Apparently, I don't know the Moon at all. Earth is a satellite of the Sun, orbital speed of Earth should then be,
\({V}_{e}=\sqrt{3{g}_{e}{r}_{e}}\) = (3*0.00980665*6371)^(0.5)
\({V}_{e}\)=13.69 kms-1
In actual fact, Earth average orbital speed is 29.78 kms-1. The formula used over-estimated Moon speed but under-estimated Earth's speed. That could only mean one thing; I am a fool.
Part of the drop in GPE should have been converted to Spin Kinetic Energy as Earth is spinning about its tilted axis. That would suggest even less calculated speed.
The other explanation is, Earth has already acquired certain velocity when it was captured by the Sun.
