1. The problem statement, all variables and given/known data A body on an orbit with semi-major axis a and eccentricity e undergoes tidal circularisation. Show that the orbit will circularise at a semi-major axis, acirc, given by acirc = 2rperi = 2a (1 − e). 2. Relevant equations No equations given, but I think the following could be useful E = -GMm/2a e2 = 1 - b2/a2 3. The attempt at a solution An earlier part of the question hints at L conservation Equating centripetal force and grav force for the circular orbit gives: L = m (GMR)0.5 Finding the velocity at the closest point in orbit r = a(1-e) E = -GMm/2a = 1/2 mv2 - GMm/a(1-e) simplifies to v2 = GM(1+e)/rp Equating L2 L2 = GMm2 rp (1+e) = GMm2 rc Finally: rc = rp (1+e) This is close to the final answer, but not quite! Somethings gone wrong somewhere but I'm sure what.. I've checked my working several times. Sorry a lot of my working lines are missing, it's quite tricky to type them all out.