Sagnac effect

The Sagnac effect is a relativistic phenomenon in which two signals travelling in opposite directions around a rotating system do not arrive at the same time. Because the Earth rotates, a measurable time difference arises in time transfer over long distances.

In precise time transfer via satellite or fibre the Sagnac correction is taken into account; it can amount to hundreds of nanoseconds and is essential for east-west and west-east paths to agree.