A satellite moves fastest at its closest approach (perigee) and slowest at its farthest point (apogee) — a circular orbit is just the special case where those two points coincide.
This is Kepler’s second law: a line from the orbited body to the satellite sweeps equal areas in equal time, which forces speed to increase as distance decreases — the same reason a pendulum swings fastest at the bottom of its arc.
Estimate with the rule, then check it against the calculator that models it properly.
Open Orbital Mechanics →