The prediction of electromagnetic waves by Maxwell and the demonstration of their existence by Hertz led several scientists to speculate that celestial objects, such as the Sun and stars, might generate radio waves. The following scientists set the groundwork for the later discovery of radio astronomy. Click on each one for a summary of his contibutions.

• James Clerk Maxwell (1870s)
• Heinrich Hertz (1888)
• Thomas Edison (1890)
• Sir Oliver Lodge (1894)
• Wilsing and Scheiner (1896)
• Charles Nordman (1900)
• Max Planck (1900)
• Oliver Heaviside (1902)
• Guglielmo Marconi (1900-1910)

In the 1860s and 1870s, James Clerk Maxwell developed the theory of electric and magnetic forces, summarized in his famous four equations.  These equations encapsulated all that had been discovered about electricity and magnetism in the experiments done over the previous few hundred years by Faraday, Volta, and many others.   They showed that electricity and magnetism were two aspects of the same force.  The equations also predicted that there should be a form of radiation, which came to be known as electromagnetic radiation.   Maxwell realized that light was a form of electromagnetic radiation.   Around 1862 he wrote, "We can scarcely avoid the conclusion that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena."

The equations predicted that electromagnetic radiation could exist with any wavelength.  The various colors of light have wavelengths less than a thousandth of a millimeter.   Much longer wavelengths are possible.  

Read more about Maxwell. (from Univ of St.Andrews, Scotland)

In 1888, Heinrich Hertz built an apparatus that could transmit and receive electromagnetic waves of about 5 meters in length. He used a coil to generate a high voltage spark between two electrodes which served as a transmitter. The detector was a loop of wire with a small gap. A spark at the transmitter produces electromagnetic waves that travel to the detector, producing a spark in the gap. He showed that the waves were polarized, and that they could interfere with each other, just as predicted by theory.

Read more about Hertz (and other pioneers of electronics)

Once Hertz had demonstrated the existence of electromagnetic radiation, the possibility of receiving such radiation from celestial objects may have occurred to many scientists. Edison seems to be the first on record to have proposed an experiment to detect radio waves from the Sun. The evidence of this is a letter sent in 1890 to Lick Observatory by Kennelly, who worked in Edison's laboratory. It describes building a detector by winding several cables around a mass of iron ore. There is no record that the experiment was actually carried out, but it could not have been a success. In hindsight, the proposed apparatus would be very insensitive, and could only detect very long wavelengths. The ionosphere would prevent such long waves from reaching the earth's surface. (The prediction of a reflecting layer in the upper atmosphere, the ionosphere, was made by Kennelly and Heaviside in 1902).

Letter reproduced from "The Evolution of Radio Astronomy", by J.S.Hey, Science History Publications, 1973. See also: C.D.Shane, Pub.Astron. Soc. Pacific 70,303, 1958.
More about Edison:

• Biography
• National Historic Site
• Another Biography
• from K-12 Teaching and Learning Center
• from the Franklin Institute

Sir Oliver Lodge made many innovations in early radio technology, inventing a better radio detector, introducing the use of tuned circuits, and inventing the loud speaker. Around 1897-1900, Lodge attempted to detect radio waves from the Sun. Read his description of the experiment.
The "spot of light" Lodge refers to is a reflecting galvanometer. The experiment was sensitive to centimer wave radiation, which can penetrate the ionosphere. In hindsight, his apparatus was probably not sensitive enough to have detected the Sun. In any case, there were too many sources of radio interference in Liverpool for the experiment to succeed.

Letter reproduced from "Classics in Radio Astronomy", by W.T.Sullivan, Reidel, 1982. Original in Lodge: "Signalling across space without wires", The Electrician Publ.Co., London, 1900.

Read more about Lodge.

Johannes Wilsing (1856-1943) and Julius Scheiner (1858-1913) were astrophysicists who have the distinction of being the first to properly write up and publish their attempt to detect radio radiation from the Sun (Ann.Phys.Chem.59,782, 1896, in German). The diagram of their experiment, on the right, is from "Classics in Radio Astronomy" by W.T. Sullivan, Reidel, 1982. They ran their experiment for eight days, and were unable to detect any signals that could be associated with the Sun. They speculated that they may have been foiled by absorption of radio waves in the atmosphere (they were wrong).

These unsuccesful attempts at finding solar radio waves may have discouraged further experiments. But it is also possible that the theoretical breakthroughs by Planck and Heaviside may have played a role.

The story goes that when Max Planck was a student at the University of Munich, his advisor recommended that he not bother majoring in Physics, because all the problems had been solved. Fortunately he did not take that advice. He later found an unsolved problem, namely the theoretical explanation of the "black body", or thermal radiation curves.

It was known that when dense objects are heated to high temperatures, they radiate energy, and that the graph of intensity versus wavelength followed a curve such as is illustrated here. The higher the temperature, the shorter the wavelength of the peak of the curve.

Planck worked out how to derive this thermal radiation curve from a theory of absorption and emission of radiation by matter. The theory required that energy had to be emitted or absorbed in small packets, or "quanta" of energy. This was a breakthrough in Physics and led to further development of the quantum theory to explain all electro-magnetic phenomena.

The spectrum of light from the Sun very closely resembles a thermal radiation curve. If one applies Planck's theory to predict the amount of radiation we might receive from the Sun in the radio part of the spectrum (wavelengths in the 10 to 100 cm range), the radiation would be very weak: much too weak to be detected by any receiver available around 1900. This theoretical prediction, along with the failure of experiments to detect the Sun, may have discouraged further attempts.

Read more about Planck. (from Univ of St.Andrews, Scotland)

Heaviside and Kennelly, in 1902, predicted that there should be an ionised layer in the upper atmosphere that would reflect radio waves. They pointed out that it would be useful for long distance communication, allowing radio signals to travel to distant parts of the earth by bouncing off the underside of this layer. The existence of the layer, now known as the Heaviside layer or the ionosphere, was demonstrated in the 1920s.

If radio waves bounce off the inside of the ionosphere, then they must also bounce off the outside. So any radio waves from outside the earth would not get through to the ground -- they would bounce back into space.

Thus the predictions by Heaviside, combined with Planck's radiation theory, probably discouraged further attempts to detect radio waves from the Sun and other celestial objects. For whatever reason, there seem to have been no attempts for 30 years, until Jansky's unexpected discovery in 1932.

Later it was learned that the reflection from the ionosphere is very dependent on the frequency (or wavelength). It reflects most of the radiation of frequency less than about 20 MHz. But the ionosphere is not a barrier to frequencies above about 50 MHz. Radio astronomy had to wait for the development of high frequency radio receivers.

Read more about Heaviside. (from Univ of St.Andrews, Scotland)

Some Biographies of Marconi:

• Nobel Prize writeup.
• by S.D.Perry