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syzygy

(Encyclopedia)syzygy sĭzˈəjē [key], in astronomy, alignment of three bodies of the solar system along a straight or nearly straight line. A planet is in syzygy with the earth and sun when it is in opposition or...

weight

(Encyclopedia)weight, measure of the force of gravity on a body (see gravitation). Since the weights of different bodies at the same location are proportional to their masses, weight is often used as a measure of m...

reflection

(Encyclopedia)CE5 Image formation by curved mirrors and lenses reflection, return of a wave from a surface that it strikes into the medium through which it has traveled. The general principles governing the ref...

Eratosthenes

(Encyclopedia)Eratosthenes ĕrətŏsˈthənēz [key], c.275–c.195 b.c., Greek scholar, b. Cyrene. A pupil of Callimachus in Athens, he became (c.240 b.c.) head of the library at Alexandria. Known for his versatil...

Reclus, Jean Jacques Élisée

(Encyclopedia)Reclus, Jean Jacques Élisée zhäN zhäk ālēzāˈ rəklüˈ [key], 1830–1905, French geographer, b. Gironde, educated mainly in Germany, where he studied under Karl Ritter. Several times he was f...

node

(Encyclopedia)node, in astronomy, point at which the orbit of a body crosses a reference plane. One reference plane that is often used is the plane of the earth's orbit around the sun (ecliptic). Since the moon's o...

shaft sinking

(Encyclopedia)shaft sinking, excavation from the surface of an opening in the earth. Shafts, which are generally vertical, are usually distinguished from tunnels, which are horizontal. Little difficulty is experien...

equatorial coordinate system

(Encyclopedia)CE5 Equatorial coordinate system equatorial coordinate system, the most commonly used astronomical coordinate system for indicating the positions of stars or other celestial objects on the celesti...

lutetium

(Encyclopedia)lutetium, formerly lutecium both: lo͞otēˈshēəm [key], metallic chemical element; symbol Lu; atomic number 71; at. wt. 174.9668; m.p. about 1,663℃; b.p. about 3,395℃; sp. gr. 9.835 at 25℃; v...

superior planet

(Encyclopedia)superior planet, planet whose orbit lies outside that of the earth. The superior planets are Mars, Jupiter, Saturn, Uranus, and Neptune. ...