The dome of St. Peter's was designed by Michelangelo, who became chief architect in 1546. At the time of his death (1564), the dome was finished as far as the drum, the base on which domes sit. The dome was vaulted between 1585 and 1590 by the architect Giacomo della Porta with the assistance of Domenico Fontana, who was probably the best engineer of the day. Fontana built the lantern the following year, and the ball was placed in 1593.
The great double dome is made of brick and is 42.3 metres in interior diameter (almost as large as the Pantheon), rising to 120 metres above the floor. In the early 18th century cracks appeared in the dome, so four iron chains were installed between the two shells to bind it. The four piers of the crossing that support the dome are each 60 feet (18 meters) across.
Uniquely, Michelangelo's dome is not a hemisphere, but a parabola: it has a vertical thrust, which is made more emphatic by the bold ribbing that springs from the paired Corinthian columns, which appear to be part of the drum, but which stand away from it like buttresses, to absorb the outward thrust of the dome's weight. Above, the vaulted dome rises to Fontana's two-stage lantern, capped with a spire.
The great dome soars above the altar and the baldacchino, sumptuously embellished with mosaic and stucco ornaments
The giant letters on a gold background, from St. Veronica to St. Helen, say "Hinc una fides mundo refulgent" (From here a single faith shines throughout the world); and from St. Longinus to St. Andrew: "Hinc sacerdotii unitas exoritur" (From here is born the unity of the priesthood).
In the four spandrels which link the square piers and the circular drum, the four Evangelists are portrayed in medallions with a diameter of 8.5 m.: Matthew with the ox, Mark with the lion, Luke with the angel and John with the eagle.
Around the base of the drum we can read the solemn words from Matthew's Gospel with which Jesus invests Peter with supreme authority. The text reads: "Tu es Petrus et super hanc petram aedificabo ecclesiam mean et tibi dabo claves regni caelorum" ("You are 'Rock' and on this rock I will build my Church, to you I will give the keys of the kingdom of heaven." Mt 16:18).
The gigantic black letters (2 m. high) of this text on a gold background are lit by the light from 16 large windows, typical of Michelangelo's style, which punctuate the drum.
Above the windows, the dome is divided into sixteen ribs and as many segments, decorated by majestic figures on six ascending concentric levels.
Starting at the bottom the figures portray:
1. busts of the 16 popes buried in the basilica;
2. majestic figures of Christ, the Virgin Mary, St. Joseph, St. John the Baptist and various Apostles;
3. in the rectangular frames, angels bearing the instruments of Jesus' Passion;
4. the faces of cherubim and seraphim in circular medallions;
5. angels, the custodians of St. Peter's tomb
6. additional faces of winged angels.
Above the 96 figures is a blue sky spangled with stars, and above it the lantern at whose base is a Latin inscription: "To the glory of St. Peter, Pope Sixtus V in the year 1590, the fifth of his pontificate".
The eye then penetrates the lantern which is 18 m. long, and as in a vision come to rest on the glorious figure of God the Father.
Many artists worked on these decorations. Clement VIII commissioned Giuseppe Cesari, known as Cavalier d'Arpino (1568-1640) to carry out the upper part of the decoration. He prepared the cartoons from 1603 to 1612. His drawings were simultaneously transposed into mosaic by the best mosaic artists of the period.
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