And returning to the topic..............
I have read reputable scientific papers proposing that the reasons for earths internal heat are threefold.
The first is from the nuclear reaction of radioactive elements in the earths core, however this one is not considered to be the major contributor of thermal energy. There is, however, some disagreement on this from the fringes of the scientific community.
The second is the tidal forces exerted by the moon. This is the one that is thought to contribute the most to the internal stresses that drive plate tectonics and therefore cause earthquakes through plate interactions. This stress also generates heat. The combination of heat and plate stress helps drive volcanic activity.
The earth has the largest moon to planet mass ratio and therefore the strongest moon induced tidal forces.
The third is the current generated by the differential spin of the various layers of the earths core and mantle through it's own magnetic field. As far as I know the amount this contributes to the internal heat is unknown and a subject of speculation.
The tilt of the earth's axis out of alignment with the orbital plane is not considered to be a major factor in anything but the seasons. The earth, at 93,000,000 miles from the sun and only 5000 miles in diameter, would be subjected to only trivial variations in force due to it's tilt on it's axis as it revolves around the sun.
If solar radiation were a contributor of any apreciable amount of the earths internal heat the surface would be unlivable and we would not be here. Just look at mercury for an example. Mercury does not rotate in relation to the sun (the same face always faces the sun) and the sunward side is extremely hot while the side away from the sun is a frozen wasteland well below any temperatures found on earth.
