The heliotrope is an instrument that uses a mirror to reflect sunlight over great distances to mark the positions of participants in a land survey. The heliotrope was invented in 1821 by the German mathematician Carl Friedrich Gauss. The word "heliotrope" is taken from the Greek: helios (Greek: Ἥλιος), meaning "sun", and tropos (Greek: τρόπος), meaning "turn". It is a fitting name for an instrument which can be turned to reflect the sun toward a given point.
The heliotrope was utilized by surveyors as a specialized form of target; it was employed during large triangulation surveys where, because of the great distance between stations (usually twenty miles or more), a regular target would appear indistinct. Heliotropes have been used repeatedly as survey targets at ranges of over 100 miles. In California, in 1878, a heliotrope on Mount Saint Helena was surveyed by B.A. Colonna of the USCGS from Mount Shasta, a distance of 192 miles (309 km).
The heliotrope was limited to use on sunny days and was further limited (in regions of high temperatures) to mornings and afternoons when atmospheric aberration least affected the instrument-man's line of sight. The heliotrope operator was called a "heliotroper" or "flasher" and would sometimes employ a second mirror for communicating with the instrument station through heliography, a signalling system using impulsed reflecting surfaces. The inventor of the Heliograph, a similar instrument specialized for signaling, was inspired by observing the use of heliotropes in the survey of India.
- Heliograph, a similar instrument, used in communication
- The Heliotrope, a New Instrument - Arts and Sciences - The Gentleman's Magazine (1822)
- Dunnington, G. Waldo (2004). Carl Friedrich Gauss: Titan of Science. New York: The Mathematical Association of America. pp. 122–127. ISBN 0-88385-547-X.
- NOAA 200th: Collections: Theodolites: heliotrope
- Abbe, Cleveland (December 1900). "Meteorology and Geodesy". Monthly Weather Review. XXVIII (12): 545–547. Retrieved 10 July 2013.
|Wikimedia Commons has media related to Heliotrope.|
- The Surveyor's Heliotrope
- Topographic, Trigonometric and Geodetic Surveying, by Herbert Michael Wilson (1912) pp. 566–574 are devoted to heliotropes
- Elemente der Vermessungskunde, (in German) by Karl Maximilian von Bauernfeind (1862) pp. 115–122 are devoted to Gauss's heliotrope, and the Stierlin and Steinheil heliotropes are described as well.
- The Heliotrope A short history.
- Transits of Venus Page with photographs of three heliotropes from 1873.
- Improvised Heliotrope this 1969 article also provides the US Army part number for a heliotrope.
- Heliotrope Heliotrope photo, description of a 192-mile record.