I am confused with some astrophysics stuff (esepically Q1), could anyone help me? I would really appreciate if you do.

**1) Calculate the gravitational potential energy of the moon relative to the earth given that the distance between their centers is 3.94x10^8 m.**
Solution:

Eg = -G(5.98x10^24)(7.35x10^22) / (3.94x10^8 )

Eg = -7.40x10^28J

This is an example that I have seen. But I really really don't understand why Eg will have a

*negative value*. (the moon is ABOVE the earth, right? So shouldn't the value of Eg be POSITIVE?) Where is the reference point of the Eg? (at the center of the earth?) For near earth surface case, Eg=mgh, and Eg of objects above the earth will be positive if you set the reference as ground level. "

Sorry I am new on this forum. The reply may be formatted wrong . Whenever two objects attract (as always in case of gravitation , till date ) , the potential energy is negative .

But , see in electrostatics , when you have to calculate potential energy between electron - electron or proton - proton , it is always positive because they are going to repel .

And you know that F = - dU/dr . So , if U is positive , then F is negative , hence repulsion and vice versa .