[xelxebar]

I just wanted to mention that the Schwartzchild metric doesn't only apply to black holes. It's the general metric for a spherically symmetric, static space-time, so it's good for roughly spherical bodies like stars and planets over small timescales.

Recently, I worked out the ISS orbit using the Schwartzchild metric as an approximation of Earth. It's cool to see the orbital period pop out and agree with real life! It's then just a small step to calculate the time dilation experienced by ISS astronauts.