What is the voltage difference between the Moon and the Earth? The Moon and Earth are approximately spherical conductive balls and the Earth has a self-capacitance of around 710uF. Is there ever a significant potential difference in the Earth-Moon system? Is there even a way to tell if there is? Presumably the Apollo astronauts didn't expect to be electrocuted on landing.
I was thinking that a huge charge would not build up because they are both bathed in a flow of charged particles from the Sun which I would have thought would equalize the voltage. Is this true?
Also what voltage difference would have to build up for a massive bolt of lightening to jump through space between them?
 A: There could easily be a potential difference between the Earth and the Moon but the charge is spread in a relatively homogeneus way on the Earth's surface and on the Moon's too.It means that a very little charge could be delivered to the Moon by the astronauts and their spaceship because their surface area is vanishingly small compared to the earth's surface. So when an astronaut steps down from the spaceship's ladder to the they can not get electrocuted.
But le's calculate an example! Let's assume that the potential difference is $10V$ the Earth's surface area is around $5.101\cdot 10^{14}m^{2}$ and an average adult's surface area is around $2m^{2}$ . So an astronaut could carry $$Q_{ASTRONAUT}=\frac{C_{Earth}A_{Astronaut}}{VA_{Earth}}\approx2.784\cdot10^{-19}\Rightarrow Around\ 2\ electrons! $$
And 2 electrons can't kill a human being. And no phenomena can produce a potential difference between the Earth and the Moon which is big enough for a discharge because there is nothing to ionise in the vacuum of space. I hope my answer was useful :)
