I believe tidal forces from the moon pulling on the earth cause a significant amount of heating too.

4.5 thousand million years... or 4.5 billion to us on this side of the pond. Of course I think Numberfile did a good on it... Yep... http://videosift.com/video/How-big-is-a-billion-Numberphile

Heh, I don't think 4.5 thousand million years is normal for anybody to conceive of. I believe the purpose behind saying it that way was to directly compare the magnitude of Kelvin's estimate to the actual estimate we have today. Ie. He estimated 20 million years but the actual age is 45 thousand million years, as in he estimated 20 units but in actuality it was 45,000 units.