Spin rotation by Earthʼs gravitational field in a “frozen-spin” ring

► Earthʼs gravity effect on particle spin in a “frozen-spin” ring is calculated. ► The coordinate systems used affect observed spin rotations in the vertical plane. ► The resulting ambiguity can be avoided by using a purely electric ring. ► The main systematic error in EDM measurement can thereby be significantly reduced. ► Spin-gravity coupling can be measured in such a ring.