The size of Heavenly Jerusalem is illustrated here:
http://www.tourofheaven.com/eternal/new-jerusalem/size.aspx
It's about the size of the Moon, coincidentally.
So what about this: What if Heavenly Jerusalem doesn't actually rest ON the Earth, but orbits about it just like the Moon. Isn't it written somewhere that there will be no need of the Sun there because the light of God will illuminate it?
A jewel-like city shining with the light of God would make a good replacement for the Sun, in the event that the shaking of the Heavens involved it's disappearance. And under this Godly illumination, the people of the Millenium might even start living the long lifespans again.
Just a what-if.
Mike B.