how does the shape of the earth affect insolation?

Answer 1

The earth is an oblate spheroid.The sun's rays fall directly over the equator, and the heat is concentrated over a small area...this causes temperatures to be high at the low latitudes.
The curvature of the earth at the higher latitudes cause the sun's rays to spread over a much larger area as they fall obliquely on the earth.This spreads out the heat over the larger area, causing the temperatures to be much lower at the poles.

Answer 2

Due to the spherical shape of the earth,the angle of incidence of the sun's rays is more over the poles.Hence the amount of heat received on a given area diminishes from tropics towards the poles.Hence poles remain snow covered most of the time in a year.Moreover this snow or ice reflects whatever insolation received over poles.Further the insolation is used only to melt the ice and not to raise the temperature above freezing point.Thus the poles experience very low temperature.
In short ,you can say that the shape of the earth makes the insolation ineffective over the poles and keeps the poles always(or most of the time in a year)snow or ice covered .

Answer 3

Ummm because the United States was so far from Europe etc... that it could isolate itself without immediately having to be involved in world affairs.