Giant weeble-wobble - GIF

[Frank S]

toma , the weight of the hemisphere could be calculated by 4/3 pi * radius ^3. (in^3) * 150 (lbs/ft^3) / 1728 (in^3/ft^3)/2 .
As far as if there is some heavier material cast inside near the bottom I would have no idea. but from what I can see of the mounting of the poles it is pretty clear they have a solidly braced mounting to them which would be an indicator of steel reinforcement.