Chronology of Eras

[Zantetsuken]

I imagine it would be very easy to determine when an Umbral era begins (usually with a large catastrophe), but how can anyone determine the beginning of an Astral Era?

Wouldn't any gap in war/destruction/etc be treated as potentially the beginning of an Astral era? Or is it just declared in retrospect after years of peaceful living?

[Delsus]

I imagine it would be very easy to determine when an Umbral era begins (usually with a large catastrophe), but how can anyone determine the beginning of an Astral Era?

Wouldn't any gap in war/destruction/etc be treated as potentially the beginning of an Astral era? Or is it just declared in retrospect after years of peaceful living?

If we go by the 6th umbral era which was a huge flood right across Eorzea, the 6th umbral era would have ended when the water receded completely.

The 7th umbral era (in lore terms) will probably be when Bahamut is either killed or imprisoned again, but in game terms may never end.

[Inaaca]

But why were the primals sealed? Who did it? Did Midgardsomr just randomly decide to seal all primals, including it's own tribe god (Bahamut)?

You know, the theory that each era was caused by a Primal may not be so far fetched. They are considered gods by their beast tribes and at no point any primal is mentioned as being "stronger" than other. Maybe each time one of the primals went apeshit crazy. Oh well, who knows.

We don't really know the reasons behind the sealing of the primals, but Bahamut wasn't sealed by Midgarsomr, he was sealed in Dalamud. We also don't really know where Odin fits in all of this.

Er no, Midgarsormr didn't seal anything.

The seal itself is what is/was under the Silvertear Lake. Midgarsormr only guards Silvertear Lake which in turn protects the concentration of aether that lies beneath (part of the Crystal Tower and the tomb of the Allagan Emperor).

The point is that the primals were sealed during Midgarsomr's reign and unable to wreak havok upon anything.