# Why is angular momentum of the Earth/Moon system conserved?

Why is the angular momentum of the Earth/Moon system conserved, apparently unaffected by external forces such as force of the Sun?

• Welcome to SE.Physics! Since this question has attracted 3 answers already, I'm guessing that other folks are getting the gist of what's being asked, however I'm a bit unclear about it personally. Are you basically asking how the conservation of angular momentum can apply to partial subsets of the universe? – Nat Sep 12 '18 at 15:13
• But momentum is conserved about centre of rotation – Ashutosh Sep 12 '18 at 19:02

Angular momentum is conserved when the net external torque is zero (you're thinking about linear momentum) via $\tau = \frac{dL}{dt}$. Anyway, as long as we assume that the earth, moon, and planets are all orbiting around in a disk (which is a decent approximation), then the position vector to the Earth/Moon system from any other of the planets is in the same plane as their perturbing gravitational forces, and furthermore these other planets are sufficiently far away from the Earth/Moon system so that the position vector and force vector are essentially parallel, meaning that the torque, $\tau = \vec{r} \times \vec{F}_{Grav}$, is zero, and thus angular momentum is conserved.