This was essentially a coronation Mass, though Popes have not been taking the tiara of late. The heir to a throne takes power as soon as he inherits, but he still needs to be confirmed in his position. Until fairly recently (past 150 years), the Pope was also absolute monarch of the Papal States.

I read in a commentary that the symbolism of the eastern patriarchs is to represent the Holy Father as Shepherd of the Universal Church including the Eastern Rites... ie not just Roman Catholics.

Regarding the tiara, I also read that the earliest forms actually were not a triple tiara but rather looked similar to a white helmet- it was only around the middle ages that they started to become tiaras when monarchs became more involved.

Yes, I rather heard someone say that he’s taking up being Bishop of Rome, and I am pedantic enough to have thought, not until he gets to St. John Lateran

Historically, after the period of papal coronations began in the mid-9th century and until the Babylonian Captivity, they were at the Lateran, and thus part of the taking of canonical possession and enthronement.

@Liam, I checked the 2005 edition and it did not seem to have any explanation for the presence of the Eastern Patriarchs (although it did have a brief commentary on the significance of the station at the tomb of St. Peter).

As far as the Mass itself goes, the Praenotanda simply says that it marks the "solemn beginning" of the Pontificate, and highlights the Petrine dimension of the Pope's role especially through the giving of the Pallium (recall the Papal pallium had 2 different forms under Benedict, until Francis changed it back to the standard one) and the Fisherman's Ring. The location is viewed in terms of people gathering at the site where the Apostle (and others) testified to the faith through martyrdom.

PLTT

Thank you. Yes, the papal pallium got an Eastern inflection during B16's papacy.

Tangent: I am rather fond of the Lateran. S Maria Maggiore is my favorite interior of the four major basilicas (the exterior encasing leaves much to be desired), but I think the early 17th century popes and their architects did a better job with what was already at the Lateran (which had suffered much damage over the centuries) than the eradication of Old St Peter's at the turn of the 16th century. Borromini's structural+artistic solution of twinning the nave columns into piers with niches for monumental statuary, while retaining the basic acoustical scale and form of the building for worship, was and is a clear success (Borromini's treatment of the aisles is also sensitive and successful). (St Peter's, for all of its visual grandeur and scale, is, to put it exceedingly charitably, not ideal acoustically; no more deep domes on high drums, please, as they decidedly unhelpful with reverb....) And I much prefer the early 18th century facade of the Lateran to the bloated early 17th century facade of St Peter's that ended up vexing Bernini so much. (St Peter's Square is da bomb compared to the piazza in front of the Lateran, of course.)

Lovely professional images of the Lateran: https://www.goteamjosh.com/blog/stjohn