Effelsberg radio telescope (right) and the control centre. This 100-metre radio telescope is located near Bonn, Germany. It is used exclusively for research in radio astronomy, both as an individual instrument and as part of interferometry networks. It is one of the world's largest fully steerable antennae, and operates in a wavelength range from 90 centimetres to 3.5 millimetres. It rotates on a 65-metre-wide circular track (lower right). This radio telescope began operating in 1972. Ground- based radio astronomy is possible because Earth's atmosphere is transparent to radio waves. Galaxies and nebulae are common sources of radio waves.