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Alfonso X, Spanish king of Castile and León

(Encyclopedia)Alfonso X (Alfonso the Wise), 1221–84, Spanish king of Castile and León (1252–84); son and successor of Ferdinand III, whose conquests of the Moors he continued, notably by taking Cádiz (1262). ...

Copernican system

(Encyclopedia)Copernican system, first modern European theory of planetary motion that was heliocentric, i.e., that placed the sun motionless at the center of the solar system with all the planets, including the ea...

Enceladus

(Encyclopedia)Enceladus ĕnsĕlˈədəs [key], in astronomy, one of the named moons, or natural satellites, of Saturn. Also known as Saturn II (or S2), Enceladus is 310 mi (500 km) in diameter, orbits Saturn at a m...

Newcomb, Simon

(Encyclopedia)Newcomb, Simon no͞oˈkəm, nyo͞oˈ– [key], 1835–1909, American astronomer, b. Nova Scotia, grad. Lawrence Scientific School, Harvard, 1858. Living in the United States from 1853, he was appointe...

Local Group

(Encyclopedia)Local Group, in astronomy, loose cluster of at least 40 nearby galaxies, including our own Milky Way galaxy, the Andromeda Galaxy, and the Magellanic Clouds. The Local Group is spread over an ellipsoi...

Laplace, Pierre Simon, marquis de

(Encyclopedia)Laplace, Pierre Simon, marquis de pyĕr sēmôNˈ märkēˈ də läpläsˈ [key], 1749–1827, French astronomer and mathematician. At 18 he went to Paris, proved his gift for mathematical analysis to...

Theodosius II

(Encyclopedia)Theodosius II, 401–50, Roman emperor of the East (408–50), son and successor of Arcadius. He preferred the study of theology and astronomy to public affairs, which he left to the guidance of his s...

solstice

(Encyclopedia)solstice sŏlˈstĭs [key] [Lat.,=sun stands still], in astronomy, either of the two points on the ecliptic that lie midway between the equinoxes (separated from them by an angular distance of 90°). ...

seeing

(Encyclopedia)seeing, in astronomy, the clarity with which stars and other celestial objects can be observed. It is primarily determined by the atmosphere of the earth. The most obvious phenomenon is twinkling, whe...

Sagan, Carl Edward

(Encyclopedia)Sagan, Carl Edward sāˈgən [key], 1934–96, American astronomer and popularizer of science, b. New York City. Early in his career he investigated radio emissions from Venus and concluded that the c...