The ribbon-cutting ceremony that officially opened the newly completed Ara Pacis Augustae or "Altar of Peace", in Imperial Rome, nine years before the first year of what would eventually become known as the "Christian Era."
I see it as an iconic moment in the history of Western Civilization, pregnant with aspiration, and marking the start of close to Two Hundred Mostly Uninterrupted Years of relative peace and prosperity in Western Europe and around the Mediterranean, a period that became renowned as the "Pax Romana."
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Augustus's reforms transformed Rome's Republican system of government to a de facto monarchy, couched in traditional Roman practices and Republican values. The "princeps" (later known as Emperor) was expected to balance the interests of the Roman military, Senate and people, and to maintain peace, security and prosperity throughout an ethnically diverse empire.
Assuming that in this far-fetched fantasy we'd be offered adequate protection , I would like to witness the January 14th 2005 landing of the Huygens probe on the surface of Saturn's moon, Titan. It's the farthest landing from Earth a spacecraft has ever made. The view of Saturn would be spectacular.
The most transformable choice would probably be something to do with space. The rock that got the dinosaurs? That was probably pretty spectacular viewed from space!