Superluminal motion in astronomy

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Abstract
Several examples of `intrinsic-type' superluminal motion in astronomy are taken. A simple signal-delay transformation is devised and shown to be sufficient to explain the superluminal effect as resulting from differential signal delay across an expanding source. The distinction between relativistic motion and relativistic kinematics is made. The key kinematical equation used to describe superluminal motion is an alternative statement of the Doppler effect. Relativistic transformations, which are relevant when intervals in different reference frames are compared, then lead to the relativistic Doppler factor (δ), which is applicable to measurements on a photographic image, for example that of a relativistic quasar jet with superluminal components.