Gravitomagnetism and relative observer clock effects

The gravitomagnetic clock effect and the Sagnac effect for circularly rotat ing orbits in stationary axisymmetric spacetimes are studied from a relativ e observer point of view, clarifying their relationships and the roles play ed by special observer families. In particular, Semerak's recent characteri zation of extremely accelerated observers in terms of the two-clock clock e ffect is shown to be complemented by a similarly special property of the si ngle-clock clock effect.