What Does Δr Represent in the Roche Limit Formula?

[PChar]

Relevant equations

r_{Roche}=(\frac{2M}{m})^{1/3}Δr

I can't seem to find a good explanation of what Δr is. My textbook uses an example of two particles, with that being the distance between their centers, but on the planetary scale, does Δr represent the orbiting body's radius or diameter?

 

[gneill]

Relevant equations

r_{Roche}=(\frac{2M}{m})^{1/3}Δr

I can't seem to find a good explanation of what Δr is. My textbook uses an example of two particles, with that being the distance between their centers, but on the planetary scale, does Δr represent the orbiting body's radius or diameter?

Δr is the radius of the primary. Wikipedia has a derivation.