# Roche Limit Formula Question

1. Feb 6, 2012

### 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?

2. Feb 6, 2012

### Staff: Mentor

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