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[vek-tawr-eez, -tohr-] /vɛkˈtɔr iz, -ˈtoʊr-/ (Show IPA),
1.
Mathematics. the length of the line segment joining a fixed point or origin to a given point.
2.
Astronomy.
1. the straight line joining two bodies in relative orbital motion, as the line from the sun to a planet at any point in its orbit.
2. the distance between two such bodies at any point in the orbit.
1745-1755
First recorded in 1745-55
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2017.
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Examples from the Web for radius vector
Historical Examples
• The angle QOP, which the radius vector makes with the fundamental plane.

• The radius vector (or line joining sun and planet) sweeps out equal areas in equal times.

Oliver Lodge
• In the revolution of a planet round the sun, the radius vector describes equal areas in equal times.

William George Hooper
• He also now answered quite satisfactorily, but too late, the question about the radius vector sent to him months before.

Oliver Lodge
• The radius vector is the imaginary straight line joining the centres of the sun and the earth or planet.

William George Hooper
• And the way it increases will be for the radius vector to lengthen, so as to sweep out a bigger area.

Oliver Lodge
• Hansen, therefore, shows how the radius vector is corrected so as to give that of the true planet.

• The anomaly is then the angle BFP which the radius vector makes with the major axis.

• The line drawn from the sun to any point of the orbit (the string from pin to pencil point) is a radius vector.

George C. Comstock
• Thus the path of the attraction between the two bodies is along the radius vector.

William George Hooper
British Dictionary definitions for radius vector

noun
1.
(maths) a line joining a point in space to the origin of polar or spherical coordinates
2.
(astronomy) an imaginary line joining a satellite to the planet or star around which it is orbiting
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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 radius vector   A line segment that joins the origin and a variable point in a system of polar or spherical coordinates.The imaginary straight line that connects the center of the Sun or another body with the center of a planet, comet, or other body that orbits it.
The American Heritage® Science Dictionary