world. And during the course of 2009 four more cardinals will cross this threshold.

That is only the tip of the iceberg. Prelates aged 75 and older currently head more than 200 local dioceses. Fifty-five of these are archdioceses. And a further 118 diocesan/archdiocesan ordinaries will reach retirement age during this year. But even more critical than the ageing of the episcopate is the fact that more than 90 dioceses and seven archdioceses are currently "vacant", that is, they do not even have an ordinary. At least 38 of these places have been vacant since 2007 or earlier, all but 10 of them because the Vatican "promoted" or "retired" the ordinary without naming a successor.

These statistics suggest that Pope Benedict and his aides do not always have an easy time finding the right man for the right job. Perhaps that is why some Vatican officials - the Pope included - are said to be intent on raising the retirement age to 78.

Over the last two centuries the appointment of bishops has emerged as one of the central duties - and means of power - of the modern papacy. This is especially true when it comes to naming archbishops (even those who are not cardinals), because these men head some of the Catholic Church's oldest, largest and most important local churches.

The Pope relies heavily on recommendations from the Congregation for the Evangelisation of Peoples (Propaganda Fide) when naming bishops for dioceses in "mission" territory such as Africa and Asia or the Congregation for Oriental Churches when approving heads of Eastern Rite dioceses. For almost everywhere else, including Westminster, the Pope is assisted by the Congregation for Bishops. The Congregation's procedures in episcopal appointments is unveiled in a chapter of the 1989 book Archbishop by Fr Thomas Reese SJ, which, Vatican officials say, remains a valid point of reference even today.

For example, the papal nuncio to Britain, Archbishop Faustino Sainz Muñoz, is the man who would have done most of the initial preparation for vetting and suggesting a successor to Cardinal Murphy-O'Connor. First of all, he would have consulted the cardinal and requested his detailed analysis of the state of the archdiocese and its needs. The nuncio would have also queried other British bishops, as well as priests and reliable lay Catholics of the archdiocese - and anyone else he deemed to be informative and useful. The process takes much time and discretion and involves a limited number of people. But, above all, it is highly confidential.

Once the nuncio has completed the consultations he draws up a detailed report, which he sends with many pages of documentation to the Congregation for Bishops. Included is the all-important terna, or list of the top three candidates. Archbishop Vincent Nichols (Birmingham) and Archbishop Peter Smith (Cardiff) are believed to have secured a place on the terna for Westminster. Speculation has been divided over whether the third candidate was Bishop Malcolm McMahon OP (Nottingham) or Bishop Arthur Roche (Leeds). Naturally, the speculation could be wrong and any other variety of names may have been on the nuncio's terna.

Whatever the case, the material is then sent to Rome (it can sometimes fill up several boxes) and the priest-staff member at the Congregation who deals with dioceses in Britain carefully checks it for accuracy or missing information. Once this process is completed, the undersecretary at the Congregation asks a cardinal member to study the material and then prepare and present a summary for the other members. This cardinal presenter is called the ponente. The presentation takes place at one of the Congregation's fortnightly meetings in Rome and afterwards the members vote on the candidate they think should be appointed to the relative see. Unfortunately not everyone is able to attend these meetings.

There are 32 members of the Congregation for Bishops - 26 cardinals and six archbishops. Fifteen of these are heads of Roman Curia offices and another eight are retired (but aged under 80) Vatican officials who live in Rome. Only seven of the members are residential bishops, while another is the Pope's Vicar for Rome. Twenty-four of them are over the age of 70.

All but five of the Congregation's 32 members did theological studies in Rome, which is helpful because the meetings are always conducted in Italian. This is reinforced by the fact that 11 of the 24 European members are from Italy - including the Congregation's prefect, Cardinal Giovanni Battista Re. The only members whose native tongue is English are four cardinals from the United States. There are also four men from Latin America and three each from Poland and Germany among the members. This is the make-up of the group of men who will have voted on the terna and suggested to Pope Benedict who should be the next Archbishop of Westminster.

Cardinal Re would have hand-delivered this information at one of the routine private audiences he has each Saturday with the Pope. Once he receives a recommendation the Pope can immediately name the new bishop or he can wait and ponder his decision for an indeterminate amount of time. Benedict XVI seems to favour the latter approach.

It is hard to know how much influence an outgoing bishop might have in naming his own successor. Obviously, it is helpful if he is held in high esteem by the prefect of the Congregation for Bishops, the members of the Congregation and, of course, the Pope. Cardinal Murphy-O'Connor had an opportunity to give his opinion a few weeks ago when he had a rare private meeting with Pope Benedict.

In any event, it is a good bet that the man who finally gets Westminster will have already been a bishop. In all but a very, very few cases Pope Benedict has chosen Latin Rite archbishops from among already-serving diocesan ordinaries or auxiliaries. Among the exceptions were two of Pope John Paul II's private secretaries - now-Cardinal Stanislaw Dziwisz (Krakow) and recently appointed Archbishop Miroslaw Mokrzycki (Lviv). The only other exceptions were Italians. The Pope directly appointed three priests to head small archdioceses in Italy but, more surprisingly, he named an Italian priest from the Catholic political organisation Communion and Liberation to be archbishop and head the Apostolic Administration in Moscow.

Westminster's succession has drawn intense media attention in Britain, and understandably so. After all, this will be Pope Benedict's first chance to name an archbishop in Great Britain. Up to now he has only had the opportunity to appoint three of the 22 diocesan bishops in England and Wales plus five auxiliaries. And he has also named two of the eight bishops in Scotland.

Movers and shakers in Westminster continue to suggest that Cardinal Murphy-O'Connor's resignation is likely to be accepted in the next few weeks and that a successor will be named the same day. However, it is never wise to try to set a time-line to episcopal appointments, especially in the current pontificate. But when the day finally arrives, perhaps Pope Benedict will also appoint a new bishop to the Diocese of Hexham and Newcastle.

Lest anyone forget, the 200,000 Catholics that make up England's most northern diocese have been without a shepherd since last March following the death of Bishop Kevin Dunn.