The difference lies in interpreting the two sides of the conflict. McPherson saw the war as the war between the evil South and the good North. He was focused on the abolition of slavery and their integration into the society as free people. Horwitz on the other hand saw South not as evil but as a faction which went to war to protect its economic interests.
Answer:
There are 3 main reasons.
1. heat from when the planet formed and accreted, which has not yet been lost.
2. frictional heating, caused by denser core material sinking into the center of the planet.
3. heat from the decay of radioactive elements.
I hope my answer helped!
The original thirteen colonies were British.
