# Tidal potential expression using eccentricity

I'm reading these excellent lecture notes about tidal dissipation, and they mention at some point that the time-dependent part of the tidal potential is approximated by (Eq. 17.1 on the third page):

$$~ e\frac{GMR^2}{a^3}$$

Where $$M$$ is the host planet mass, $$R$$ is the satellite radius and $$a$$ is the orbital radius (which I guess it's the semi-major axis since the orbit is eccentric?), and $$e$$ is the eccentricity as far as I understand.

Can someone shed some light on how they got to this approximation for the tidal potential?