Direction of movement and Doppler shift in radio frequencies

In cosmology, a positive Doppler shift in radio waves means an object is moving away from us.

This happens because the waves get redshifted — their wavelength stretches, and the frequency decreases — which corresponds to a shift toward the red end of the spectrum. This is typical for galaxies in an expanding universe, where most distant objects exhibit a positive Doppler shift due to the overall expansion of space (Hubble’s law).

A negative Doppler shift, on the other hand, would mean the object is moving toward us, causing blueshift — the wavelength shortens, and the frequency increases.