The author describes a new method of using the VLA for measuring small changes in the structures of bright radio sources. As a test of the method, he has measured the angular expansion of NGC 7027, with a signal-to-noise ratio of 30, in a period of only 2.8 yr. Based on a simple model for the dynamics of the nebula, a distance of 940±200 pc is derived. The ability to detect velocities as low as 20 km s⁻¹ at a distance of 1 kpc opens up many exciting areas of research.
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C. R. Masson (1986) studied this question.