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Clark, Alvan

(Encyclopedia)Clark, Alvan, 1804–87, American astronomer and maker of astronomical lenses, b. Ashfield, Mass. In 1846 the firm of Alvan Clark & Sons was established at Cambridgeport, Mass.; it became famous a...

hour angle

(Encyclopedia)hour angle, in astronomy, a coordinate in the equatorial coordinate system. The hour angle of a celestial body is the angular distance, expressed in hours, minutes, and seconds (one hour equals 15 deg...

Freundlich, Erwin Finlay

(Encyclopedia)Freundlich, Erwin Finlay froindˈlĭkh [key], 1885–1964, German astronomer. Freundlich obtained a doctorate in mathematics at Göttingen, then joined the Royal Observatory at Berlin, where he worked...

optical sensing

(Encyclopedia)optical sensing, in general, any method by which information that occurs as variations in the intensity, or some other property, of light is translated into an electric signal. This is usually accompl...

Abbe, Ernst

(Encyclopedia)Abbe, Ernst ĕrnst äˈbə [key], 1840–1905, German physicist. He was appointed professor at the Univ. of Jena in 1870 and director of its astronomical and meteorological observatories in 1878. From...

radar

(Encyclopedia)radar, system or technique for detecting the position, movement, and nature of a remote object by means of radio waves reflected from its surface. Although most radar units use microwave frequencies, ...

isopleth

(Encyclopedia)isopleth īˈsəplĕthˌ [key], line drawn on a map through all points of equal value of some measurable quantity. In many meteorologic, oceanographic, or geologic studies some physical or chemical pr...

geologic timescale

(Encyclopedia)CE5 geologic timescale, a chronological scale of earth's history used to measure the relative or absolute age of any part of geologic time. Of the numerous timescales, the most common is based on g...

Hevelius, Johannes

(Encyclopedia)Hevelius, Johannes yōhänˈəs hāvāˈlēo͝os [key], 1611–87, Polish astronomer, b. Danzig. From a finely equipped observatory in his house at Danzig, assisted by his wife Elizabeth, he made valu...

Elephant Butte Dam

(Encyclopedia)Elephant Butte Dam, main unit of the Rio Grande project of the U.S. Bureau of Reclamation on the Rio Grande, SW N.Mex.; completed 1916. The dam, with its large reservoir, is used for flood control, hy...