Karl Guthe Jansky - Radio Astronomy

Radio Astronomy

At Bell Telephone Laboratories Jansky built an antenna designed to receive radio waves at a frequency of 20.5 MHz (wavelength about 14.6 meters). It was mounted on a turntable that allowed it to be rotated in any direction, earning it the name "Jansky's merry-go-round". It had a diameter of approximately 100 ft. and stood 20 ft. tall. By rotating the antenna on a set of four Ford Model-T tires, the direction of a received signal could be pinpointed. A small shed to the side of the antenna housed an analog pen-and-paper recording system.

After recording signals from all directions for several months, Jansky eventually categorized them into three types of static: nearby thunderstorms, distant thunderstorms, and a faint steady hiss of unknown origin. He spent over a year investigating the source of the third type of static. The location of maximum intensity rose and fell once a day, leading Jansky to initially surmise that he was detecting radiation from the Sun.

After a few months of following the signal, however, the brightest point moved away from the position of the Sun. Jansky also determined that the signal repeated on a cycle of 23 hours and 56 minutes, the period of the Earth's rotation relative to the cosmos (sidereal day), instead of the 24-hour solar day. By comparing his observations with optical astronomical maps, Jansky concluded that the radiation was coming from the Milky Way and was strongest in the direction of the center the galaxy, in the constellation of Sagittarius.

His discovery was widely publicized, appearing in the New York Times of May 5, 1933. He published his classic paper "Electrical disturbances apparently of extraterrestrial origin" in Proc. IRE in 1933. This paper was re-printed in Proc. IEEE in 1984 (for their centennial issue, where they note the research most likely would have won a Nobel prize, had not the author died young) and again in 1998, for the first centennial of radio. Jansky wanted to follow up on this discovery and investigate the radio waves from the Milky Way in further detail. He submitted a proposal to Bell Labs to build a 30 meter diameter dish antenna with greater sensitivity that would allow more careful measurements of the structure and strength of the radio emission. Bell Labs, however, rejected his request for funding on the grounds that the detected emission would not significantly affect their planned transatlantic communications system. Jansky was re-assigned to another project and did no further work in the field of astronomy.