Human eye can detect electromagnetic radiation within a narrow range from 400 nm to 700 nm, but astronomical objects emit light of all wavelengths from picometer-length gamma rays to a meter-length radio waves. Radio astronomy deals with celestial electromagnetic radiation at radio wavelengths, from 0.3 mm ($\nu=c/\lambda=1$ THz) to 30 m (10 MHz).

Atmospheric opacity and quantum noise determine the boundary between far-infrared and radio astronomy at 1 THz and the ionosphere reflect back all waves longer than 30 m. Below 2 MHz even our Galaxy becomes an absorber, its ionized gas prevents radio radiation from propagating.

If we had radio eyes and looked at the sky, the Sun would still be very bright, but different. Its non-thermal emissions would dominate over the thermal blackbody radiation, the corona would be visible as a hazy envelope. The planets would be seen not in reflected light, but in their own invisible light. The Milky Way would shine not because of its stars, but mainly because of the innumerable high-speed electrons in its interstellar medium. Instead of stars, the night-sky would be filled with compact active galaxies.

Only radio and optical astronomy can be done from the ground efficiently.