Location of the tabernacle in Catholic churches?
To understand why many churches have their tabernacle in places other than over the classical position of behind the main altar we will have to take a walk down memory lane and then try to understand the present situation from an historical perspective.
We must acknowledge that we are living in a very modern society and as such the Church must do all in her power to promote reverence towards the Blessed Sacrament, as well as putting safeguards in place To avoid any irreverence being done towards the Blessed Sacrament.
Now, I am going to let Fr. George Saunders do a little explaining on the subject, as his explanation is quite balanced and has a good historical background also.
To approach this question fairly and adequately, we need to understand some of the liturgical laws through history surrounding tabernacles. Actually the first norms governing tabernacles were promulgated in the Middle Ages. Until this time, no uniform custom regarding where tabernacles were located in churches existed.
The Fourth Lateran Council (1215) decreed that the Blessed Sacrament be kept in a secure receptacle and placed in a clean, conspicuous place. The Synods of Cologne (1281) and Munster (1279) stipulated that the Blessed Sacrament be kept above the altar, sometimes in tabernacles shaped like doves and suspended by chains. (An example of this type of tabernacle is on exhibit in the medieval collection of the National Gallery of Art.)
Overall, during these times, the Blessed Sacrament was reserved in four possible ways: in a locked cabinet in the sacristy, a custom originating in the early Church; in a cabinet in the wall of the choir area, or in a cabinet called the "Sacrament House," which was constructed like a tower and attached to a wall near the altar; in a "dove" receptacle suspended from the baldachino above the altar; and in a tabernacle on the altar itself or in the reredos of the altar.
In the sixteenth century, the Blessed Sacrament became customarily reserved in a tabernacle that was placed on the altar or part of the reredos. However, only in 1863 did the Sacred Congregation of Rites prohibit the use of suspended doves and sacrament houses.
The liturgical reforms of the Second Vatican Council prompted a "rethinking" of the location of the tabernacle in the church. Two important points must always be kept in mind: First, reverence for the holy Eucharist must be preserved and promoted. The "Constitution on the Sacred Liturgy" reminded us that the holy Eucharist is "a sacrament of love, a sign of unity, a bond of charity, a paschal banquet in which Christ is consumed, the mind is filled with grace and a pledge of future glory is given to us" (No. 46).
Second, the significance of the offering of the Mass itself, where the holy Eucharist is confected, must be preserved and promoted. The "Dogmatic Constitution on the Church" asserted, "Taking part in the Eucharistic sacrifice, the source and summit of the Christian life, they offer the divine victim to God and themselves along with it" (No.11).
Accordingly, the "Instruction on the Worship of the Eucharistic Mystery" (1967 issued regulations (later incorporated into the new "Code of Canon Law") concerning tabernacles (cf. No. 52-57 and Canons 934 944): The holy Eucharist may be reserved only on one altar or one place in any church, and a vigil lamp must bum at all times to indicate and honor the presence of our Lord in the Blessed Sacrament. This tabernacle must be immovable, made of solid and opaque material, and locked to prevent theft or desecration of the Blessed Sacrament. The tabernacle "should be placed in a part of the Church that is prominent, conspicuous, beautifully decorated, and suitable for prayer" (Canon 938).
Here is where some confusion emerges. To promote prayer and devotion, the "Instruction" stated "It is therefore recommended that, as far as possible, the tabernacle be placed in a chapel distinct from the middle or central part of the church, above all in those churches where marriages and funerals take place frequently, and in places which are much visited for their artistic or historical treasures" (No. 53).
For example, at St. Patrick's Cathedral in New York City, which has a constant flow of tourists, the Blessed Sacrament is reserved in Our Lady's Chapel located behind the main altar, this beautiful chapel provides a quiet place for the faithful to pray without the distraction of the comings and goings of people. A similar situation exists at the Basilica of the National Shrine of the Immaculate Conception in Washington.
However, this recommendation does not necessitate the interiors of "old" churches be destroyed to move the tabernacle. The "Instruction" stated, "In adapting churches, care will be taken not to destroy treasures of sacred art" (No. 24). Moreover, any renovation should be done with "prudence."
Moreover, the "Instruction" recommendation does not prohibit having the tabernacle in the center of the church, stating, "the Blessed Sacrament should be reserved in a solid, inviolable tabernacle in the middle of the main altar or on a side altar, but in a truly prominent place" (No. 54). The tabernacle can be located in the "center of the church," perhaps on an elevated area behind the altar so as not to diminish the attention to the Eucharistic sacrifice. Actually, I think the visual alignment of the tabernacle and altar emphasizes best both reverence for the Holy Eucharist and the significance of the sacrifice of the Mass.
From a purely educational perspective, the goodness of having the tabernacle in the body of the church either in the center, or at least to the side, is that it fosters devotion to the Blessed Sacrament. For instance, people genuflect in reverence to the Blessed Sacrament. Since the one day most parishioners visit their church is on Sunday, having the tabernacle visible in a prominent and conspicuous location makes them aware of the Eucharistic presence of our Lord. The people are more mindful that church itself is the "House of God" and a sacred space, not just a meeting house. In an age of doubt and disbelief, we need to do all we can to promote and foster devotion to our Lord in the Blessed Sacrament. - Placing the Tabernacle
For those visiting St. Peter’s Basilica in Rome, you will notice that the tabernacle is not over the main altar, but in a small side chapel in order to promote more reverence to the Blessed Sacrament. The amount of tourists that the Basilica has every day, makes this a viable solution for maintaining proper reverence in the presence of the Blessed Sacrament.
The Sacred Congregation of Rites, 21 August, 1863, forbade the use of these Sacrament-Houses. Thus they disappeared, or ceased to be used.
The legislation of the Gode on the tabernacle is contained in the three canons 1268, 1269, and 1271. The first speaks of the place where the Blessed Sacrament should be reserved, or where the tabernacle should be placed; the second, of the tabernacle itself, of its position, of its construction and its ornamentation; and the third, of the lamp before the tabernacle.
The three paragraphs of canon 1268 that bear on the subject, read as follows:
The Blessed Sacrament cannot be reserved continually or regularly on more than one altar in the same church.
The Blessed Sacrament is to be reserved in the most prominent place of honor, and therefore, generally on the main altar, unless there is one more conveniently located and better suited for the veneration and worship of this august Sacrament. The rules regarding the last three days of Holy Week, however, must be observed.
In cathedral, collegiate, and conventual churches, in which choir functions are held at the main altar, the Blessed Sacrament is, as a rule, to be reserved in another chapel or on another than the high altar, in order not to interfere with the services. - The Tabernacle: Its History, Structure and Custody