Heliograph

A heliograph (from Ancient Greek ἥλιος (hḗlios) 'sun' and γράφειν (gráphein) 'to write') is a solar telegraph[1] system that signals by flashes of sunlight (generally using Morse code from the 1840s) reflected by a mirror.

[2] The heliograph was a simple but effective instrument for instantaneous optical communication over long distances during the late 19th and early 20th centuries.

It was very portable, did not require any power source, and was relatively secure since it was invisible to those not near the axis of operation, and the beam was very narrow, spreading only 50 ft (15 m) per 1 mi (1.6 km) of range.

Stations at higher altitudes benefit from thinner, clearer air, and are required in any event for great ranges, to clear the curvature of the Earth.

The world record distance was established by a detachment of U.S. Army signal sergeants by the inter-operation of stations in North America on Mount Ellen, (Utah), and Mount Uncompahgre, (Colorado), 183 mi (295 km) apart on 17 September 1894, with Army Signal Corps heliographs carrying mirrors only 8 in2 (52 cm2).

[12] The German professor Carl Friedrich Gauss (1777–1855), of the University of Göttingen developed and used a predecessor of the heliograph (the heliotrope) in 1821.

[2][13] His device directed a controlled beam of sunlight to a distant station to be used as a marker for geodetic survey work, and was suggested as a means of telegraphic communications.

For example, one author in 1919 chose to "hazard the theory"[16] that the Italian mainland signals from the capital of Rome that ancient Roman emperor Tiberius (42 B.C.-A.D.37, reigned A.D.14 to 37), watched for from his imperial retreat on the island of Capri.

[22] In a letter dated 3 June 1778, John Norris, High Sheriff of Buckinghamshire, England, notes: "Did this day heliograph intelligence from Dr [Benjamin] Franklin in Paris to Wycombe".

In pre-radio times heliography was often the only means of communication that could span ranges of as much as 100 mi (160 km) with a lightweight portable instrument.

Miles (1839–1925), set up a network of 27 heliograph stations in the Arizona and New Mexico territories of the old Southwest during the extended campaign and hunt for the native Apache renegade chief / guerrilla warfare leader Geronimo (1829–1909).

[33] The network of communication begun by General Miles in 1886, and continued by unsung and now unfortunately relatively unknown Lieutenant W. A. Glassford, was perfected in 1889 at ranges of 85, 88, 95 and 125 mi (137, 142, 153 and 201 km) over a rugged and broken country, which was the stronghold of the Apache, Commanche and other hostile native Indian tribes.

For night communications, the British used some large Aldis lamps, brought inland on railroad cars, and equipped with leaf-type shutters for keying a beam of light into dots and dashes.

With land wire telegraph lines cut, the only contact with the outside world was via light-beam communication, helio by day, and Aldis lamps at night.

[38] During World War II (1939–1945), Union of South Africa and Royal Australian military forces used the heliograph while fighting enemy Nazi German and Fascist Italian forces along the southern coast of the Mediterranean Sea in Libya and western Egypt with fellow defending British military in the desert North African campaign in 1940, 1941 and 1942.

[2] The heliograph remained standard equipment for military signallers in the Royal Australian and British armies until the 1940s, where it was considered a "low probability of intercept" type of communication.

By 1884, all active units of the "Mangin apparatus" (a dual-mode French Army military field optical telegraph that could use either lantern or sunlight) were equipped with clockwork heliostats.

[40][41][42] Proposals to automate both the modulation of the sunbeam (by clockwork) and the detection (by electrical selenium photodetectors, or photographic means) date back to at least 1882.

[43] In 1961, the United States Air Force was working on a space heliograph to signal between satellites[44] In May 2012, "Solar Beacon" robotic mirrors designed at the University of California at Berkeley were mounted on the twin towers of the Golden Gate Bridge at the entrance to San Francisco Bay, and a web site set up[45] where the public could schedule times for the mirrors to signal with sun-flashes, entering the time and their latitude, longitude and altitude.