Dipleidoscope A dipleidoscope is an instrument used to determine true

noon Noon (also known as noontime or midday) is 12 o'clock in the daytime. It is written as 12 noon, 12:00 m. (for '' meridiem'', literally 12:00 midday), 12 p.m. (for ''post meridiem'', literally "after midday"), 12 pm, or 12:00 (using a 24-hour cl ...

; its name comes from the Greek for ''double image viewer''. It consists of a small

telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

and a

prism PRISM is a code name for a program under which the United States National Security Agency (NSA) collects internet communications from various U.S. internet companies. The program is also known by the SIGAD . PRISM collects stored internet ...

that creates a double image of the sun. When the two images overlap, it is local true noon. The instrument is capable of determining true noon to within ten seconds. The dipleidoscope was invented by

Giovanni Battista Amici Giovanni Battista Amici (; 25 March 1786 – 10 April 1863) was an Italian astronomer, microscopist, and botanist. Amici was born in Modena, in present-day Italy. After studying at Bologna, he became professor of mathematics at Modena, and in 18 ...

in the first half of the 19th century.

Edward John Dent Edward Dent (1790–1853) was a famous English watchmaker noted for his highly accurate clocks and marine chronometers. He founded the Dent company. Early years Edward John Dent, son of John and Elizabeth Dent, was born in London on 19 Augu ...

, a chronometer and clockmaker in London, was working in the 1830s on a simple contrivance that would allow the public to set clocks correctly based on the transit of the sun (more complex and expensive transit telescopes had been developed by

Ole Rømer Ole Christensen Rømer (; 25 September 1644 – 19 September 1710) was a Danes, Danish astronomer who, in 1676, first demonstrated that light travels at a finite speed. Rømer also invented the modern thermometer showing the temperature between ...

in 1690). By 1840, he felt he had come to a suitable design using shadows, however when he communicated his ideas to J.M. Bloxam (a barrister), he found he had also been working on his own design using reflections, which Dent felt was superior. The two formed a partnership and worked together on the device, and after a further 2 years of work, they finalised the design and patented it (GB Patent 9793 of 1843), with Dent manufacturing and selling it as Dent's Dipleidoscope.'Dent's Dipleidoscope, or transit instrument for the correction of time-keepers', The Practical Mechanic and Engineer's Magazine, Jan 1845, pp100-101 The instrument could use the moon as well as the sun and when correctly calibrated and aligned the accuracy was said to be less than a second. Dent exhibited the device at the Great Exhibition of 1851. After Edward Dent died in 1853, his son Frederick William Dent took over manufacture. The significance of this device relates in part to the development of the railways, when an absolute knowledge of the time became more important, whereas previously it was often sufficient that an entire rural community would use the parish clock, and this would periodically be set by 'the announcement of the guard of the mail coach' or similar. The instrument came with a detailed instruction booklet, which had a substantial section on correcting local time to

Greenwich Mean Time Greenwich Mean Time (GMT) is the local mean time at the Royal Observatory, Greenwich, Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being ...

(as used by the railways).

References

External links

A dipleidoscope of the National Observatory of Athens
Optical instruments Clocks Italian inventions {{optics-stub