(*)
Gaseous isotopes are separated through a type of motion named for this shape. Kepler proved that planetary orbits are elliptical, rather than this shape. (*) Centrifuges move samples along a path with this shape. For 10 points, name this geometric shape whose circumference is equal to twice its radius times pi.
Gaseous isotopes are separated through a type of motion named for this shape.
circles (accept circular paths; accept circular motion; accept rings; do not accept "ellipses")